[clang][C++20] P0960R3 and P1975R0: Allow initializing aggregates from a parenthesized list of values

[ayzhao]

Summary

This patch implements P0960R3, which allows initialization of aggregates
via parentheses.

As an example:

struct S { int i, j; };
S s1(1, 1);

int arr1[2](1, 2);

This patch also implements P1975R0, which fixes the wording of P0960R3
for single-argument parenthesized lists so that statements like the
following are allowed:

S s2(1);
S s3 = static_cast<S>(1);
S s4 = (S)1;

int (&&arr2)[] = static_cast<int[]>(1);
int (&&arr3)[2] = static_cast<int[2]>(1);

This patch was originally authored by @0x59616e and completed by
@ayzhao.

Fixes #54040, Fixes #54041

Co-authored-by: Sheng <[e-mail]>

Full write up : https://discourse.llvm.org/t/c-20-rfc-suggestion-desired-regarding-the-implementation-of-p0960r3/63744

[shafik]

clang/include/clang/AST/ExprCXX.h

4778

Should we prefer makeArrayRef?

clang/lib/CodeGen/CGExprAgg.cpp

1626

clang/lib/Sema/SemaInit.cpp

5272

I feel like this function could use a lot of comments, I found it hard to follow the flow of logic and I think I get it mostly now but comments would have helped a lot.

5282

Or some other clarifying comment.

5319

[ayzhao]

In D129531#3873872, @royjacobson wrote:

Thanks for working on it! It looks really good. Please remember to update the feature test macro (__cpp_aggregate_paren_init).

Done

Also, I think there's no test coverage for the ASTReader/Writer changes? I would like to see some as well.

I added a precompiled header test. Currently, the ASTReader/Writer change is broken with the following error:

                                                                                                                         "hunter-bidens-laptop." 15:58 24-Oct-22
$ bin/llvm-lit -vv --debug ../clang/test/PCH/cxx_paren_init.cpp
[553/553] Creating executable symlink bin/clang
llvm-lit: /llvm-project/llvm/utils/lit/lit/discovery.py:62: note: loading suite config '/llvm-project/build/tools/clang/test/lit.site.cfg.py'
llvm-lit: /llvm-project/llvm/utils/lit/lit/LitConfig.py:116: note: load_config from '/llvm-project/clang/test/lit.cfg.py'
llvm-lit: /llvm-project/llvm/utils/lit/lit/llvm/config.py:456: note: using clang: /llvm-project/build/bin/clang
llvm-lit: /llvm-project/llvm/utils/lit/lit/TestingConfig.py:129: note: ... loaded config '/llvm-project/clang/test/lit.cfg.py'
llvm-lit: /llvm-project/llvm/utils/lit/lit/TestingConfig.py:129: note: ... loaded config '/llvm-project/build/tools/clang/test/lit.site.cfg.py'
llvm-lit: /llvm-project/llvm/utils/lit/lit/discovery.py:136: note: resolved input '../clang/test/PCH/cxx_paren_init.cpp' to 'Clang'::('PCH', 'cxx_paren_init.cpp')
-- Testing: 1 tests, 1 workers --
FAIL: Clang :: PCH/cxx_paren_init.cpp (1 of 1)
******************** TEST 'Clang :: PCH/cxx_paren_init.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   /llvm-project/build/bin/clang -cc1 -internal-isystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -emit-pc
h -o /llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.tmp /llvm-project/clang/test/PCH/cxx_paren_init.h
: 'RUN: at line 2';   /llvm-project/build/bin/clang -cc1 -internal-isystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -include
-pch /llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.tmp -fsyntax-only -S -o -
--
Exit Code: 139

Command Output (stderr):
--
+ : 'RUN: at line 1'
+ /llvm-project/build/bin/clang -cc1 -internal-isystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -emit-pch -o /dev/shm/ayzhao
_llvm/llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.tmp /llvm-project/clang/test/PCH/cxx_paren_init.h
+ : 'RUN: at line 2'
+ /llvm-project/build/bin/clang -cc1 -internal-isystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -include-pch /dev/shm/ayzhao
_llvm/llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.tmp -fsyntax-only -S -o -
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace, preprocessed source, and associated run script.
Stack dump:
0.      Program arguments: /llvm-project/build/bin/clang -cc1 -internal-isystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -in
clude-pch /llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.tmp -fsyntax-only -S -o -
1.      <eof> parser at end of file
2.      /llvm-project/clang/test/PCH/cxx_paren_init.h:4:3: LLVM IR generation of declaration 'foo'
3.      /llvm-project/clang/test/PCH/cxx_paren_init.h:4:3: Generating code for declaration 'foo'
 #0 0x000055ac3d88eeea llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /src/llvm_tmpfs/llvm-project/llvm/lib/Support/Unix/Signals.inc:569:11
 #1 0x000055ac3d88f09b PrintStackTraceSignalHandler(void*) /src/llvm_tmpfs/llvm-project/llvm/lib/Support/Unix/Signals.inc:636:1
 #2 0x000055ac3d88d6e6 llvm::sys::RunSignalHandlers() /src/llvm_tmpfs/llvm-project/llvm/lib/Support/Signals.cpp:104:5
 #3 0x000055ac3d88f7c5 SignalHandler(int) /src/llvm_tmpfs/llvm-project/llvm/lib/Support/Unix/Signals.inc:407:1
 #4 0x00007fbe3c83daf0 (/lib/x86_64-linux-gnu/libc.so.6+0x3daf0)
 #5 0x000055ac3dd9e32c clang::Stmt::getStmtClass() const /src/llvm_tmpfs/llvm-project/clang/include/clang/AST/Stmt.h:1166:44
 #6 0x000055ac3e40c735 clang::ImplicitValueInitExpr::classof(clang::Stmt const*) /src/llvm_tmpfs/llvm-project/clang/include/clang/AST/Expr.h:5527:30
 #7 0x000055ac3e40c715 llvm::isa_impl<clang::ImplicitValueInitExpr, clang::Expr, void>::doit(clang::Expr const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:64:46
 #8 0x000055ac3e40c6f3 llvm::isa_impl_cl<clang::ImplicitValueInitExpr, clang::Expr const*>::doit(clang::Expr const*) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:110:5
 #9 0x000055ac3e40c678 llvm::isa_impl_wrap<clang::ImplicitValueInitExpr, clang::Expr const*, clang::Expr const*>::doit(clang::Expr const* const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:137:5
#10 0x000055ac3e40c652 llvm::isa_impl_wrap<clang::ImplicitValueInitExpr, clang::Expr const* const, clang::Expr const*>::doit(clang::Expr const* const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:127:5
#11 0x000055ac3e40c625 llvm::CastIsPossible<clang::ImplicitValueInitExpr, clang::Expr const*, void>::isPossible(clang::Expr const* const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:255:5
#12 0x000055ac3e40d3d2 llvm::CastInfo<clang::ImplicitValueInitExpr, clang::Expr* const, void>::isPossible(clang::Expr* const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:510:5
#13 0x000055ac3e40d385 bool llvm::isa<clang::ImplicitValueInitExpr, clang::Expr*>(clang::Expr* const&) /src/llvm_tmpfs/llvm-project/llvm/include/llvm/Support/Casting.h:549:3
#14 0x000055ac3e4088ac (anonymous namespace)::AggExprEmitter::EmitInitializationToLValue(clang::Expr*, clang::CodeGen::LValue) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:1536:44
#15 0x000055ac3e402d1e (anonymous namespace)::AggExprEmitter::VisitCXXParenListInitExpr(clang::CXXParenListInitExpr*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:1628:5
#16 0x000055ac3e3fe04d clang::StmtVisitorBase<std::add_pointer, (anonymous namespace)::AggExprEmitter, void>::Visit(clang::Stmt*) /src/llvm_tmpfs/llvm-project/build/tools/clang/include/clang/AST/StmtNodes.inc:877:1
#17 0x000055ac3e3fba35 (anonymous namespace)::AggExprEmitter::Visit(clang::Expr*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:108:3
#18 0x000055ac3e40a9ed (anonymous namespace)::AggExprEmitter::VisitCastExpr(clang::CastExpr*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:864:5
#19 0x000055ac3e40992d clang::StmtVisitorBase<std::add_pointer, (anonymous namespace)::AggExprEmitter, void>::VisitExplicitCastExpr(clang::ExplicitCastExpr*) /src/llvm_tmpfs/llvm-project/build/tools/clang/include/clang/AST/StmtNodes.inc:973:1
#20 0x000055ac3e403c1d clang::StmtVisitorBase<std::add_pointer, (anonymous namespace)::AggExprEmitter, void>::VisitCXXFunctionalCastExpr(clang::CXXFunctionalCastExpr*) /src/llvm_tmpfs/llvm-project/build/tools/clang/include/clang/AST/StmtNodes.inc:989:1
#21 0x000055ac3e3fe17f clang::StmtVisitorBase<std::add_pointer, (anonymous namespace)::AggExprEmitter, void>::Visit(clang::Stmt*) /src/llvm_tmpfs/llvm-project/build/tools/clang/include/clang/AST/StmtNodes.inc:989:1
#22 0x000055ac3e3fba35 (anonymous namespace)::AggExprEmitter::Visit(clang::Expr*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:108:3
#23 0x000055ac3e3fb655 clang::CodeGen::CodeGenFunction::EmitAggExpr(clang::Expr const*, clang::CodeGen::AggValueSlot) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGExprAgg.cpp:2046:1
#24 0x000055ac3dfab89b clang::CodeGen::CodeGenFunction::EmitReturnStmt(clang::ReturnStmt const&) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGStmt.cpp:0:7
#25 0x000055ac3dfa8491 clang::CodeGen::CodeGenFunction::EmitStmt(clang::Stmt const*, llvm::ArrayRef<clang::Attr const*>) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGStmt.cpp:153:75
#26 0x000055ac3dfb1e5f clang::CodeGen::CodeGenFunction::EmitCompoundStmtWithoutScope(clang::CompoundStmt const&, bool, clang::CodeGen::AggValueSlot) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CGStmt.cpp:496:3
#27 0x000055ac3df8295d clang::CodeGen::CodeGenFunction::EmitFunctionBody(clang::Stmt const*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenFunction.cpp:1239:5
#28 0x000055ac3df83732 clang::CodeGen::CodeGenFunction::GenerateCode(clang::GlobalDecl, llvm::Function*, clang::CodeGen::CGFunctionInfo const&) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenFunction.cpp:1448:3
#29 0x000055ac3de1cd02 clang::CodeGen::CodeGenModule::EmitGlobalFunctionDefinition(clang::GlobalDecl, llvm::GlobalValue*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:5321:3
#30 0x000055ac3de13639 clang::CodeGen::CodeGenModule::EmitGlobalDefinition(clang::GlobalDecl, llvm::GlobalValue*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:3596:12
#31 0x000055ac3de18b3a clang::CodeGen::CodeGenModule::EmitGlobal(clang::GlobalDecl) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:3337:5
#32 0x000055ac3de11d50 clang::CodeGen::CodeGenModule::EmitTopLevelDecl(clang::Decl*) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenModule.cpp:6180:5
#33 0x000055ac3ef4e7b0 (anonymous namespace)::CodeGeneratorImpl::HandleTopLevelDecl(clang::DeclGroupRef) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/ModuleBuilder.cpp:188:73
#34 0x000055ac3ef470e0 clang::BackendConsumer::HandleTopLevelDecl(clang::DeclGroupRef) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:232:12
#35 0x000055ac3ef4724b clang::BackendConsumer::HandleInterestingDecl(clang::DeclGroupRef) /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:260:5
#36 0x000055ac3f013cc3 clang::ASTReader::PassInterestingDeclToConsumer(clang::Decl*) /src/llvm_tmpfs/llvm-project/clang/lib/Serialization/ASTReader.cpp:7740:1
#37 0x000055ac3f0ebeba clang::ASTReader::PassInterestingDeclsToConsumer() /src/llvm_tmpfs/llvm-project/clang/lib/Serialization/ASTReaderDecl.cpp:3965:3
#38 0x000055ac3f013e27 clang::ASTReader::StartTranslationUnit(clang::ASTConsumer*) /src/llvm_tmpfs/llvm-project/clang/lib/Serialization/ASTReader.cpp:0:5
#39 0x000055ac417efb74 clang::ParseAST(clang::Sema&, bool, bool) /src/llvm_tmpfs/llvm-project/clang/lib/Parse/ParseAST.cpp:151:20
#40 0x000055ac3ed606ac clang::ASTFrontendAction::ExecuteAction() /src/llvm_tmpfs/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1164:1
#41 0x000055ac3ef432f4 clang::CodeGenAction::ExecuteAction() /src/llvm_tmpfs/llvm-project/clang/lib/CodeGen/CodeGenAction.cpp:1145:5
#42 0x000055ac3ed600ac clang::FrontendAction::Execute() /src/llvm_tmpfs/llvm-project/clang/lib/Frontend/FrontendAction.cpp:1059:7
#43 0x000055ac3ec8f16c clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) /src/llvm_tmpfs/llvm-project/clang/lib/Frontend/CompilerInstance.cpp:1043:23
#44 0x000055ac3ef2c007 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) /src/llvm_tmpfs/llvm-project/clang/lib/FrontendTool/ExecuteCompilerInvocation.cpp:266:8
#45 0x000055ac3b82a5e0 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) /src/llvm_tmpfs/llvm-project/clang/tools/driver/cc1_main.cpp:250:13
#46 0x000055ac3b81c49c ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&) /src/llvm_tmpfs/llvm-project/clang/tools/driver/driver.cpp:316:5
#47 0x000055ac3b81b325 clang_main(int, char**) /src/llvm_tmpfs/llvm-project/clang/tools/driver/driver.cpp:388:5
#48 0x000055ac3b84a8b2 main /src/llvm_tmpfs/llvm-project/build/tools/clang/tools/driver/clang-driver.cpp:11:35
#49 0x00007fbe3c82920a __libc_start_call_main ./csu/../sysdeps/nptl/libc_start_call_main.h:58:16
#50 0x00007fbe3c8292bc call_init ./csu/../csu/libc-start.c:128:20
#51 0x00007fbe3c8292bc __libc_start_main ./csu/../csu/libc-start.c:376:5
#52 0x000055ac3b81aba1 _start (/llvm-project/build/bin/clang+0x54d9ba1)
/llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.cpp.script: line 2: 1747518 Segmentation fault      /llvm-project/build/bin/clang -cc1 -internal-is
ystem /llvm-project/build/lib/clang/16.0.0/include -nostdsysteminc -x c++ -std=c++20 -include-pch /llvm-project/build/tools/clang/test/PCH/Output/cxx_paren_init.c
pp.tmp -fsyntax-only -S -o -

--

********************
********************
Failed Tests (1):
  Clang :: PCH/cxx_paren_init.cpp


Testing Time: 48.40s
  Failed: 1

I'm currently looking into this - I suspect it has to do with CXXParenListInitExpr storing all it's subexpressions in an llvm::TrailingObjects base class, but I have yet to confirm one way or another.

[ayzhao]

some comment fixes, add a test for serialization which doesn't work yet

[ayzhao]

remove whitespace change and disable test

[ilya-biryukov]

clang/lib/Serialization/ASTReaderStmt.cpp

2175

FYI: I think this is where the crash comes from.
We should allocate the trailing objects first.
E.g. see how PragmaCommentDecl::CreateDeserialized does this.

[ayzhao]

clang/lib/Serialization/ASTReaderStmt.cpp

2175

This sounds like it could be the solution - thanks for looking at it!

Currently, I'm working on the refactor that shafik@ suggested, which was to inherit from InitListExpr. Hopefully, that refactor will fix this issue as InitListExpr stores it's subexpressions in a class member instead of using llvm::TrailingObjects.

[ilya-biryukov]

clang/lib/Serialization/ASTReaderStmt.cpp

2175

Are we trying to share code between two implementations? If so, I suggest to consider alternatives, e.g. creating a new base class and inheriting both InitListExpr and CXXParentInitListExpr to share the common code.

Inheriting CXXParentInitListExpr from InitListExpr breaks Liskov substitution principle and will likely to lead to bugs that are hard to chase. InitListExpr is widely used and means`{}. CXXParenInitListExpr` is not {} and we do not know in advance which code is going to work for both and which code is only valid for {}. Reviewing all callsites that use InitListExpr does not seem plausible. Note that in addition to Clang, there are also uses in ast-matchers and in clang-tidy checks (quite a few of those are downstream).

[ayzhao]

Fix PCH test and address some code review comments

[ayzhao]

In D129531#3880875, @ayzhao wrote:

In D129531#3873872, @royjacobson wrote:

Thanks for working on it! It looks really good. Please remember to update the feature test macro (__cpp_aggregate_paren_init).

Done

Also, I think there's no test coverage for the ASTReader/Writer changes? I would like to see some as well.

I added a precompiled header test. Currently, the ASTReader/Writer change is broken with the following error:

...

I'm currently looking into this - I suspect it has to do with CXXParenListInitExpr storing all it's subexpressions in an llvm::TrailingObjects base class, but I have yet to confirm one way or another.

The PCH test is fixed now, so we now have coverage for the ASTReader/Writer changes.

clang/lib/AST/Expr.cpp

3782

See the response from @ilya-biryukov for why I decided against inheriting from InitListExpr.

clang/lib/AST/ExprClassification.cpp

449

IMO; no.

I believe the comment for InitListExpr refers to statements like:

int a;
int &b{a};

This doesn't apply to CXXParenListInitExprClass because the corresponding statement with parentheses wouldn't be parsed as a CXXParenListInitExpr in the first place.

clang/lib/AST/StmtPrinter.cpp

2469

See the response from @ilya-biryukov for why I decided against inheriting from InitListExpr.

clang/lib/Sema/TreeTransform.h

14039

This is an extra change, but clang-format would always make it.

clang/lib/Serialization/ASTReaderStmt.cpp

2175

So I figured out why this was failing. When I created the CXXParenListInitExpr object on the heap in ASTReader::ReadStmtFromStream(...) below, I just called new without allocating any space for the TrailingObjects. This is fixed now, and the PCH test passes.

Inheriting CXXParentInitListExpr from InitListExpr breaks Liskov substitution principle and will likely to lead to bugs that are hard to chase. InitListExpr is widely used and means`{}. CXXParenInitListExpr` is not {} and we do not know in advance which code is going to work for both and which code is only valid for {}. Reviewing all callsites that use InitListExpr does not seem plausible. Note that in addition to Clang, there are also uses in ast-matchers and in clang-tidy checks (quite a few of those are downstream).

+1, I am convinced that inheriting from InitListExpr is the wrong thing to do here. Additionally, when I tried to inherit from InitListExpr, I found that there were a lot of things in that class that were specific to InitListExpr that I had to wrap with an if statement to make it work with CXXParenListInitExpr, and that the amount of code that I had to add considerably decreases readability.

[ayzhao]

we don't need CHECK-DAG here

[ayzhao]

add C++20 extension diagnostics and some other code review feedback

[ayzhao]

Friendly ping for reviewers as all comments should be addressed by now.

In D129531#3873775, @cor3ntin wrote:

It starting to look great.
Should we add an extension warning? Note that I'm not suggesting to support that in earlier modes, just an off-by-default warning that says "this is a c++20 feature". but we are a bit inconsistent with those.

Done

can you rename test/SemaCXX/P0960R3.cpp to something like test/SemaCXX/cxx20-paren-list-init.cpp (and same for other tests?) it's more consistent with other existing tests.

Done

clang/lib/Sema/SemaInit.cpp

5272

I added some comments, did a little refactoring, and renamed some variables. PTAL and LMKWYT.

[cor3ntin]

I added a few more comments, mostly nitpicks

clang/lib/AST/ExprConstant.cpp

9978–9979

9982

Maybe that message could be a bit more clear.

"Union should have exactly 1 initializer in in C++ paren list" or something like that.

clang/lib/CodeGen/CGExprAgg.cpp

1603

There are some overlaps with VisitInitListExpr, I wonder if we should try to factor out some of the code, especially the base class initialization

1608

Maybe it would be cleaner to use a structured binding here.

1624

Should we assert that the base is not virtual?

clang/lib/Sema/SemaInit.cpp

5291

clang/lib/Sema/TreeTransform.h

14047

ArgumentChanged is not used, and is defaulted by the function, so you should be able to remove it

[ayzhao]

address some comments and implement disallowing flexible array members

[ayzhao]

Status update:

While investigating @cor3ntin's comment about refactoring VisitInitListExpr and VisitCXXParenListInitExpr as they share common code, I discovered that flexible array members would cause this patch to explode. Specifically, the following code:

struct S {
    int b;
    int a[];
};

S s2(1, {1, 2});

was causing this assertion to fail.

I took a look at GCC, and it turns out that GCC doesn't allow flexible array members at all in paren list initialization of aggregates, even in contexts where the corresponding designated initializer expression succeeds: https://godbolt.org/z/E73433erb

For now, I updated this implementation to follow GCC's behavior and disallow flexible array members, as IIRC in C++ they are a GCC extension and we should therefore follow GCC behavior. However, I am open to other thoughts.

[ayzhao]

clang-format + rebase

[ayzhao]

Merge CXXParenListInitExpr with VisitInitListExpr, also friendly ping as all comments are addressed

[cor3ntin]

I think this is basically good to go, but I'll let other people give it a look too

clang/lib/AST/ExprConstant.cpp

9991–9994

clang/lib/CodeGen/CGExprAgg.cpp

91

Could you give E a better name?

484–486

Ditto

[ayzhao]

addressed code review comments

[ayzhao]

rebase + update commit message

[ayzhao]

Friendly ping, does anyone else have any more comments?

[ayzhao]

whoops, uploaded the wrong commit. should be fixed now

[ayzhao]

remove extra parens

[ayzhao]

s/pro20/post20/g

[cor3ntin]

There is a C++ standard meeting this week so some folks won't be able to review it. Feel free to re-ping next week if nobody else replies.
Because it's a fairly big change and I've been known to miss glaring things, I don't feel comfortable to approve it. But it looks great to me.
Either way we need to make sure this ships in Clang 16.

[shafik]

I am in the committee meeting this week but if you ping next week I should have time.

[ilya-biryukov]

Sorry for not reviewing this sooner. This looks very good overall, but I still have some NITs and a few major comments. For the major points see:

• the comment about the filler and the syntactic/semantic form for the newly added expression,
• the comment about relying on getFailedCandidateSet().size().

clang/lib/AST/ExprCXX.cpp

1778

NIT: add newline

clang/lib/AST/ExprConstant.cpp

9991

NIT: maybe add braces as the other branches have them? This seems to be in the style guide:

readability is also harmed if an if/else chain does not use braced bodies for either all or none of its members

10974

Could we add an assertion that the array size and the InitExprs.size() are the same?
E.g. it can differ in the case of InitListExpr (probably due to the filler optimization?), it would be nice to add a sanity check here.

clang/lib/AST/JSONNodeDumper.cpp

853

NIT: maybe use the same formatting as other switch cases for constistency?

clang/lib/CodeGen/CGExprAgg.cpp

484–486

NIT: maybe add a comment that ExprToVisit is either InitListExpr or CXXParenInitListExpr?

585

• Should we have a filler for CXXParenInitListExpr too? It seems like an important optimization and could have large effect on compile times if we don't

• Same question for semantic and syntactic-form (similar to InitListExpr): should we have it here? I am not sure if it's semantically required (probably not?), but that's definitely something that clang-tidy and other source tools will rely on.

We should probably share the code there, I suggest moving it to a shared base class and using it where appropriate instead of the derived classes.

clang/lib/Sema/SemaInit.cpp

6200

It seems shaky to rely on the size of getFailedCandidateSet.
What are we trying to achieve here? Is there a more direct way to specify it?

6219

NIT: maybe move the full comment into the body of the function?
It describes in detail what the body of the function does and it would be easier to understand whether the code fits the intention in the standard.
Having the first part of the comment here would still be useful, probably.

clang/lib/Serialization/ASTReaderStmt.cpp

2174

Could we assign the CXXParenInitListExpr.NumExprs on construction and add the assertion sanity-checking the numbers here:

unsigned NumExprs = Record.readInt();
(void)NumExprs; // to avoid unused warnings in NDEBUG builds.
assert(NumExprs == E->NumExprs);

(Looked up how DesignatedInitExpr does a similar thing, probably good for consistency in addition to added safety)

[ayzhao]

address code review comments

clang/lib/AST/JSONNodeDumper.cpp

853

Unfortunately clang-format insists that these be on separate lines.

clang/lib/CodeGen/CGExprAgg.cpp

585

Should we have a filler for CXXParenInitListExpr too? It seems like an important optimization and could have large effect on compile times if we don't

This feels like premature optimization - presumably, wouldn't this only be an issue with extraordinarily large (say, O(1000)) arrays?

Same question for semantic and syntactic-form (similar to InitListExpr): should we have it here? I am not sure if it's semantically required (probably not?), but that's definitely something that clang-tidy and other source tools will rely on

IIRC this doesn't apply to paren list aggregate expressions, as the syntatic form would be the enclosing ParenListExpr.

clang/lib/Sema/SemaInit.cpp

6200

Done.

As mentioned in the above comment, we try to parse this as a CXXParenListInit if the only failed overload constructor candidates are the default constructor, the default copy constructor, and the default move constructor).

6219

Done. Most of this comment is spread out throughout the body of the function anyways.

[ilya-biryukov]

Thanks for addressing the comments and sorry for a wait before the comments.
Please see the comment about syntactic form, I might be holding it wrong, but it looks like we actually loose the syntactic form completely in this case.

clang/lib/AST/JSONNodeDumper.cpp

853

Ah, that's unfortunate. I normally just revert the effect of clang-format for those lines, but up to you.
Also fine to keep as is or even format the other lines according to the style guide (it's just 3 more lines, so should not be a big deal).

clang/lib/CodeGen/CGExprAgg.cpp

585

This feels like premature optimization - presumably, wouldn't this only be an issue with extraordinarily large (say, O(1000)) arrays?

Yes, this should only happen with large arrays. Normally I would agree, but it's surprising that changing {} to () in the compiler would lead to performance degradation.
In that sense, this premature optimization is already implemented, we are rather degrading performance for a different syntax to do the same thing.

Although we could also land without it, but in that case could you open a GH issue and add a FIXME to track the implementation of this particular optimization?
This should increase the chances of users finding the root cause of the issue if they happen to hit it.

IIRC this doesn't apply to paren list aggregate expressions, as the syntatic form would be the enclosing ParenListExpr.

Do we even have the enclosing ParenListExpr? If I dump the AST with clang -fsyntax-only -Xclang=-ast-dump ... for the following code:

struct pair { int a; int b = 2; };
int main() {
  pair(1); pair p(1);
  pair b{1}; pair{1};
}

I get

`-FunctionDecl 0x557d79717e98 <line:2:1, line:5:1> line:2:5 main 'int ()'
  `-CompoundStmt 0x557d797369d0 <col:12, line:5:1>
    |-CXXFunctionalCastExpr 0x557d79718528 <line:3:3, col:9> 'pair':'pair' functional cast to pair <NoOp>
    | `-CXXParenListInitExpr 0x557d79718500 <col:3> 'pair':'pair'
    |   |-IntegerLiteral 0x557d79718010 <col:8> 'int' 1
    |   `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
    |-DeclStmt 0x557d79718650 <line:3:12, col:21>
    | `-VarDecl 0x557d79718568 <col:12, col:17> col:17 p 'pair':'pair' parenlistinit
    |   `-CXXParenListInitExpr 0x557d79718610 <col:17> 'pair':'pair'
    |     |-IntegerLiteral 0x557d797185d0 <col:19> 'int' 1
    |     `-IntegerLiteral 0x557d79717e18 <line:1:30> 'int' 2
    |-DeclStmt 0x557d797187d8 <line:4:3, col:12>
    | `-VarDecl 0x557d79718680 <col:3, col:11> col:8 b 'pair':'pair' listinit
    |   `-InitListExpr 0x557d79718750 <col:9, col:11> 'pair':'pair'
    |     |-IntegerLiteral 0x557d797186e8 <col:10> 'int' 1
    |     `-CXXDefaultInitExpr 0x557d797187a0 <col:11> 'int'
    `-CXXFunctionalCastExpr 0x557d797369a8 <col:14, col:20> 'pair':'pair' functional cast to pair <NoOp>
      `-InitListExpr 0x557d79718868 <col:18, col:20> 'pair':'pair'
        |-IntegerLiteral 0x557d79718800 <col:19> 'int' 1
        `-CXXDefaultInitExpr 0x557d797188b8 <col:20> 'int'

It feels like the ParentListExpr is replaced during semantic analysis and there is no way to get it back. I also tried running clang-query and trying to match parenListExpr() and go 0 results. Is it just missing in the AST dump and I run clang-query incorrectly or do we actually not have the syntactic form of this expression after all?

[ilya-biryukov]

clang/lib/CodeGen/CGExprAgg.cpp

585

Another important thing to address from the dump: notice how braced initialization creates CXXDefaultInitExpr and CXXParenListInitExpr copies the default argument expression directly. It's really important to use the former form, here's the example that currently breaks:

#include <iostream>

struct func_init { int some_int; const char* fn = __builtin_FUNCTION(); };

int main() {
  func_init a(10);
  std::cout << "From paren init:" << a.fn << std::endl;

  func_init b{10};
  std::cout << "From braced init: " << b.fn << std::endl;
}

The program above is expected to report main for both cases, but instead we get:

From paren init:
From braced init: main

[ayzhao]

implement CXXDefaultInitExpr and ImplicitValueInitExpr, array fillers, and semantic vs syntatic forms.

clang/lib/CodeGen/CGExprAgg.cpp

585

The following have now been implemented:

• CXXDefaultInitExpr and ImplicitValueInitExpr, which includes adding a test for __builtin_FUNCTION()
• Array fillers
• Semantic forms vs syntactic forms

[ilya-biryukov]

Thanks!

I have two major comments and also inline NITs. Not sure if we should block on those, just wanted to hear your opinions:

• InitListExpr and CXXParenInitListExpr have some code in common. Code duplication is substantial and I think sharing the common implementation is warranted. E.g. if some code would want to change something with ArrayFiller or make a use of it, the work will probably need to be duplicated. To reiterate what I mentioned earlier, I believe deriving CXXParenInitListExpr from InitListExpr is not a very good option, but we might explore other possibilities: a base class or factoring out common code in a separate struct. I believe this could be done with a follow-up change, though, as the amount of changes required does not seem too big, I would be happy with first landing this patch and then cleaning up with a follow-up change.
• Did we explore the possibility of modelling this differently and somehow avoiding making CXXParenInitListExpr an expression. This seems to borrow from InitListExpr, but maybe for the wrong reasons. Braced initializers can actually occur in expressions positions, e.g. int a = {123}. However, CXXParenInistListExpr cannot occur in expression positions outside initializations, e.g. having CXXFunctionalCastExpr for aggregate_struct(123) feels somewhat wrong to me and seems to lead to confusing error messages too. Maybe we should model it as a new expression that contains both the type and the argument list? I.e. more similar to CXXConstructExpr?

clang/lib/Sema/SemaInit.cpp

5455

NIT: s/expressionse/expressions

9780

NIT: add break

clang/lib/Sema/TreeTransform.h

14044

reinterpret_cast seems redundant here or am I missing something?

clang/lib/Serialization/ASTReaderStmt.cpp

2172

NIT: ExpectedNumExprs

2181

NIT: maybe add a comment that dependence does not depend on filler or syntactic form. This might be non-obvious for someone who did not look into the implementation.

Alternatively, you could run this at the end of this function, it should not be confusing to anyone that certain properties get computed *after* the expression was fully deserialized.

clang/test/CXX/class/class.compare/class.spaceship/p1.cpp

106 ↗

(On Diff #479758)

Do you have any idea why did this diagnostic change?

clang/test/CXX/drs/dr2xx.cpp

161 ↗

(On Diff #479758)

This might be related to other diagnostic change.
Why do we see the new behavior here? Do you think we should keep the old behavior maybe?
I feel that both errors are communicating the same thing and probably keeping the current state is acceptable, just want to better understand how we got there.

It definitely has something to do with the initialization code, but I struggle to understand what exactly changed

clang/test/CXX/temp/temp.decls/temp.variadic/p4.cpp

131

Given how pervasive this error is right now, I feel that we want to add a name of the struct to this message.
This case is also a good example of how this diagnostic can be really low quality with templates: it's unclear which exact base class causes a problem here from the compiler output,.

Maybe open a GH issue for that? It seems like an independent task that will also affect braced initializers and may need test file updates.

clang/test/PCH/cxx_paren_init.cpp

30

NIT: add a newline

clang/test/PCH/cxx_paren_init.h

6

NIT: add a newline

[ayzhao]

In D129531#3971392, @ilya-biryukov wrote:

Thanks!

I have two major comments and also inline NITs. Not sure if we should block on those, just wanted to hear your opinions:

• InitListExpr and CXXParenInitListExpr have some code in common. Code duplication is substantial and I think sharing the common implementation is warranted. E.g. if some code would want to change something with ArrayFiller or make a use of it, the work will probably need to be duplicated. To reiterate what I mentioned earlier, I believe deriving CXXParenInitListExpr from InitListExpr is not a very good option, but we might explore other possibilities: a base class or factoring out common code in a separate struct. I believe this could be done with a follow-up change, though, as the amount of changes required does not seem too big, I would be happy with first landing this patch and then cleaning up with a follow-up change.
• Did we explore the possibility of modelling this differently and somehow avoiding making CXXParenInitListExpr an expression. This seems to borrow from InitListExpr, but maybe for the wrong reasons. Braced initializers can actually occur in expressions positions, e.g. int a = {123}. However, CXXParenInistListExpr cannot occur in expression positions outside initializations, e.g. having CXXFunctionalCastExpr for aggregate_struct(123) feels somewhat wrong to me and seems to lead to confusing error messages too. Maybe we should model it as a new expression that contains both the type and the argument list? I.e. more similar to CXXConstructExpr?

As discussed offline, let's keep the current implementation as is for now.

[ayzhao]

use NumUserSpecifiedExprs instead of syntatic/semantic forms, also address some comments

[ayzhao]

clang/lib/CodeGen/CGExprAgg.cpp

585

Instead of implementing separate syntactic and semantic forms, I think it might be simpler and cleaner to store an unsigned NumUserSpecifiedExprs.

The reason for implementing it this way is that the purpose of having separate syntactic and semantic forms is to determine what the form in the code was originally as written and what the form is after semantic analysis. For InitListExpr, we need separate objects because there can be a significant difference in the two forms (e.g. designated initializers, etc.). However, CXXParenListInitExpr is much simpler - the first k expressions are user-specified, while the remaining are default/compiler-generated. Therefore, we can eliminate the complexity required for maintaining two forms (say, in TreeTransform) by just storing k.

N.B. this also resolves a build failure with an earlier diff where the libc++ bots were failing.

There is also precedence for this kind of structure - function calls with default arguments use similar logic instead of creating two Expr objects.

[ayzhao]

add missing EOF newline

[ayzhao]

restore original diagnostics

clang/test/CXX/class/class.compare/class.spaceship/p1.cpp

106 ↗

(On Diff #479758)

Fixed - I needed to add a line to SemaCast.cpp

clang/test/CXX/temp/temp.decls/temp.variadic/p4.cpp

131

Created https://github.com/llvm/llvm-project/issues/59367

[ayzhao]

clang/lib/Sema/SemaInit.cpp

5380

So the libc++ test compile failures are due to this line.

One example of a failing unit test is range.take.while/ctor.view.pass. Clang calls this function twice in TreeTransform.h - once with VerifyOnly set to true, once with it set to false.

For some reason, when this function tries to value-initialize the member MoveOnly mo in View, Seq.Failed() returns false after TryValueInitialization(...), but the resulting ExprResult is nullptr, causing the segfault we see when we push nullptr to InitExprs and pass InitExprs to the constructor of CXXParenListInitExpr. One way to be fix this is to move the line ExprResult ER = Seq.Perform(...) out of the if (!VerifyOnly) block and check for ER.isInvalid() instead of Seq.Failed(), but that results in test failures due to excess diagnostic messages in Seq.Perform(...)

I'm still looking into this, but if anyone has any ideas, they would be very welcome.

To repro the buildbot failures, just build clang with this patch, and then in a separate build directory, build the target check-cxx using the previously built clang.

[ayzhao]

rebase + (hopefully) fix libc++ c++2b test failures

clang/lib/Sema/SemaInit.cpp

5380

I was able to get the above workaround to pass the test by clearing the diagnostics after calling Seq.Perform(...).

IMO, this should be OK for now, but I'm open to better ideas if anyone has any.

[ayzhao]

clang/lib/AST/JSONNodeDumper.cpp

853

Yeah, I would definitely prefer to keep the current style. The problem though is that the buildbots run clang format and will fail if the patch isn't formatted correctly.

[ilya-biryukov]

clang/include/clang/AST/ExprCXX.h

4823

NIT: maybe just use return ArrayFiller? The code is much simpler and since functions are very close it's pretty much impossible that anyone would refactor one of those without looking at the other.

clang/lib/AST/ExprConstant.cpp

10006

NIT: !Args.empty() ? Args[0] : &VIE
Args.size() also works, but arguably captures the intent of the code a bit less clearly.

clang/lib/CodeGen/CGExprAgg.cpp

585

That looks fine indeed, I agree that CXXParenInitListExpr is simpler.

clang/lib/Sema/SemaInit.cpp

5281

NIT: reminder to remove this and other debug prints

5326

NIT: maybe rename to SubSeq or something similar?
Having both Seq and Sequence in scope with the same type is error-prone and hard to comprehend.

5380

Clearing all the diagnostics is a nuclear options and definitely seems off here. We should not call Perform() when VerifyOnly is true to avoid producing the diagnostics in the first place.

It's fine for the call with VerifyOnly = true to return no errors and later produce diagnostics with VerifyOnly = false, I believe this is what InitListChecker is already doing.
I have been playing around with the old version of the code, but couldn't fix it fully. I do have a small example that breaks, we should add it to the test and it should also be easier to understand what's going on:

struct MoveOnly
{
  MoveOnly(int data = 1);
  MoveOnly(const MoveOnly&) = delete;
  MoveOnly(MoveOnly&&) = default;
};

struct View {
  int a;
  MoveOnly mo;
};

void test() {
  View{0};
  View(0); // should work, but crashes and produces invalid diagnostics.
}

In general, my approach would be to try mimicing what InitListChecker is doing as much as possible, trimming all the unnecessary complexity that braced-init-lists entail.
Hope it's helpful.

[ayzhao]

address comments + rebase

[ayzhao]

clang/lib/Sema/SemaInit.cpp

5380

So it looks like all I had to do was remove the call to TryValueInitialization(...) and just check for Seq.Failed(). This is also what we do in InitListChecker.

The check-cxx target appears to work for me locally, so fingers crossed that the build passes.

I do have a small example that breaks, we should add it to the test and it should also be easier to understand what's going on:

Done.

[ilya-biryukov]

This LG, but not accepting as I believe libc++ is still broken if built with modules, right?
I have run check-cxx locally and it seems to work, but I suspect that's not using modules by default. I have clicked through the links in phrabricator and the errors seem to come from the latest uploaded diff.
Let me know if I'm missing something.

clang/lib/Sema/SemaInit.cpp

5380

Good catch! I have also noticed that TryValueInitialization is missing in InitListChecker, but somehow thought it's just doing the same thing without reusing the code.

[ilya-biryukov]

Since the errors only shows up in modular builds, I would look closely for bugs inside ASTReader/ASTWriter.
Also, it seems that ArrayFiller is not taken in to account in computeDependence and maybe it should be. I am not 100% sure, though: if ArrayFiller is only used for non-dependent code, it should not case this bug. It also does not explain the variation between modular and non-modular builds.

[ayzhao]

rebase + some compiler warning fixes

[ayzhao]

In D129531#3988996, @ilya-biryukov wrote:

Since the errors only shows up in modular builds, I would look closely for bugs inside ASTReader/ASTWriter.
Also, it seems that ArrayFiller is not taken in to account in computeDependence and maybe it should be. I am not 100% sure, though: if ArrayFiller is only used for non-dependent code, it should not case this bug. It also does not explain the variation between modular and non-modular builds.

These test failures are a little weird because they seem to alternate between being able to reliably reproduce vs not being able to reproduce at all.

One thing that I did notice is that the tests fail with RelWithDebInfo builds, but pass with all of the other builds (Debug, Release, and MinSizeRel). Right now though, I'm unable to reproduce the libc++ failure even with RelWithDebInfo.

Incidentally, I just rebased this patch, and it seems like the dr2xx.cpp test is also failing in a similar manner (fails on RelWithDebInfo but passes on other builds).

[ayzhao]

fix memory access issues identified by UBSan

[ayzhao]

In D129531#3990050, @ayzhao wrote:

In D129531#3988996, @ilya-biryukov wrote:

Since the errors only shows up in modular builds, I would look closely for bugs inside ASTReader/ASTWriter.

These test failures are a little weird because they seem to alternate between being able to reliably reproduce vs not being able to reproduce at all.

One thing that I did notice is that the tests fail with RelWithDebInfo builds, but pass with all of the other builds (Debug, Release, and MinSizeRel). Right now though, I'm unable to reproduce the libc++ failure even with RelWithDebInfo.

Incidentally, I just rebased this patch, and it seems like the dr2xx.cpp test is also failing in a similar manner (fails on RelWithDebInfo but passes on other builds).

All of the builds are green now - it turns out that I had to resolve 2 instances of UB in this patch.

Also, it seems that ArrayFiller is not taken in to account in computeDependence and maybe it should be. I am not 100% sure, though: if ArrayFiller is only used for non-dependent code, it should not case this bug. It also does not explain the variation between modular and non-modular builds

I don't think this should be an issue - InitListExpr doesn't take ArrayFiller into account in computeDependence(...) either: https://github.com/llvm/llvm-project/blob/c8647738cd654d9ecfdc047e480d05a997d3127b/clang/lib/AST/ComputeDependence.cpp#L636-L641

[ilya-biryukov]

LGTM! Thanks a lot for driving this to conclusion! Please note that there is one small NIT you may want to fix before landing this.
UB definitely explains why we saw the bugs. It's good that we caught it before this landed.

In D129531#3992998, @ayzhao wrote:

I don't think this should be an issue - InitListExpr doesn't take ArrayFiller into account in computeDependence(...) either: https://github.com/llvm/llvm-project/blob/c8647738cd654d9ecfdc047e480d05a997d3127b/clang/lib/AST/ComputeDependence.cpp#L636-L641

Yeah, makes sense, if InitListExpr does not take into account we are probably good here as well.
My intuition is that we can only figure out what to set in ArrayFiller when the expressions are non-dependent, therefore the filler itself should not add any extra "dependence" to the expression.
If that's the case, it would be nice to add an assertion. But that's unrelated to this change, we can do it some other day.

clang/lib/Sema/SemaInit.cpp

6199

NIT: use getFailureKind(), it asserts that Failed() == true and would have catched this particular UB.

[ayzhao]

use getFailureKind() + rebase

[shafik]

clang/include/clang/AST/ExprCXX.h

4760

It looks like we use this idiom in several places, it may be worth it to sink this as a member function of QualType

clang/lib/CodeGen/CGExprAgg.cpp

1609

This is second time I see you looking for the initialized field for a union while InitListExpr has getInitializedFieldInUnion(). Would it be too painful to also do that for CXXParenListInitExpr to avoid the code duplication?

clang/lib/Sema/SemaInit.cpp

5411

nit

[ayzhao]

clang/include/clang/AST/ExprCXX.h

4760

note: this patch was already merged, so I created a new patch addressing the revisions at D140159

It turns out there's a method getValueKindForType(...) that does the exact same thing, so I used that instead.

[hokein]

FYI, this patch triggers a crash in chromium, see https://github.com/llvm/llvm-project/issues/59675 for details.

[rsmith]

clang/lib/Sema/SemaInit.cpp

6198–6199

For when you look at re-landing this, we encountered a regression for this case:

struct A {};
struct B : A { Noncopyable nc; };

Here, given B b, a B(b) direct-initialization should be rejected because it selects a deleted copy constructor of B, but it looks like Clang instead performed aggregate initialization, as if by B{(const A&)b, {}}. This is pretty bad -- copies that should be disallowed end up working but not actually copying any members of the derived class!

The general problem is that Clang's overload resolution incorrectly handles deleted functions, treating them as an overload resolution failure rather than a success. In this particular case, what happens is constructor overload resolution produces OR_Deleted, which gets mapped into FK_ConstructorOverloadFailed despite the fact that overload resolution succeeded and picked a viable function.

I guess we can split FK_*OverloadFailed into separate "failed" and "succeeded but found a deleted function" cases?

[ayzhao]

clang/lib/Sema/SemaInit.cpp

6198–6199

For when you look at re-landing this, we encountered a regression for this case:

struct A {};
struct B : A { Noncopyable nc; };

Here, given B b, a B(b) direct-initialization should be rejected because it selects a deleted copy constructor of B, but it looks like Clang instead performed aggregate initialization, as if by B{(const A&)b, {}}. This is pretty bad -- copies that should be disallowed end up working but not actually copying any members of the derived class!

The general problem is that Clang's overload resolution incorrectly handles deleted functions, treating them as an overload resolution failure rather than a success. In this particular case, what happens is constructor overload resolution produces OR_Deleted, which gets mapped into FK_ConstructorOverloadFailed despite the fact that overload resolution succeeded and picked a viable function.

I guess we can split FK_*OverloadFailed into separate "failed" and "succeeded but found a deleted function" cases?

This was the cause of https://github.com/llvm/llvm-project/issues/59675 as well. The reland at D141546 should handle this case.

clang/docs/ReleaseNotes.rst

- Clang now correctly delays the instantiation of function constraints until
   `Issue 44178 <https://github.com/llvm/llvm-project/issues/44178>`_.
 - Clang implements DR2621, correcting a defect in ``using enum`` handling.  The
   name is found via ordinary lookup so typedefs are found.
 - Implemented `P0634r3 <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0634r3.html>`_,
   which removes the requirement for the ``typename`` keyword in certain contexts.
 - Implemented The Equality Operator You Are Looking For (`P2468 <http://wg21.link/p2468r2>`_).
 - Implemented `P2113R0: Proposed resolution for 2019 comment CA 112 <https://wg21.link/P2113R0>`_
   ([temp.func.order]p6.2.1 is not implemented, matching GCC).
 - Implemented `P0857R0 <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0857r0.html>`_,
   which specifies constrained lambdas and constrained template *template-parameter*\s.
 - Required parameter pack to be provided at the end of the concept parameter list. This
   fixes `Issue 48182 <https://github.com/llvm/llvm-project/issues/48182>`_.
 
 - Do not hide templated base members introduced via using-decl in derived class
   (useful specially for constrained members). Fixes `GH50886 <https://github.com/llvm/llvm-project/issues/50886>`_.
 - Implemented CWG2635 as a Defect Report, which prohibits structured bindings from being constrained.
 
+- Implemented `P0960R3: <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p0960r3.html>`_
+  and `P1975R0: <https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1975r0.html>`_,
+  which allows parenthesized aggregate-initialization.
+
 C++2b Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Support label at end of compound statement (`P2324 <https://wg21.link/p2324r2>`_).
 - Implemented `P1169R4: static operator() <https://wg21.link/P1169R4>`_ and `P2589R1: static operator[] <https://wg21.link/P2589R1>`_.
 - Implemented "char8_t Compatibility and Portability Fix" (`P2513R3 <https://wg21.link/P2513R3>`_).
   This change was applied to C++20 as a Defect Report.
 - Implemented "Permitting static constexpr variables in constexpr functions" (`P2647R1 <https://wg21.link/P2647R1>_`).

clang/include/clang-c/Index.h

enum CXCursorKind {
    */
   CXCursor_ConceptSpecializationExpr = 153,
 
   /**
    * Expression that references a C++20 concept.
    */
   CXCursor_RequiresExpr = 154,
 
-  CXCursor_LastExpr = CXCursor_RequiresExpr,
+  /**
+   * Expression that references a C++20 parenthesized list aggregate
+   * initializer.
+   */
+  CXCursor_CXXParenListInitExpr = 155,
+
+  CXCursor_LastExpr = CXCursor_CXXParenListInitExpr,
 
   /* Statements */
   CXCursor_FirstStmt = 200,
   /**
    * A statement whose specific kind is not exposed via this
    * interface.
    *
    * Unexposed statements have the same operations as any other kind of

clang/include/clang/AST/ASTNodeTraverser.h

public:
   }
 
   void VisitInitListExpr(const InitListExpr *ILE) {
     if (auto *Filler = ILE->getArrayFiller()) {
       Visit(Filler, "array_filler");
     }
   }
 
+  void VisitCXXParenListInitExpr(const CXXParenListInitExpr *PLIE) {
+    if (auto *Filler = PLIE->getArrayFiller()) {
+      Visit(Filler, "array_filler");
+    }
+  }
+
   void VisitBlockExpr(const BlockExpr *Node) { Visit(Node->getBlockDecl()); }
 
   void VisitOpaqueValueExpr(const OpaqueValueExpr *Node) {
     if (Expr *Source = Node->getSourceExpr())
       Visit(Source);
   }
 
   void VisitGenericSelectionExpr(const GenericSelectionExpr *E) {

clang/include/clang/AST/ComputeDependence.h

 class CXXTemporaryObjectExpr;
 class CXXDefaultInitExpr;
 class CXXDefaultArgExpr;
 class LambdaExpr;
 class CXXUnresolvedConstructExpr;
 class CXXDependentScopeMemberExpr;
 class MaterializeTemporaryExpr;
 class CXXFoldExpr;
+class CXXParenListInitExpr;
 class TypeTraitExpr;
 class ConceptSpecializationExpr;
 class SYCLUniqueStableNameExpr;
 class PredefinedExpr;
 class CallExpr;
 class OffsetOfExpr;
 class MemberExpr;
 class ShuffleVectorExpr;

 ExprDependence computeDependence(CXXDefaultInitExpr *E);
 ExprDependence computeDependence(CXXDefaultArgExpr *E);
 ExprDependence computeDependence(LambdaExpr *E,
                                  bool ContainsUnexpandedParameterPack);
 ExprDependence computeDependence(CXXUnresolvedConstructExpr *E);
 ExprDependence computeDependence(CXXDependentScopeMemberExpr *E);
 ExprDependence computeDependence(MaterializeTemporaryExpr *E);
 ExprDependence computeDependence(CXXFoldExpr *E);
+ExprDependence computeDependence(CXXParenListInitExpr *E);
 ExprDependence computeDependence(TypeTraitExpr *E);
 ExprDependence computeDependence(ConceptSpecializationExpr *E,
                                  bool ValueDependent);
 
 ExprDependence computeDependence(SYCLUniqueStableNameExpr *E);
 ExprDependence computeDependence(PredefinedExpr *E);
 ExprDependence computeDependence(CallExpr *E, llvm::ArrayRef<Expr *> PreArgs);
 ExprDependence computeDependence(OffsetOfExpr *E);

clang/include/clang/AST/Decl.h

public:
   enum InitializationStyle {
     /// C-style initialization with assignment
     CInit,
 
     /// Call-style initialization (C++98)
     CallInit,
 
     /// Direct list-initialization (C++11)
-    ListInit
+    ListInit,
+
+    /// Parenthesized list-initialization (C++20)
+    ParenListInit
   };
 
   /// Kinds of thread-local storage.
   enum TLSKind {
     /// Not a TLS variable.
     TLS_None,
 
     /// TLS with a known-constant initializer.

clang/include/clang/AST/ExprCXX.h

child_range children() {
     return child_range(SubExprs, SubExprs + SubExpr::Count);
   }
 
   const_child_range children() const {
     return const_child_range(SubExprs, SubExprs + SubExpr::Count);
   }
 };
 
+/// Represents a list-initialization with parenthesis.
+///
+/// As per P0960R3, this is a C++20 feature that allows aggregate to
+/// be initialized with a parenthesized list of values:
+/// ```
+/// struct A {
+///   int a;
+///   double b;
+/// };
+///
+/// void foo() {
+///   A a1(0);        // Well-formed in C++20
+///   A a2(1.5, 1.0); // Well-formed in C++20
+/// }
+/// ```
+/// It has some sort of similiarity to braced
+/// list-initialization, with some differences such as
+/// it allows narrowing conversion whilst braced
+/// list-initialization doesn't.
+/// ```
+/// struct A {
+///   char a;
+/// };
+/// void foo() {
+///   A a(1.5); // Well-formed in C++20
+///   A b{1.5}; // Ill-formed !
+/// }
+/// ```
+class CXXParenListInitExpr final
+    : public Expr,
+      private llvm::TrailingObjects<CXXParenListInitExpr, Expr *> {
+  friend class TrailingObjects;
+  friend class ASTStmtReader;
+  friend class ASTStmtWriter;
+
+  unsigned NumExprs;
+  unsigned NumUserSpecifiedExprs;
+  SourceLocation InitLoc, LParenLoc, RParenLoc;
+  Expr *ArrayFiller = nullptr;
+
+  CXXParenListInitExpr(ArrayRef<Expr *> Args, QualType T,
+                       unsigned NumUserSpecifiedExprs, SourceLocation InitLoc,
+                       SourceLocation LParenLoc, SourceLocation RParenLoc)
+      : Expr(CXXParenListInitExprClass, T,
+             T->isLValueReferenceType()   ? VK_LValue
+             : T->isRValueReferenceType() ? VK_XValue
+                                          : VK_PRValue,
+             OK_Ordinary),
+        NumExprs(Args.size()), NumUserSpecifiedExprs(NumUserSpecifiedExprs),
+        InitLoc(InitLoc), LParenLoc(LParenLoc), RParenLoc(RParenLoc) {
+    std::copy(Args.begin(), Args.end(), getTrailingObjects<Expr *>());
+    assert(NumExprs >= NumUserSpecifiedExprs &&
+           "number of user specified inits is greater than the number of "
+           "passed inits");
+    setDependence(computeDependence(this));
+  }
+
+  size_t numTrailingObjects(OverloadToken<Expr *>) const { return NumExprs; }
+
+public:
+  static CXXParenListInitExpr *
+  Create(ASTContext &C, ArrayRef<Expr *> Args, QualType T,
+         unsigned NumUserSpecifiedExprs, SourceLocation InitLoc,
+         SourceLocation LParenLoc, SourceLocation RParenLoc);
+
+  static CXXParenListInitExpr *CreateEmpty(ASTContext &C, unsigned numExprs,
+                                           EmptyShell Empty);
+
+  explicit CXXParenListInitExpr(EmptyShell Empty, unsigned NumExprs)
+      : Expr(CXXParenListInitExprClass, Empty), NumExprs(NumExprs),
+        NumUserSpecifiedExprs(0) {}
+
+  void updateDependence() { setDependence(computeDependence(this)); }
+
+  ArrayRef<Expr *> getInitExprs() {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(), NumExprs);
+  }
+
+  const ArrayRef<Expr *> getInitExprs() const {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(), NumExprs);
+  }
+
+  ArrayRef<Expr *> getUserSpecifiedInitExprs() {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(),
+                              NumUserSpecifiedExprs);
+  }
+
+  const ArrayRef<Expr *> getUserSpecifiedInitExprs() const {
+    return llvm::makeArrayRef(getTrailingObjects<Expr *>(),
+                              NumUserSpecifiedExprs);
+  }
+
+  SourceLocation getBeginLoc() const LLVM_READONLY { return LParenLoc; }
+
+  SourceLocation getEndLoc() const LLVM_READONLY { return RParenLoc; }
+
+  SourceLocation getInitLoc() const LLVM_READONLY { return InitLoc; }
+
+  SourceRange getSourceRange() const LLVM_READONLY {
+    return SourceRange(getBeginLoc(), getEndLoc());
+  }
+
+  void setArrayFiller(Expr *E) { ArrayFiller = E; }
+
+  Expr *getArrayFiller() { return ArrayFiller; }
+
+  const Expr *getArrayFiller() const { return ArrayFiller; }
+
+  child_range children() {
+    Stmt **Begin = reinterpret_cast<Stmt **>(getTrailingObjects<Expr *>());
+    return child_range(Begin, Begin + NumExprs);
+  }
+
+  const_child_range children() const {
+    Stmt *const *Begin =
+        reinterpret_cast<Stmt *const *>(getTrailingObjects<Expr *>());
+    return const_child_range(Begin, Begin + NumExprs);
+  }
+
+  static bool classof(const Stmt *T) {
+    return T->getStmtClass() == CXXParenListInitExprClass;
+  }
+};
+
 /// Represents an expression that might suspend coroutine execution;
 /// either a co_await or co_yield expression.
 ///
 /// Evaluation of this expression first evaluates its 'ready' expression. If
 /// that returns 'false':
 ///  -- execution of the coroutine is suspended
 ///  -- the 'suspend' expression is evaluated
 ///     -- if the 'suspend' expression returns 'false', the coroutine is

clang/include/clang/AST/RecursiveASTVisitor.h

 DEF_TRAVERSE_STMT(CXXNoexceptExpr, {})
 DEF_TRAVERSE_STMT(PackExpansionExpr, {})
 DEF_TRAVERSE_STMT(SizeOfPackExpr, {})
 DEF_TRAVERSE_STMT(SubstNonTypeTemplateParmPackExpr, {})
 DEF_TRAVERSE_STMT(SubstNonTypeTemplateParmExpr, {})
 DEF_TRAVERSE_STMT(FunctionParmPackExpr, {})
 DEF_TRAVERSE_STMT(CXXFoldExpr, {})
 DEF_TRAVERSE_STMT(AtomicExpr, {})
+DEF_TRAVERSE_STMT(CXXParenListInitExpr, {})
 
 DEF_TRAVERSE_STMT(MaterializeTemporaryExpr, {
   if (S->getLifetimeExtendedTemporaryDecl()) {
     TRY_TO(TraverseLifetimeExtendedTemporaryDecl(
         S->getLifetimeExtendedTemporaryDecl()));
     ShouldVisitChildren = false;
   }
 })
 // For coroutines expressions, traverse either the operand

clang/include/clang/Basic/DiagnosticSemaKinds.td

 def err_reference_bind_init_list : Error<
   "reference to type %0 cannot bind to an initializer list">;
 def err_init_list_bad_dest_type : Error<
   "%select{|non-aggregate }0type %1 cannot be initialized with an initializer "
   "list">;
 def warn_cxx20_compat_aggregate_init_with_ctors : Warning<
   "aggregate initialization of type %0 with user-declared constructors "
   "is incompatible with C++20">, DefaultIgnore, InGroup<CXX20Compat>;
+def warn_cxx17_compat_aggregate_init_paren_list : Warning<
+  "aggregate initialization of type %0 from a parenthesized list of values "
+  "is a C++20 extension">, DefaultIgnore, InGroup<CXX20>;
 
 def err_reference_bind_to_bitfield : Error<
   "%select{non-const|volatile}0 reference cannot bind to "
   "bit-field%select{| %1}2">;
 def err_reference_bind_to_vector_element : Error<
   "%select{non-const|volatile}0 reference cannot bind to vector element">;
 def err_reference_bind_to_matrix_element : Error<
   "%select{non-const|volatile}0 reference cannot bind to matrix element">;

clang/include/clang/Basic/StmtNodes.td

 def PackExpansionExpr : StmtNode<Expr>;
 def SizeOfPackExpr : StmtNode<Expr>;
 def SubstNonTypeTemplateParmExpr : StmtNode<Expr>;
 def SubstNonTypeTemplateParmPackExpr : StmtNode<Expr>;
 def FunctionParmPackExpr : StmtNode<Expr>;
 def MaterializeTemporaryExpr : StmtNode<Expr>;
 def LambdaExpr : StmtNode<Expr>;
 def CXXFoldExpr : StmtNode<Expr>;
+def CXXParenListInitExpr: StmtNode<Expr>;
 
 // C++ Coroutines TS expressions
 def CoroutineSuspendExpr : StmtNode<Expr, 1>;
 def CoawaitExpr : StmtNode<CoroutineSuspendExpr>;
 def DependentCoawaitExpr : StmtNode<Expr>;
 def CoyieldExpr : StmtNode<CoroutineSuspendExpr>;
 
 // C++20 Concepts expressions

clang/include/clang/Sema/Initialization.h

enum StepKind {
     /// Perform initialization via a constructor taking a single
     /// std::initializer_list argument.
     SK_StdInitializerListConstructorCall,
 
     /// Initialize an OpenCL sampler from an integer.
     SK_OCLSamplerInit,
 
     /// Initialize an opaque OpenCL type (event_t, queue_t, etc.) with zero
-    SK_OCLZeroOpaqueType
+    SK_OCLZeroOpaqueType,
+
+    /// Initialize an aggreagate with parenthesized list of values.
+    /// This is a C++20 feature.
+    SK_ParenthesizedListInit
   };
 
   /// A single step in the initialization sequence.
   class Step {
   public:
     /// The kind of conversion or initialization step we are taking.
     StepKind Kind;
 
enum FailureKind {
     FK_PlaceholderType,
 
     /// Trying to take the address of a function that doesn't support
     /// having its address taken.
     FK_AddressOfUnaddressableFunction,
 
     /// List-copy-initialization chose an explicit constructor.
     FK_ExplicitConstructor,
+
+    /// Parenthesized list initialization failed at some point.
+    /// This is a C++20 feature.
+    FK_ParenthesizedListInitFailed,
   };
 
 private:
   /// The reason why initialization failed.
   FailureKind Failure;
 
   /// The failed result of overload resolution.
   OverloadingResult FailedOverloadResult;
public:
   /// Add a step to initialize an OpenCL sampler from an integer
   /// constant.
   void AddOCLSamplerInitStep(QualType T);
 
   /// Add a step to initialzie an OpenCL opaque type (event_t, queue_t, etc.)
   /// from a zero constant.
   void AddOCLZeroOpaqueTypeStep(QualType T);
 
+  void AddParenthesizedListInitStep(QualType T);
+
   /// Add steps to unwrap a initializer list for a reference around a
   /// single element and rewrap it at the end.
   void RewrapReferenceInitList(QualType T, InitListExpr *Syntactic);
 
   /// Note that this initialization sequence failed.
   void SetFailed(FailureKind Failure) {
     SequenceKind = FailedSequence;
     this->Failure = Failure;

clang/include/clang/Serialization/ASTBitCodes.h

enum StmtCode {
   EXPR_USER_DEFINED_LITERAL,
 
   /// A CXXStdInitializerListExpr record.
   EXPR_CXX_STD_INITIALIZER_LIST,
 
   /// A CXXBoolLiteralExpr record.
   EXPR_CXX_BOOL_LITERAL,
 
+  /// A CXXParenListInitExpr record.
+  EXPR_CXX_PAREN_LIST_INIT,
+
   EXPR_CXX_NULL_PTR_LITERAL, // CXXNullPtrLiteralExpr
   EXPR_CXX_TYPEID_EXPR,      // CXXTypeidExpr (of expr).
   EXPR_CXX_TYPEID_TYPE,      // CXXTypeidExpr (of type).
   EXPR_CXX_THIS,             // CXXThisExpr
   EXPR_CXX_THROW,            // CXXThrowExpr
   EXPR_CXX_DEFAULT_ARG,      // CXXDefaultArgExpr
   EXPR_CXX_DEFAULT_INIT,     // CXXDefaultInitExpr
   EXPR_CXX_BIND_TEMPORARY,   // CXXBindTemporaryExpr

clang/lib/AST/ComputeDependence.cpp

ExprDependence clang::computeDependence(CXXFoldExpr *E) {
   auto D = ExprDependence::TypeValueInstantiation;
   for (const auto *C : {E->getLHS(), E->getRHS()}) {
     if (C)
       D |= C->getDependence() & ~ExprDependence::UnexpandedPack;
   }
   return D;
 }
 
+ExprDependence clang::computeDependence(CXXParenListInitExpr *E) {
+  auto D = ExprDependence::None;
+  for (const auto *A : E->getInitExprs())
+    D |= A->getDependence();
+  return D;
+}
+
 ExprDependence clang::computeDependence(TypeTraitExpr *E) {
   auto D = ExprDependence::None;
   for (const auto *A : E->getArgs())
     D |= toExprDependenceAsWritten(A->getType()->getDependence()) &
          ~ExprDependence::Type;
   return D;
 }
 

clang/lib/AST/Expr.cpp

bool Expr::HasSideEffects(const ASTContext &Ctx,
   case CXXPseudoDestructorExprClass:
   case CXXRewrittenBinaryOperatorClass:
   case CXXStdInitializerListExprClass:
   case SubstNonTypeTemplateParmExprClass:
   case MaterializeTemporaryExprClass:
   case ShuffleVectorExprClass:
   case ConvertVectorExprClass:
   case AsTypeExprClass:
+  case CXXParenListInitExprClass:
     // These have a side-effect if any subexpression does.
     break;
 
   case UnaryOperatorClass:
     if (cast<UnaryOperator>(this)->isIncrementDecrementOp())
       return true;
     break;
 
bool Expr::HasSideEffects(const ASTContext &Ctx,
   case ObjCPropertyRefExprClass:
   // FIXME: Classify these cases better.
     if (IncludePossibleEffects)
       return true;
     break;
   }
 
   // Recurse to children.
   for (const Stmt *SubStmt : children())
     if (SubStmt &&
         cast<Expr>(SubStmt)->HasSideEffects(Ctx, IncludePossibleEffects))
       return true;
 
   return false;
 }
 
 FPOptions Expr::getFPFeaturesInEffect(const LangOptions &LO) const {

clang/lib/AST/ExprCXX.cpp

CUDAKernelCallExpr *CUDAKernelCallExpr::CreateEmpty(const ASTContext &Ctx,
                                                     EmptyShell Empty) {
   // Allocate storage for the trailing objects of CallExpr.
   unsigned SizeOfTrailingObjects = CallExpr::sizeOfTrailingObjects(
       /*NumPreArgs=*/END_PREARG, NumArgs, HasFPFeatures);
   void *Mem = Ctx.Allocate(sizeof(CUDAKernelCallExpr) + SizeOfTrailingObjects,
                            alignof(CUDAKernelCallExpr));
   return new (Mem) CUDAKernelCallExpr(NumArgs, HasFPFeatures, Empty);
 }
+
+CXXParenListInitExpr *
+CXXParenListInitExpr::Create(ASTContext &C, ArrayRef<Expr *> Args, QualType T,
+                             unsigned NumUserSpecifiedExprs,
+                             SourceLocation InitLoc, SourceLocation LParenLoc,
+                             SourceLocation RParenLoc) {
+  void *Mem = C.Allocate(totalSizeToAlloc<Expr *>(Args.size()));
+  return new (Mem) CXXParenListInitExpr(Args, T, NumUserSpecifiedExprs, InitLoc,
+                                        LParenLoc, RParenLoc);
+}
+
+CXXParenListInitExpr *CXXParenListInitExpr::CreateEmpty(ASTContext &C,
+                                                        unsigned NumExprs,
+                                                        EmptyShell Empty) {
+  void *Mem = C.Allocate(totalSizeToAlloc<Expr *>(NumExprs),
+                         alignof(CXXParenListInitExpr));
+  return new (Mem) CXXParenListInitExpr(Empty, NumExprs);
+}

clang/lib/AST/ExprClassification.cpp

case Expr::InitListExprClass:
     return ClassifyInternal(Ctx, cast<InitListExpr>(E)->getInit(0));
 
   case Expr::CoawaitExprClass:
   case Expr::CoyieldExprClass:
     return ClassifyInternal(Ctx, cast<CoroutineSuspendExpr>(E)->getResumeExpr());
   case Expr::SYCLUniqueStableNameExprClass:
     return Cl::CL_PRValue;
     break;
+
+  case Expr::CXXParenListInitExprClass:
+    if (isa<ArrayType>(E->getType()))
+      return Cl::CL_ArrayTemporary;
+    return Cl::CL_ClassTemporary;
   }
 
   llvm_unreachable("unhandled expression kind in classification");
 }
 
 /// ClassifyDecl - Return the classification of an expression referencing the
 /// given declaration.
 static Cl::Kinds ClassifyDecl(ASTContext &Ctx, const Decl *D) {

clang/lib/AST/ExprConstant.cpp

public:
     bool VisitCXXConstructExpr(const CXXConstructExpr *E) {
       return VisitCXXConstructExpr(E, E->getType());
     }
     bool VisitLambdaExpr(const LambdaExpr *E);
     bool VisitCXXInheritedCtorInitExpr(const CXXInheritedCtorInitExpr *E);
     bool VisitCXXConstructExpr(const CXXConstructExpr *E, QualType T);
     bool VisitCXXStdInitializerListExpr(const CXXStdInitializerListExpr *E);
     bool VisitBinCmp(const BinaryOperator *E);
+    bool VisitCXXParenListInitExpr(const CXXParenListInitExpr *E);
+    bool VisitCXXParenListOrInitListExpr(const Expr *ExprToVisit,
+                                         ArrayRef<Expr *> Args);
   };
 }
 
 /// Perform zero-initialization on an object of non-union class type.
 /// C++11 [dcl.init]p5:
 ///  To zero-initialize an object or reference of type T means:
 ///    [...]
 ///    -- if T is a (possibly cv-qualified) non-union class type,
case CK_UncheckedDerivedToBase: {
     return true;
   }
   }
 }
 
 bool RecordExprEvaluator::VisitInitListExpr(const InitListExpr *E) {
   if (E->isTransparent())
     return Visit(E->getInit(0));
+  return VisitCXXParenListOrInitListExpr(E, E->inits());
+}
 
-  const RecordDecl *RD = E->getType()->castAs<RecordType>()->getDecl();
+bool RecordExprEvaluator::VisitCXXParenListOrInitListExpr(
+    const Expr *ExprToVisit, ArrayRef<Expr *> Args) {
+  const RecordDecl *RD =
+      ExprToVisit->getType()->castAs<RecordType>()->getDecl();
   if (RD->isInvalidDecl()) return false;
   const ASTRecordLayout &Layout = Info.Ctx.getASTRecordLayout(RD);
   auto *CXXRD = dyn_cast<CXXRecordDecl>(RD);
 
   EvalInfo::EvaluatingConstructorRAII EvalObj(
       Info,
       ObjectUnderConstruction{This.getLValueBase(), This.Designator.Entries},
       CXXRD && CXXRD->getNumBases());
 
   if (RD->isUnion()) {
-    const FieldDecl *Field = E->getInitializedFieldInUnion();
+    const FieldDecl *Field;
+    if (auto *ILE = dyn_cast<InitListExpr>(ExprToVisit)) {
+      Field = ILE->getInitializedFieldInUnion();
+    } else if (isa<CXXParenListInitExpr>(ExprToVisit)) {
+      assert(Args.size() == 1 &&
+             "Unions should have exactly 1 initializer in a C++ paren list");
+
+      for (const FieldDecl *FD : RD->fields()) {
+        if (FD->isUnnamedBitfield())
+          continue;
+
+        Field = FD;
+        break;
+      }
+    } else {
+      llvm_unreachable(
+          "Expression is neither an init list nor a C++ paren list");
+    }
+
     Result = APValue(Field);
     if (!Field)
       return true;
 
     // If the initializer list for a union does not contain any elements, the
     // first element of the union is value-initialized.
     // FIXME: The element should be initialized from an initializer list.
     //        Is this difference ever observable for initializer lists which
     //        we don't build?
     ImplicitValueInitExpr VIE(Field->getType());
-    const Expr *InitExpr = E->getNumInits() ? E->getInit(0) : &VIE;
+    const Expr *InitExpr = Args.empty() ? &VIE : Args[0];
 
     LValue Subobject = This;
     if (!HandleLValueMember(Info, InitExpr, Subobject, Field, &Layout))
       return false;
 
     // Temporarily override This, in case there's a CXXDefaultInitExpr in here.
     ThisOverrideRAII ThisOverride(*Info.CurrentCall, &This,
                                   isa<CXXDefaultInitExpr>(InitExpr));
if (!Result.hasValue())
     Result = APValue(APValue::UninitStruct(), CXXRD ? CXXRD->getNumBases() : 0,
                      std::distance(RD->field_begin(), RD->field_end()));
   unsigned ElementNo = 0;
   bool Success = true;
 
   // Initialize base classes.
   if (CXXRD && CXXRD->getNumBases()) {
     for (const auto &Base : CXXRD->bases()) {
-      assert(ElementNo < E->getNumInits() && "missing init for base class");
-      const Expr *Init = E->getInit(ElementNo);
+      assert(ElementNo < Args.size() && "missing init for base class");
+      const Expr *Init = Args[ElementNo];
 
       LValue Subobject = This;
       if (!HandleLValueBase(Info, Init, Subobject, CXXRD, &Base))
         return false;
 
       APValue &FieldVal = Result.getStructBase(ElementNo);
       if (!EvaluateInPlace(FieldVal, Info, Subobject, Init)) {
         if (!Info.noteFailure())
bool RecordExprEvaluator::VisitCXXParenListOrInitListExpr(
   for (const auto *Field : RD->fields()) {
     // Anonymous bit-fields are not considered members of the class for
     // purposes of aggregate initialization.
     if (Field->isUnnamedBitfield())
       continue;
 
     LValue Subobject = This;
 
-    bool HaveInit = ElementNo < E->getNumInits();
+    bool HaveInit = ElementNo < Args.size();
 
     // FIXME: Diagnostics here should point to the end of the initializer
     // list, not the start.
-    if (!HandleLValueMember(Info, HaveInit ? E->getInit(ElementNo) : E,
+    if (!HandleLValueMember(Info, HaveInit ? Args[ElementNo] : ExprToVisit,
                             Subobject, Field, &Layout))
       return false;
 
     // Perform an implicit value-initialization for members beyond the end of
     // the initializer list.
     ImplicitValueInitExpr VIE(HaveInit ? Info.Ctx.IntTy : Field->getType());
-    const Expr *Init = HaveInit ? E->getInit(ElementNo++) : &VIE;
+    const Expr *Init = HaveInit ? Args[ElementNo++] : &VIE;
 
     if (Field->getType()->isIncompleteArrayType()) {
       if (auto *CAT = Info.Ctx.getAsConstantArrayType(Init->getType())) {
         if (!CAT->getSize().isZero()) {
           // Bail out for now. This might sort of "work", but the rest of the
           // code isn't really prepared to handle it.
           Info.FFDiag(Init, diag::note_constexpr_unsupported_flexible_array);
           return false;
public:
     bool VisitCXXConstructExpr(const CXXConstructExpr *E,
                                const LValue &Subobject,
                                APValue *Value, QualType Type);
     bool VisitStringLiteral(const StringLiteral *E,
                             QualType AllocType = QualType()) {
       expandStringLiteral(Info, E, Result, AllocType);
       return true;
     }
+    bool VisitCXXParenListInitExpr(const CXXParenListInitExpr *E);
+    bool VisitCXXParenListOrInitListExpr(const Expr *ExprToVisit,
+                                         ArrayRef<Expr *> Args,
+                                         const Expr *ArrayFiller,
+                                         QualType AllocType = QualType());
   };
 } // end anonymous namespace
 
 static bool EvaluateArray(const Expr *E, const LValue &This,
                           APValue &Result, EvalInfo &Info) {
   assert(!E->isValueDependent());
   assert(E->isPRValue() && E->getType()->isArrayType() &&
          "not an array prvalue");
if (!SL)
       return Error(E);
     return VisitStringLiteral(SL, AllocType);
   }
   // Any other transparent list init will need proper handling of the
   // AllocType; we can't just recurse to the inner initializer.
   assert(!E->isTransparent() &&
          "transparent array list initialization is not string literal init?");
 
+  return VisitCXXParenListOrInitListExpr(E, E->inits(), E->getArrayFiller(),
+                                         AllocType);
+}
+
+bool ArrayExprEvaluator::VisitCXXParenListOrInitListExpr(
+    const Expr *ExprToVisit, ArrayRef<Expr *> Args, const Expr *ArrayFiller,
+    QualType AllocType) {
+  const ConstantArrayType *CAT = Info.Ctx.getAsConstantArrayType(
+      AllocType.isNull() ? ExprToVisit->getType() : AllocType);
+
   bool Success = true;
 
   assert((!Result.isArray() || Result.getArrayInitializedElts() == 0) &&
          "zero-initialized array shouldn't have any initialized elts");
   APValue Filler;
   if (Result.isArray() && Result.hasArrayFiller())
     Filler = Result.getArrayFiller();
 
-  unsigned NumEltsToInit = E->getNumInits();
+  unsigned NumEltsToInit = Args.size();
   unsigned NumElts = CAT->getSize().getZExtValue();
-  const Expr *FillerExpr = E->hasArrayFiller() ? E->getArrayFiller() : nullptr;
 
   // If the initializer might depend on the array index, run it for each
   // array element.
-  if (NumEltsToInit != NumElts && MaybeElementDependentArrayFiller(FillerExpr))
+  if (NumEltsToInit != NumElts && MaybeElementDependentArrayFiller(ArrayFiller))
     NumEltsToInit = NumElts;
 
   LLVM_DEBUG(llvm::dbgs() << "The number of elements to initialize: "
                           << NumEltsToInit << ".\n");
 
   Result = APValue(APValue::UninitArray(), NumEltsToInit, NumElts);
 
   // If the array was previously zero-initialized, preserve the
   // zero-initialized values.
   if (Filler.hasValue()) {
     for (unsigned I = 0, E = Result.getArrayInitializedElts(); I != E; ++I)
       Result.getArrayInitializedElt(I) = Filler;
     if (Result.hasArrayFiller())
       Result.getArrayFiller() = Filler;
   }
 
   LValue Subobject = This;
-  Subobject.addArray(Info, E, CAT);
+  Subobject.addArray(Info, ExprToVisit, CAT);
   for (unsigned Index = 0; Index != NumEltsToInit; ++Index) {
-    const Expr *Init =
-        Index < E->getNumInits() ? E->getInit(Index) : FillerExpr;
+    const Expr *Init = Index < Args.size() ? Args[Index] : ArrayFiller;
     if (!EvaluateInPlace(Result.getArrayInitializedElt(Index),
                          Info, Subobject, Init) ||
         !HandleLValueArrayAdjustment(Info, Init, Subobject,
                                      CAT->getElementType(), 1)) {
       if (!Info.noteFailure())
         return false;
       Success = false;
     }
   }
 
   if (!Result.hasArrayFiller())
     return Success;
 
   // If we get here, we have a trivial filler, which we can just evaluate
   // once and splat over the rest of the array elements.
-  assert(FillerExpr && "no array filler for incomplete init list");
+  assert(ArrayFiller && "no array filler for incomplete init list");
   return EvaluateInPlace(Result.getArrayFiller(), Info, Subobject,
-                         FillerExpr) && Success;
+                         ArrayFiller) &&
+         Success;
 }
 
 bool ArrayExprEvaluator::VisitArrayInitLoopExpr(const ArrayInitLoopExpr *E) {
   LValue CommonLV;
   if (E->getCommonExpr() &&
       !Evaluate(Info.CurrentCall->createTemporary(
                     E->getCommonExpr(),
                     getStorageType(Info.Ctx, E->getCommonExpr()),
bool ArrayExprEvaluator::VisitCXXConstructExpr(const CXXConstructExpr *E,
 
   if (!Type->isRecordType())
     return Error(E);
 
   return RecordExprEvaluator(Info, Subobject, *Value)
              .VisitCXXConstructExpr(E, Type);
 }
 
+bool ArrayExprEvaluator::VisitCXXParenListInitExpr(
+    const CXXParenListInitExpr *E) {
+  assert(dyn_cast<ConstantArrayType>(E->getType()) &&
+         "Expression result is not a constant array type");
+
+  return VisitCXXParenListOrInitListExpr(E, E->getInitExprs(),
+                                         E->getArrayFiller());
+}
+
 //===----------------------------------------------------------------------===//
 // Integer Evaluation
 //
 // As a GNU extension, we support casting pointers to sufficiently-wide integer
 // types and back in constant folding. Integer values are thus represented
 // either as an integer-valued APValue, or as an lvalue-valued APValue.
 //===----------------------------------------------------------------------===//
 
auto OnSuccess = [&](CmpResult CR, const BinaryOperator *E) {
     return CheckConstantExpression(Info, E->getExprLoc(), E->getType(), Result,
                                    ConstantExprKind::Normal);
   };
   return EvaluateComparisonBinaryOperator(Info, E, OnSuccess, [&]() {
     return ExprEvaluatorBaseTy::VisitBinCmp(E);
   });
 }
 
+bool RecordExprEvaluator::VisitCXXParenListInitExpr(
+    const CXXParenListInitExpr *E) {
+  return VisitCXXParenListOrInitListExpr(E, E->getInitExprs());
+}
+
 bool IntExprEvaluator::VisitBinaryOperator(const BinaryOperator *E) {
   // We don't support assignment in C. C++ assignments don't get here because
   // assignment is an lvalue in C++.
   if (E->isAssignmentOp()) {
     Error(E);
     if (!Info.noteFailure())
       return false;
   }
#include "clang/AST/StmtNodes.inc"
   case Expr::PseudoObjectExprClass:
   case Expr::AtomicExprClass:
   case Expr::LambdaExprClass:
   case Expr::CXXFoldExprClass:
   case Expr::CoawaitExprClass:
   case Expr::DependentCoawaitExprClass:
   case Expr::CoyieldExprClass:
   case Expr::SYCLUniqueStableNameExprClass:
+  case Expr::CXXParenListInitExprClass:
     return ICEDiag(IK_NotICE, E->getBeginLoc());
 
   case Expr::InitListExprClass: {
     // C++03 [dcl.init]p13: If T is a scalar type, then a declaration of the
     // form "T x = { a };" is equivalent to "T x = a;".
     // Unless we're initializing a reference, T is a scalar as it is known to be
     // of integral or enumeration type.
     if (E->isPRValue())
#include "clang/AST/StmtNodes.inc"
   }
   case Expr::BuiltinBitCastExprClass: {
     if (!checkBitCastConstexprEligibility(nullptr, Ctx, cast<CastExpr>(E)))
       return ICEDiag(IK_NotICE, E->getBeginLoc());
     return CheckICE(cast<CastExpr>(E)->getSubExpr(), Ctx);
   }
   }
 
   llvm_unreachable("Invalid StmtClass!");
 }
 
 /// Evaluate an expression as a C++11 integral constant expression.
 static bool EvaluateCPlusPlus11IntegralConstantExpr(const ASTContext &Ctx,
                                                     const Expr *E,
                                                     llvm::APSInt *Value,
                                                     SourceLocation *Loc) {
   if (!E->getType()->isIntegralOrUnscopedEnumerationType()) {

clang/lib/AST/ItaniumMangle.cpp

#include "clang/AST/StmtNodes.inc"
   case Expr::MSPropertyRefExprClass:
   case Expr::MSPropertySubscriptExprClass:
   case Expr::TypoExprClass: // This should no longer exist in the AST by now.
   case Expr::RecoveryExprClass:
   case Expr::OMPArraySectionExprClass:
   case Expr::OMPArrayShapingExprClass:
   case Expr::OMPIteratorExprClass:
   case Expr::CXXInheritedCtorInitExprClass:
+  case Expr::CXXParenListInitExprClass:
     llvm_unreachable("unexpected statement kind");
 
   case Expr::ConstantExprClass:
     E = cast<ConstantExpr>(E)->getSubExpr();
     goto recurse;
 
   // FIXME: invent manglings for all these.
   case Expr::BlockExprClass:

clang/lib/AST/JSONNodeDumper.cpp

void JSONNodeDumper::VisitVarDecl(const VarDecl *VD) {
   attributeOnlyIfTrue("inline", VD->isInline());
   attributeOnlyIfTrue("constexpr", VD->isConstexpr());
   attributeOnlyIfTrue("modulePrivate", VD->isModulePrivate());
   if (VD->hasInit()) {
     switch (VD->getInitStyle()) {
     case VarDecl::CInit: JOS.attribute("init", "c");  break;
     case VarDecl::CallInit: JOS.attribute("init", "call"); break;
     case VarDecl::ListInit: JOS.attribute("init", "list"); break;
+    case VarDecl::ParenListInit:
+      JOS.attribute("init", "paren-list");
+      break;
     }
   }
   attributeOnlyIfTrue("isParameterPack", VD->isParameterPack());
 }
 
 void JSONNodeDumper::VisitFieldDecl(const FieldDecl *FD) {
   VisitNamedDecl(FD);
   JOS.attribute("type", createQualType(FD->getType()));

clang/lib/AST/StmtPrinter.cpp

void StmtPrinter::VisitCXXFoldExpr(CXXFoldExpr *E) {
   OS << "...";
   if (E->getRHS()) {
     OS << " " << BinaryOperator::getOpcodeStr(E->getOperator()) << " ";
     PrintExpr(E->getRHS());
   }
   OS << ")";
 }
 
+void StmtPrinter::VisitCXXParenListInitExpr(CXXParenListInitExpr *Node) {
+  OS << "(";
+  llvm::interleaveComma(Node->getInitExprs(), OS,
+                        [&](Expr *E) { PrintExpr(E); });
+  OS << ")";
+}
+
 void StmtPrinter::VisitConceptSpecializationExpr(ConceptSpecializationExpr *E) {
   NestedNameSpecifierLoc NNS = E->getNestedNameSpecifierLoc();
   if (NNS)
     NNS.getNestedNameSpecifier()->print(OS, Policy);
   if (E->getTemplateKWLoc().isValid())
     OS << "template ";
   OS << E->getFoundDecl()->getName();
   printTemplateArgumentList(OS, E->getTemplateArgsAsWritten()->arguments(),
                             Policy,
                             E->getNamedConcept()->getTemplateParameters());

clang/lib/AST/StmtProfile.cpp

void StmtProfiler::VisitMaterializeTemporaryExpr(
   VisitExpr(S);
 }
 
 void StmtProfiler::VisitCXXFoldExpr(const CXXFoldExpr *S) {
   VisitExpr(S);
   ID.AddInteger(S->getOperator());
 }
 
+void StmtProfiler::VisitCXXParenListInitExpr(const CXXParenListInitExpr *S) {
+  VisitExpr(S);
+}
+
 void StmtProfiler::VisitCoroutineBodyStmt(const CoroutineBodyStmt *S) {
   VisitStmt(S);
 }
 
 void StmtProfiler::VisitCoreturnStmt(const CoreturnStmt *S) {
   VisitStmt(S);
 }
 

clang/lib/AST/TextNodeDumper.cpp

case VarDecl::CInit:
       OS << " cinit";
       break;
     case VarDecl::CallInit:
       OS << " callinit";
       break;
     case VarDecl::ListInit:
       OS << " listinit";
       break;
+    case VarDecl::ParenListInit:
+      OS << " parenlistinit";
     }
   }
   if (D->needsDestruction(D->getASTContext()))
     OS << " destroyed";
   if (D->isParameterPack())
     OS << " pack";
 
   if (D->hasInit()) {

clang/lib/CodeGen/CGExprAgg.cpp

public:
   void EmitFinalDestCopy(QualType type, const LValue &src,
                          ExprValueKind SrcValueKind = EVK_NonRValue);
   void EmitFinalDestCopy(QualType type, RValue src);
   void EmitCopy(QualType type, const AggValueSlot &dest,
                 const AggValueSlot &src);
 
   void EmitMoveFromReturnSlot(const Expr *E, RValue Src);
 
-  void EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
-                     QualType ArrayQTy, InitListExpr *E);
+  void EmitArrayInit(Address DestPtr, llvm::ArrayType *AType, QualType ArrayQTy,
+                     Expr *ExprToVisit, ArrayRef<Expr *> Args,
+                     Expr *ArrayFiller);
 
   AggValueSlot::NeedsGCBarriers_t needsGC(QualType T) {
     if (CGF.getLangOpts().getGC() && TypeRequiresGCollection(T))
       return AggValueSlot::NeedsGCBarriers;
     return AggValueSlot::DoesNotNeedGCBarriers;
   }
 
   bool TypeRequiresGCollection(QualType T);
public:
   void VisitObjCIvarRefExpr(ObjCIvarRefExpr *E) {
     EmitAggLoadOfLValue(E);
   }
 
   void VisitDesignatedInitUpdateExpr(DesignatedInitUpdateExpr *E);
   void VisitAbstractConditionalOperator(const AbstractConditionalOperator *CO);
   void VisitChooseExpr(const ChooseExpr *CE);
   void VisitInitListExpr(InitListExpr *E);
+  void VisitCXXParenListOrInitListExpr(Expr *ExprToVisit, ArrayRef<Expr *> Args,
+                                       FieldDecl *InitializedFieldInUnion,
+                                       Expr *ArrayFiller);
   void VisitArrayInitLoopExpr(const ArrayInitLoopExpr *E,
                               llvm::Value *outerBegin = nullptr);
   void VisitImplicitValueInitExpr(ImplicitValueInitExpr *E);
   void VisitNoInitExpr(NoInitExpr *E) { } // Do nothing.
   void VisitCXXDefaultArgExpr(CXXDefaultArgExpr *DAE) {
     CodeGenFunction::CXXDefaultArgExprScope Scope(CGF, DAE);
     Visit(DAE->getExpr());
   }
if (E->isGLValue()) {
       LValue LV = CGF.EmitPseudoObjectLValue(E);
       return EmitFinalDestCopy(E->getType(), LV);
     }
 
     CGF.EmitPseudoObjectRValue(E, EnsureSlot(E->getType()));
   }
 
   void VisitVAArgExpr(VAArgExpr *E);
+  void VisitCXXParenListInitExpr(CXXParenListInitExpr *E);
+  void VisitCXXParenListOrInitListExpr(Expr *ExprToVisit, ArrayRef<Expr *> Args,
+                                       Expr *ArrayFiller);
 
   void EmitInitializationToLValue(Expr *E, LValue Address);
   void EmitNullInitializationToLValue(LValue Address);
   //  case Expr::ChooseExprClass:
   void VisitCXXThrowExpr(const CXXThrowExpr *E) { CGF.EmitCXXThrowExpr(E); }
   void VisitAtomicExpr(AtomicExpr *E) {
     RValue Res = CGF.EmitAtomicExpr(E);
     EmitFinalDestCopy(E->getType(), Res);
static bool isTrivialFiller(Expr *E) {
   if (auto *Cons = dyn_cast_or_null<CXXConstructExpr>(E))
     return Cons->getConstructor()->isDefaultConstructor() &&
            Cons->getConstructor()->isTrivial();
 
   // FIXME: Are there other cases where we can avoid emitting an initializer?
   return false;
 }
 
-/// Emit initialization of an array from an initializer list.
+/// Emit initialization of an array from an initializer list. ExprToVisit must
+/// be either an InitListEpxr a CXXParenInitListExpr.
 void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
-                                   QualType ArrayQTy, InitListExpr *E) {
-  uint64_t NumInitElements = E->getNumInits();
+                                   QualType ArrayQTy, Expr *ExprToVisit,
+                                   ArrayRef<Expr *> Args, Expr *ArrayFiller) {
+  uint64_t NumInitElements = Args.size();
 
   uint64_t NumArrayElements = AType->getNumElements();
   assert(NumInitElements <= NumArrayElements);
 
   QualType elementType =
       CGF.getContext().getAsArrayType(ArrayQTy)->getElementType();
 
   // DestPtr is an array*.  Construct an elementType* by drilling
void AggExprEmitter::EmitArrayInit(Address DestPtr, llvm::ArrayType *AType,
   // Consider initializing the array by copying from a global. For this to be
   // more efficient than per-element initialization, the size of the elements
   // with explicit initializers should be large enough.
   if (NumInitElements * elementSize.getQuantity() > 16 &&
       elementType.isTriviallyCopyableType(CGF.getContext())) {
     CodeGen::CodeGenModule &CGM = CGF.CGM;
     ConstantEmitter Emitter(CGF);
     LangAS AS = ArrayQTy.getAddressSpace();
-    if (llvm::Constant *C = Emitter.tryEmitForInitializer(E, AS, ArrayQTy)) {
+    if (llvm::Constant *C =
+            Emitter.tryEmitForInitializer(ExprToVisit, AS, ArrayQTy)) {
       auto GV = new llvm::GlobalVariable(
           CGM.getModule(), C->getType(),
           CGM.isTypeConstant(ArrayQTy, /* ExcludeCtorDtor= */ true),
           llvm::GlobalValue::PrivateLinkage, C, "constinit",
           /* InsertBefore= */ nullptr, llvm::GlobalVariable::NotThreadLocal,
           CGM.getContext().getTargetAddressSpace(AS));
       Emitter.finalize(GV);
       CharUnits Align = CGM.getContext().getTypeAlignInChars(ArrayQTy);
if (i > 0) {
       // Tell the cleanup that it needs to destroy up to this
       // element.  TODO: some of these stores can be trivially
       // observed to be unnecessary.
       if (endOfInit.isValid()) Builder.CreateStore(element, endOfInit);
     }
 
     LValue elementLV = CGF.MakeAddrLValue(
         Address(element, llvmElementType, elementAlign), elementType);
-    EmitInitializationToLValue(E->getInit(i), elementLV);
+    EmitInitializationToLValue(Args[i], elementLV);
   }
 
   // Check whether there's a non-trivial array-fill expression.
-  Expr *filler = E->getArrayFiller();
-  bool hasTrivialFiller = isTrivialFiller(filler);
+  bool hasTrivialFiller = isTrivialFiller(ArrayFiller);
 
   // Any remaining elements need to be zero-initialized, possibly
   // using the filler expression.  We can skip this if the we're
   // emitting to zeroed memory.
   if (NumInitElements != NumArrayElements &&
       !(Dest.isZeroed() && hasTrivialFiller &&
         CGF.getTypes().isZeroInitializable(elementType))) {
 
// Emit the actual filler expression.
       // C++1z [class.temporary]p5:
       //   when a default constructor is called to initialize an element of
       //   an array with no corresponding initializer [...] the destruction of
       //   every temporary created in a default argument is sequenced before
       //   the construction of the next array element, if any
       CodeGenFunction::RunCleanupsScope CleanupsScope(CGF);
       LValue elementLV = CGF.MakeAddrLValue(
           Address(currentElement, llvmElementType, elementAlign), elementType);
-      if (filler)
-        EmitInitializationToLValue(filler, elementLV);
+      if (ArrayFiller)
+        EmitInitializationToLValue(ArrayFiller, elementLV);
       else
         EmitNullInitializationToLValue(elementLV);
     }
 
     // Move on to the next element.
     llvm::Value *nextElement = Builder.CreateInBoundsGEP(
         llvmElementType, currentElement, one, "arrayinit.next");
 
void AggExprEmitter::EmitNullInitializationToLValue(LValue lv) {
   } else {
     // There's a potential optimization opportunity in combining
     // memsets; that would be easy for arrays, but relatively
     // difficult for structures with the current code.
     CGF.EmitNullInitialization(lv.getAddress(CGF), lv.getType());
   }
 }
 
+void AggExprEmitter::VisitCXXParenListInitExpr(CXXParenListInitExpr *E) {
+  ArrayRef<Expr *> InitExprs = E->getInitExprs();
+  FieldDecl *InitializedFieldInUnion = nullptr;
+  if (E->getType()->isUnionType()) {
+    auto *RD =
+        dyn_cast<CXXRecordDecl>(E->getType()->castAs<RecordType>()->getDecl());
+    for (FieldDecl *FD : RD->fields()) {
+      if (FD->isUnnamedBitfield())
+        continue;
+      InitializedFieldInUnion = FD;
+      break;
+    }
+  }
+
+  VisitCXXParenListOrInitListExpr(E, InitExprs, InitializedFieldInUnion,
+                                  E->getArrayFiller());
+}
+
 void AggExprEmitter::VisitInitListExpr(InitListExpr *E) {
+  if (E->hadArrayRangeDesignator())
+    CGF.ErrorUnsupported(E, "GNU array range designator extension");
+
+  if (E->isTransparent())
+    return Visit(E->getInit(0));
+
+  VisitCXXParenListOrInitListExpr(
+      E, E->inits(), E->getInitializedFieldInUnion(), E->getArrayFiller());
+}
+
+void AggExprEmitter::VisitCXXParenListOrInitListExpr(
+    Expr *ExprToVisit, ArrayRef<Expr *> InitExprs,
+    FieldDecl *InitializedFieldInUnion, Expr *ArrayFiller) {
 #if 0
   // FIXME: Assess perf here?  Figure out what cases are worth optimizing here
   // (Length of globals? Chunks of zeroed-out space?).
   //
   // If we can, prefer a copy from a global; this is a lot less code for long
   // globals, and it's easier for the current optimizers to analyze.
-  if (llvm::Constant* C = CGF.CGM.EmitConstantExpr(E, E->getType(), &CGF)) {
+  if (llvm::Constant *C =
+          CGF.CGM.EmitConstantExpr(ExprToVisit, ExprToVisit->getType(), &CGF)) {
     llvm::GlobalVariable* GV =
     new llvm::GlobalVariable(CGF.CGM.getModule(), C->getType(), true,
                              llvm::GlobalValue::InternalLinkage, C, "");
-    EmitFinalDestCopy(E->getType(), CGF.MakeAddrLValue(GV, E->getType()));
+    EmitFinalDestCopy(ExprToVisit->getType(),
+                      CGF.MakeAddrLValue(GV, ExprToVisit->getType()));
     return;
   }
 #endif
-  if (E->hadArrayRangeDesignator())
-    CGF.ErrorUnsupported(E, "GNU array range designator extension");
 
-  if (E->isTransparent())
-    return Visit(E->getInit(0));
+  AggValueSlot Dest = EnsureSlot(ExprToVisit->getType());
 
-  AggValueSlot Dest = EnsureSlot(E->getType());
-
-  LValue DestLV = CGF.MakeAddrLValue(Dest.getAddress(), E->getType());
+  LValue DestLV = CGF.MakeAddrLValue(Dest.getAddress(), ExprToVisit->getType());
 
   // Handle initialization of an array.
-  if (E->getType()->isArrayType()) {
+  if (ExprToVisit->getType()->isArrayType()) {
     auto AType = cast<llvm::ArrayType>(Dest.getAddress().getElementType());
-    EmitArrayInit(Dest.getAddress(), AType, E->getType(), E);
+    EmitArrayInit(Dest.getAddress(), AType, ExprToVisit->getType(), ExprToVisit,
+                  InitExprs, ArrayFiller);
     return;
   }
 
-  assert(E->getType()->isRecordType() && "Only support structs/unions here!");
+  assert(ExprToVisit->getType()->isRecordType() &&
+         "Only support structs/unions here!");
 
   // Do struct initialization; this code just sets each individual member
   // to the approprate value.  This makes bitfield support automatic;
   // the disadvantage is that the generated code is more difficult for
   // the optimizer, especially with bitfields.
-  unsigned NumInitElements = E->getNumInits();
-  RecordDecl *record = E->getType()->castAs<RecordType>()->getDecl();
+  unsigned NumInitElements = InitExprs.size();
+  RecordDecl *record = ExprToVisit->getType()->castAs<RecordType>()->getDecl();
 
   // We'll need to enter cleanup scopes in case any of the element
   // initializers throws an exception.
   SmallVector<EHScopeStack::stable_iterator, 16> cleanups;
   llvm::Instruction *cleanupDominator = nullptr;
   auto addCleanup = [&](const EHScopeStack::stable_iterator &cleanup) {
     cleanups.push_back(cleanup);
     if (!cleanupDominator) // create placeholder once needed
       cleanupDominator = CGF.Builder.CreateAlignedLoad(
           CGF.Int8Ty, llvm::Constant::getNullValue(CGF.Int8PtrTy),
           CharUnits::One());
   };
 
   unsigned curInitIndex = 0;
 
   // Emit initialization of base classes.
   if (auto *CXXRD = dyn_cast<CXXRecordDecl>(record)) {
-    assert(E->getNumInits() >= CXXRD->getNumBases() &&
+    assert(NumInitElements >= CXXRD->getNumBases() &&
            "missing initializer for base class");
     for (auto &Base : CXXRD->bases()) {
       assert(!Base.isVirtual() && "should not see vbases here");
       auto *BaseRD = Base.getType()->getAsCXXRecordDecl();
       Address V = CGF.GetAddressOfDirectBaseInCompleteClass(
           Dest.getAddress(), CXXRD, BaseRD,
           /*isBaseVirtual*/ false);
       AggValueSlot AggSlot = AggValueSlot::forAddr(
           V, Qualifiers(),
           AggValueSlot::IsDestructed,
           AggValueSlot::DoesNotNeedGCBarriers,
           AggValueSlot::IsNotAliased,
           CGF.getOverlapForBaseInit(CXXRD, BaseRD, Base.isVirtual()));
-      CGF.EmitAggExpr(E->getInit(curInitIndex++), AggSlot);
+      CGF.EmitAggExpr(InitExprs[curInitIndex++], AggSlot);
 
       if (QualType::DestructionKind dtorKind =
               Base.getType().isDestructedType()) {
         CGF.pushDestroy(dtorKind, V, Base.getType());
         addCleanup(CGF.EHStack.stable_begin());
       }
     }
   }
 
   // Prepare a 'this' for CXXDefaultInitExprs.
   CodeGenFunction::FieldConstructionScope FCS(CGF, Dest.getAddress());
 
   if (record->isUnion()) {
     // Only initialize one field of a union. The field itself is
     // specified by the initializer list.
-    if (!E->getInitializedFieldInUnion()) {
+    if (!InitializedFieldInUnion) {
       // Empty union; we have nothing to do.
 
 #ifndef NDEBUG
       // Make sure that it's really an empty and not a failure of
       // semantic analysis.
       for (const auto *Field : record->fields())
         assert(Field->isUnnamedBitfield() && "Only unnamed bitfields allowed");
 #endif
       return;
     }
 
     // FIXME: volatility
-    FieldDecl *Field = E->getInitializedFieldInUnion();
+    FieldDecl *Field = InitializedFieldInUnion;
 
     LValue FieldLoc = CGF.EmitLValueForFieldInitialization(DestLV, Field);
     if (NumInitElements) {
       // Store the initializer into the field
-      EmitInitializationToLValue(E->getInit(0), FieldLoc);
+      EmitInitializationToLValue(InitExprs[0], FieldLoc);
     } else {
       // Default-initialize to null.
       EmitNullInitializationToLValue(FieldLoc);
     }
 
     return;
   }
 
   // Here we iterate over the fields; this makes it simpler to both
   // default-initialize fields and skip over unnamed fields.
   for (const auto *field : record->fields()) {
     // We're done once we hit the flexible array member.
     if (field->getType()->isIncompleteArrayType())
       break;
 
     // Always skip anonymous bitfields.
     if (field->isUnnamedBitfield())
       continue;
 
     // We're done if we reach the end of the explicit initializers, we
     // have a zeroed object, and the rest of the fields are
     // zero-initializable.
     if (curInitIndex == NumInitElements && Dest.isZeroed() &&
-        CGF.getTypes().isZeroInitializable(E->getType()))
+        CGF.getTypes().isZeroInitializable(ExprToVisit->getType()))
       break;
 
 
     LValue LV = CGF.EmitLValueForFieldInitialization(DestLV, field);
     // We never generate write-barries for initialized fields.
     LV.setNonGC(true);
 
     if (curInitIndex < NumInitElements) {
       // Store the initializer into the field.
-      EmitInitializationToLValue(E->getInit(curInitIndex++), LV);
+      EmitInitializationToLValue(InitExprs[curInitIndex++], LV);
     } else {
       // We're out of initializers; default-initialize to null
       EmitNullInitializationToLValue(LV);
     }
 
     // Push a destructor if necessary.
     // FIXME: if we have an array of structures, all explicitly
     // initialized, we can end up pushing a linear number of cleanups.

clang/lib/Frontend/InitPreprocessor.cpp

static void InitializeCPlusPlusFeatureTestMacros(const LangOptions &LangOpts,
   }
   if (LangOpts.AlignedAllocation && !LangOpts.AlignedAllocationUnavailable)
     Builder.defineMacro("__cpp_aligned_new", "201606L");
   if (LangOpts.RelaxedTemplateTemplateArgs)
     Builder.defineMacro("__cpp_template_template_args", "201611L");
 
   // C++20 features.
   if (LangOpts.CPlusPlus20) {
-    // Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
+    Builder.defineMacro("__cpp_aggregate_paren_init", "201902L");
 
     // P0848 is implemented, but we're still waiting for other concepts
     // issues to be addressed before bumping __cpp_concepts up to 202002L.
     // Refer to the discussion of this at https://reviews.llvm.org/D128619.
     Builder.defineMacro("__cpp_concepts", "201907L");
     Builder.defineMacro("__cpp_conditional_explicit", "201806L");
     //Builder.defineMacro("__cpp_consteval", "201811L");
     Builder.defineMacro("__cpp_constexpr_dynamic_alloc", "201907L");

clang/lib/Sema/SemaCast.cpp

static bool tryDiagnoseOverloadedCast(Sema &S, CastType CT,
   InitializationSequence sequence(S, entity, initKind, src);
 
   assert(sequence.Failed() && "initialization succeeded on second try?");
   switch (sequence.getFailureKind()) {
   default: return false;
 
   case InitializationSequence::FK_ConstructorOverloadFailed:
   case InitializationSequence::FK_UserConversionOverloadFailed:
+  case InitializationSequence::FK_ParenthesizedListInitFailed:
     break;
   }
 
   OverloadCandidateSet &candidates = sequence.getFailedCandidateSet();
 
   unsigned msg = 0;
   OverloadCandidateDisplayKind howManyCandidates = OCD_AllCandidates;
 

clang/lib/Sema/SemaDecl.cpp

if (Result.isInvalid()) {
       VDecl->setInvalidDecl();
       return;
     }
     Init = Result.get();
   }
 
   // Perform the initialization.
   ParenListExpr *CXXDirectInit = dyn_cast<ParenListExpr>(Init);
+  bool IsParenListInit = false;
   if (!VDecl->isInvalidDecl()) {
     InitializedEntity Entity = InitializedEntity::InitializeVariable(VDecl);
     InitializationKind Kind = InitializationKind::CreateForInit(
         VDecl->getLocation(), DirectInit, Init);
 
     MultiExprArg Args = Init;
     if (CXXDirectInit)
       Args = MultiExprArg(CXXDirectInit->getExprs(),
if (Result.isInvalid()) {
       auto RecoveryExpr =
           CreateRecoveryExpr(Init->getBeginLoc(), Init->getEndLoc(), Args);
       if (RecoveryExpr.get())
         VDecl->setInit(RecoveryExpr.get());
       return;
     }
 
     Init = Result.getAs<Expr>();
+    IsParenListInit = !InitSeq.steps().empty() &&
+                      InitSeq.step_begin()->Kind ==
+                          InitializationSequence::SK_ParenthesizedListInit;
   }
 
   // Check for self-references within variable initializers.
   // Variables declared within a function/method body (except for references)
   // are handled by a dataflow analysis.
   // This is undefined behavior in C++, but valid in C.
   if (getLangOpts().CPlusPlus)
     if (!VDecl->hasLocalStorage() || VDecl->getType()->isRecordType() ||
void Sema::AddInitializerToDecl(Decl *RealDecl, Expr *Init, bool DirectInit) {
   // special case code.
 
   // C++ 8.5p11:
   // The form of initialization (using parentheses or '=') is generally
   // insignificant, but does matter when the entity being initialized has a
   // class type.
   if (CXXDirectInit) {
     assert(DirectInit && "Call-style initializer must be direct init.");
-    VDecl->setInitStyle(VarDecl::CallInit);
+    VDecl->setInitStyle(IsParenListInit ? VarDecl::ParenListInit
+                                        : VarDecl::CallInit);
   } else if (DirectInit) {
     // This must be list-initialization. No other way is direct-initialization.
     VDecl->setInitStyle(VarDecl::ListInit);
   }
 
   if (LangOpts.OpenMP &&
       (LangOpts.OpenMPIsDevice || !LangOpts.OMPTargetTriples.empty()) &&
       VDecl->isFileVarDecl())

clang/lib/Sema/SemaExceptionSpec.cpp

CanThrowResult Sema::canThrow(const Stmt *S) {
   case Expr::ObjCIsaExprClass:
   case Expr::ObjCIvarRefExprClass:
   case Expr::ParenExprClass:
   case Expr::ParenListExprClass:
   case Expr::ShuffleVectorExprClass:
   case Expr::StmtExprClass:
   case Expr::ConvertVectorExprClass:
   case Expr::VAArgExprClass:
+  case Expr::CXXParenListInitExprClass:
     return canSubStmtsThrow(*this, S);
 
   case Expr::CompoundLiteralExprClass:
   case Expr::CXXConstCastExprClass:
   case Expr::CXXAddrspaceCastExprClass:
   case Expr::CXXReinterpretCastExprClass:
   case Expr::BuiltinBitCastExprClass:
       // FIXME: Properly determine whether a variably-modified type can throw.

clang/lib/Sema/SemaInit.cpp

void InitializationSequence::Step::Destroy() {
   case SK_ParenthesizedArrayInit:
   case SK_PassByIndirectCopyRestore:
   case SK_PassByIndirectRestore:
   case SK_ProduceObjCObject:
   case SK_StdInitializerList:
   case SK_StdInitializerListConstructorCall:
   case SK_OCLSamplerInit:
   case SK_OCLZeroOpaqueType:
+  case SK_ParenthesizedListInit:
     break;
 
   case SK_ConversionSequence:
   case SK_ConversionSequenceNoNarrowing:
     delete ICS;
   }
 }
 
 bool InitializationSequence::isDirectReferenceBinding() const {
   // There can be some lvalue adjustments after the SK_BindReference step.
   for (const Step &S : llvm::reverse(Steps)) {
     if (S.Kind == SK_BindReference)
       return true;
     if (S.Kind == SK_BindReferenceToTemporary)
bool InitializationSequence::isAmbiguous() const {
   case FK_Incomplete:
   case FK_ArrayTypeMismatch:
   case FK_NonConstantArrayInit:
   case FK_ListInitializationFailed:
   case FK_VariableLengthArrayHasInitializer:
   case FK_PlaceholderType:
   case FK_ExplicitConstructor:
   case FK_AddressOfUnaddressableFunction:
+  case FK_ParenthesizedListInitFailed:
     return false;
 
   case FK_ReferenceInitOverloadFailed:
   case FK_UserConversionOverloadFailed:
   case FK_ConstructorOverloadFailed:
   case FK_ListConstructorOverloadFailed:
     return FailedOverloadResult == OR_Ambiguous;
   }

 
 void InitializationSequence::AddOCLZeroOpaqueTypeStep(QualType T) {
   Step S;
   S.Kind = SK_OCLZeroOpaqueType;
   S.Type = T;
   Steps.push_back(S);
 }
 
+void InitializationSequence::AddParenthesizedListInitStep(QualType T) {
+  Step S;
+  S.Kind = SK_ParenthesizedListInit;
+  S.Type = T;
+  Steps.push_back(S);
+}
+
 void InitializationSequence::RewrapReferenceInitList(QualType T,
                                                      InitListExpr *Syntactic) {
   assert(Syntactic->getNumInits() == 1 &&
          "Can only rewrap trivial init lists.");
   Step S;
   S.Kind = SK_UnwrapInitList;
   S.Type = Syntactic->getInit(0)->getType();
   Steps.insert(Steps.begin(), S);
static void TryConstructorInitialization(Sema &S,
   //     through overload resolution.
   DeclContext::lookup_result Ctors = S.LookupConstructors(DestRecordDecl);
 
   OverloadingResult Result = OR_No_Viable_Function;
   OverloadCandidateSet::iterator Best;
   bool AsInitializerList = false;
 
   // C++11 [over.match.list]p1, per DR1467:
   //   When objects of non-aggregate type T are list-initialized, such that
   //   8.5.4 [dcl.init.list] specifies that overload resolution is performed
   //   according to the rules in this section, overload resolution selects
   //   the constructor in two phases:
   //
   //   - Initially, the candidate functions are the initializer-list
   //     constructors of the class T and the argument list consists of the
   //     initializer list as a single argument.
   if (IsListInit) {
     AsInitializerList = true;
 
static void TryConstructorInitialization(Sema &S,
   if (Result == OR_No_Viable_Function) {
     AsInitializerList = false;
     Result = ResolveConstructorOverload(S, Kind.getLocation(), UnwrappedArgs,
                                         CandidateSet, DestType, Ctors, Best,
                                         CopyInitialization, AllowExplicit,
                                         /*OnlyListConstructors=*/false,
                                         IsListInit);
   }
   if (Result) {
     Sequence.SetOverloadFailure(
         IsListInit ? InitializationSequence::FK_ListConstructorOverloadFailed
                    : InitializationSequence::FK_ConstructorOverloadFailed,
         Result);
 
     if (Result != OR_Deleted)
       return;
   }
 
   bool HadMultipleCandidates = (CandidateSet.size() > 1);
 
   // In C++17, ResolveConstructorOverload can select a conversion function
   // instead of a constructor.
   if (auto *CD = dyn_cast<CXXConversionDecl>(Best->Function)) {
     // Add the user-defined conversion step that calls the conversion function.
     QualType ConvType = CD->getConversionType();
     assert(S.Context.hasSameUnqualifiedType(ConvType, DestType) &&
            "should not have selected this conversion function");
     Sequence.AddUserConversionStep(CD, Best->FoundDecl, ConvType,
                                    HadMultipleCandidates);
     if (!S.Context.hasSameType(ConvType, DestType))
       Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
     if (IsListInit)
       Sequence.RewrapReferenceInitList(Entity.getType(), ILE);
     return;
   }
 
   CXXConstructorDecl *CtorDecl = cast<CXXConstructorDecl>(Best->Function);
   if (Result != OR_Deleted) {
     // C++11 [dcl.init]p6:
     //   If a program calls for the default initialization of an object
     //   of a const-qualified type T, T shall be a class type with a
     //   user-provided default constructor.
     // C++ core issue 253 proposal:
static void TryDefaultInitialization(Sema &S,
 
   // If the destination type has a lifetime property, zero-initialize it.
   if (DestType.getQualifiers().hasObjCLifetime()) {
     Sequence.AddZeroInitializationStep(Entity.getType());
     return;
   }
 }
 
+static void TryOrBuildParenListInitialization(
+    Sema &S, const InitializedEntity &Entity, const InitializationKind &Kind,
+    ArrayRef<Expr *> Args, InitializationSequence &Sequence, bool VerifyOnly,
+    ExprResult *Result = nullptr) {
+  unsigned ArgIndexToProcess = 0;
+  SmallVector<Expr *, 4> InitExprs;
+  QualType ResultType;
+  Expr *ArrayFiller = nullptr;
+
+  // Process entities (i.e. array members, base classes, or class fields) by
+  // adding an initialization expression to InitExprs for each entity to
+  // initialize.
+  auto ProcessEntities = [&](auto Range) -> bool {
+    for (InitializedEntity SubEntity : Range) {
+      // Unions should only have one initializer expression.
+      // If there are more initializers than it will be caught when we check
+      // whether Index equals Args.size().
+      if (ArgIndexToProcess == 1 && Entity.getType()->isUnionType())
+        return true;
+
+      // Unnamed bitfields should not be initialized at all, either with an arg
+      // or by default.
+      if (SubEntity.getKind() == InitializedEntity::EK_Member &&
+          cast<FieldDecl>(SubEntity.getDecl())->isUnnamedBitfield())
+        continue;
+
+      if (ArgIndexToProcess < Args.size()) {
+        // There are still expressions in Args that haven't been processed.
+        // Let's match them to the current entity to initialize.
+        Expr *E = Args[ArgIndexToProcess++];
+
+        // Incomplete array types indicate flexible array members. Do not allow
+        // paren list initializations of structs with these members, as GCC
+        // doesn't either.
+        if (SubEntity.getKind() == InitializedEntity::EK_Member) {
+          auto *FD = cast<FieldDecl>(SubEntity.getDecl());
+          if (FD->getType()->isIncompleteArrayType()) {
+            if (!VerifyOnly) {
+              S.Diag(E->getBeginLoc(), diag::err_flexible_array_init)
+                  << SourceRange(E->getBeginLoc(), E->getEndLoc());
+              S.Diag(FD->getLocation(), diag::note_flexible_array_member) << FD;
+            }
+            Sequence.SetFailed(
+                InitializationSequence::FK_ParenthesizedListInitFailed);
+            return false;
+          }
+        }
+
+        InitializationKind SubKind = InitializationKind::CreateForInit(
+            E->getExprLoc(), /*isDirectInit=*/false, E);
+        InitializationSequence SubSeq(S, SubEntity, SubKind, E);
+
+        if (SubSeq.Failed()) {
+          if (!VerifyOnly)
+            SubSeq.Diagnose(S, SubEntity, SubKind, E);
+          else
+            Sequence.SetFailed(
+                InitializationSequence::FK_ParenthesizedListInitFailed);
+
+          return false;
+        }
+        if (!VerifyOnly) {
+          ExprResult ER = SubSeq.Perform(S, SubEntity, SubKind, E);
+          InitExprs.push_back(ER.get());
+        }
+      } else {
+        // We've processed all of the args, but there are still entities that
+        // have to be initialized.
+        if (SubEntity.getKind() == InitializedEntity::EK_Member) {
+          // C++ [dcl.init]p17.6.2.2
+          //   The remaining elements are initialized with their default member
+          //   initializers, if any
+          auto *FD = cast<FieldDecl>(SubEntity.getDecl());
+          if (Expr *ICE = FD->getInClassInitializer(); ICE && !VerifyOnly) {
+            ExprResult DIE = S.BuildCXXDefaultInitExpr(FD->getLocation(), FD);
+            if (DIE.isInvalid())
+              return false;
+            S.checkInitializerLifetime(SubEntity, DIE.get());
+            InitExprs.push_back(DIE.get());
+            continue;
+          };
+        }
+        // Remaining class elements without default member initializers and
+        // array elements are value initialized:
+        //
+        // C++ [dcl.init]p17.6.2.2
+        //   The remaining elements...otherwise are value initialzed
+        //
+        // C++ [dcl.init]p17.5
+        //   if the destination type is an array, the object is initialized as
+        // . follows. Let x1, . . . , xk be the elements of the expression-list
+        //   ...Let n denote the array size...the ith array element is...value-
+        //   initialized for each k < i <= n.
+        InitializationKind SubKind = InitializationKind::CreateValue(
+            Kind.getLocation(), Kind.getLocation(), Kind.getLocation(), true);
+        InitializationSequence SubSeq(S, SubEntity, SubKind, std::nullopt);
+        if (SubSeq.Failed()) {
+          if (!VerifyOnly)
+            SubSeq.Diagnose(S, SubEntity, SubKind, std::nullopt);
+          return false;
+        }
+        if (!VerifyOnly) {
+          ExprResult ER = SubSeq.Perform(S, SubEntity, SubKind, std::nullopt);
+          if (SubEntity.getKind() == InitializedEntity::EK_ArrayElement) {
+            ArrayFiller = ER.get();
+            return true;
+          }
+          InitExprs.push_back(ER.get());
+        }
+      }
+    }
+    return true;
+  };
+
+  if (const ArrayType *AT =
+          S.getASTContext().getAsArrayType(Entity.getType())) {
+
+    SmallVector<InitializedEntity, 4> ElementEntities;
+    uint64_t ArrayLength;
+    // C++ [dcl.init]p17.5
+    //   if the destination type is an array, the object is initialized as
+    //   follows. Let x1, . . . , xk be the elements of the expression-list. If
+    //   the destination type is an array of unknown bound, it is define as
+    //   having k elements.
+    if (const ConstantArrayType *CAT =
+            S.getASTContext().getAsConstantArrayType(Entity.getType()))
+      ArrayLength = CAT->getSize().getZExtValue();
+    else
+      ArrayLength = Args.size();
+
+    if (ArrayLength >= Args.size()) {
+      for (uint64_t I = 0; I < ArrayLength; ++I)
+        ElementEntities.push_back(
+            InitializedEntity::InitializeElement(S.getASTContext(), I, Entity));
+
+      if (!ProcessEntities(ElementEntities))
+        return;
+
+      ResultType = S.Context.getConstantArrayType(
+          AT->getElementType(), llvm::APInt(32, ArrayLength), nullptr,
+          ArrayType::Normal, 0);
+    }
+  } else if (auto *RT = Entity.getType()->getAs<RecordType>()) {
+    const CXXRecordDecl *RD = cast<CXXRecordDecl>(RT->getDecl());
+
+    auto BaseRange = map_range(RD->bases(), [&S](auto &base) {
+      return InitializedEntity::InitializeBase(S.getASTContext(), &base, false);
+    });
+    auto FieldRange = map_range(RD->fields(), [](auto *field) {
+      return InitializedEntity::InitializeMember(field);
+    });
+
+    if (!ProcessEntities(BaseRange))
+      return;
+
+    if (!ProcessEntities(FieldRange))
+      return;
+
+    ResultType = Entity.getType();
+  }
+
+  // Not all of the args have been processed, so there must've been more args
+  // then were required to initialize the element.
+  if (ArgIndexToProcess < Args.size()) {
+    Sequence.SetFailed(InitializationSequence::FK_ParenthesizedListInitFailed);
+    if (!VerifyOnly) {
+      QualType T = Entity.getType();
+      int InitKind = T->isArrayType() ? 0 : T->isUnionType() ? 3 : 4;
+      S.Diag(Kind.getLocation(), diag::err_excess_initializers)
+          << InitKind << Args[ArgIndexToProcess]->getSourceRange();
+    }
+    return;
+  }
+
+  if (VerifyOnly) {
+    Sequence.setSequenceKind(InitializationSequence::NormalSequence);
+    Sequence.AddParenthesizedListInitStep(Entity.getType());
+  } else if (Result) {
+    SourceRange SR = Kind.getParenOrBraceRange();
+    auto *CPLIE = CXXParenListInitExpr::Create(
+        S.getASTContext(), InitExprs, ResultType, Args.size(),
+        Kind.getLocation(), SR.getBegin(), SR.getEnd());
+    CPLIE->setArrayFiller(ArrayFiller);
+    *Result = CPLIE;
+    S.Diag(Kind.getLocation(),
+           diag::warn_cxx17_compat_aggregate_init_paren_list)
+        << Kind.getLocation() << SR << ResultType;
+  }
+
+  return;
+}
+
 /// Attempt a user-defined conversion between two types (C++ [dcl.init]),
 /// which enumerates all conversion functions and performs overload resolution
 /// to select the best.
 static void TryUserDefinedConversion(Sema &S,
                                      QualType DestType,
                                      const InitializationKind &Kind,
                                      Expr *Initializer,
                                      InitializationSequence &Sequence,
static bool canPerformArrayCopy(const InitializedEntity &Entity) {
 
   default:
     break;
   }
 
   return false;
 }
 
+static bool onlyHasDefaultedCtors(OverloadCandidateSet &OCS) {
+  if (OCS.size() != 3)
+    return false;
+
+  bool HasDefaultCtor = false, HasCopyCtor = false, HasMoveCtor = false;
+  for (const auto &Candidate : OCS) {
+    if (auto *Ctor = dyn_cast_or_null<CXXConstructorDecl>(Candidate.Function);
+        Ctor != nullptr && Ctor->isDefaulted()) {
+      if (Ctor->isDefaultConstructor())
+        HasDefaultCtor = true;
+      else if (Ctor->isCopyConstructor())
+        HasCopyCtor = true;
+      else if (Ctor->isMoveConstructor())
+        HasMoveCtor = true;
+    }
+  }
+  return HasDefaultCtor && HasCopyCtor && HasMoveCtor;
+}
+
 void InitializationSequence::InitializeFrom(Sema &S,
                                             const InitializedEntity &Entity,
                                             const InitializationKind &Kind,
                                             MultiExprArg Args,
                                             bool TopLevelOfInitList,
                                             bool TreatUnavailableAsInvalid) {
   ASTContext &Context = S.Context;
 
if (const ArrayType *DestAT = Context.getAsArrayType(DestType)) {
     // Note: as a GNU C++ extension, we allow list-initialization of a
     // class member of array type from a parenthesized initializer list.
     else if (S.getLangOpts().CPlusPlus &&
              Entity.getKind() == InitializedEntity::EK_Member &&
              Initializer && isa<InitListExpr>(Initializer)) {
       TryListInitialization(S, Entity, Kind, cast<InitListExpr>(Initializer),
                             *this, TreatUnavailableAsInvalid);
       AddParenthesizedArrayInitStep(DestType);
-    } else if (DestAT->getElementType()->isCharType())
+    } else if (S.getLangOpts().CPlusPlus20 && !TopLevelOfInitList &&
+               Kind.getKind() == InitializationKind::IK_Direct)
+      TryOrBuildParenListInitialization(S, Entity, Kind, Args, *this,
+                                        /*VerifyOnly=*/true);
+    else if (DestAT->getElementType()->isCharType())
       SetFailed(FK_ArrayNeedsInitListOrStringLiteral);
     else if (IsWideCharCompatible(DestAT->getElementType(), Context))
       SetFailed(FK_ArrayNeedsInitListOrWideStringLiteral);
     else
       SetFailed(FK_ArrayNeedsInitList);
 
     return;
   }
void InitializationSequence::InitializeFrom(Sema &S,
   if (DestType->isRecordType()) {
     //     - If the initialization is direct-initialization, or if it is
     //       copy-initialization where the cv-unqualified version of the
     //       source type is the same class as, or a derived class of, the
     //       class of the destination, constructors are considered. [...]
     if (Kind.getKind() == InitializationKind::IK_Direct ||
         (Kind.getKind() == InitializationKind::IK_Copy &&
          (Context.hasSameUnqualifiedType(SourceType, DestType) ||
-          S.IsDerivedFrom(Initializer->getBeginLoc(), SourceType, DestType))))
-      TryConstructorInitialization(S, Entity, Kind, Args,
-                                   DestType, DestType, *this);
+          S.IsDerivedFrom(Initializer->getBeginLoc(), SourceType, DestType)))) {
+      TryConstructorInitialization(S, Entity, Kind, Args, DestType, DestType,
+                                   *this);
+
+      // We fall back to the "no matching constructor" path if the
+      // failed candidate set has functions other than the three default
+      // constructors. For example, conversion function.
+      if (const auto *RD =
+              dyn_cast<CXXRecordDecl>(DestType->getAs<RecordType>()->getDecl());
+          S.getLangOpts().CPlusPlus20 && RD && RD->isAggregate() && Failed() &&
+          getFailureKind() == FK_ConstructorOverloadFailed &&
+          onlyHasDefaultedCtors(getFailedCandidateSet())) {
+        // C++20 [dcl.init] 17.6.2.2:
+        //   - Otherwise, if no constructor is viable, the destination type is
+        //   an
+        //      aggregate class, and the initializer is a parenthesized
+        //      expression-list.
+        TryOrBuildParenListInitialization(S, Entity, Kind, Args, *this,
+                                          /*VerifyOnly=*/true);
+      }
+    } else {
       //     - Otherwise (i.e., for the remaining copy-initialization cases),
-    //       user-defined conversion sequences that can convert from the source
-    //       type to the destination type or (when a conversion function is
-    //       used) to a derived class thereof are enumerated as described in
-    //       13.3.1.4, and the best one is chosen through overload resolution
-    //       (13.3).
-    else
+      //       user-defined conversion sequences that can convert from the
+      //       source type to the destination type or (when a conversion
+      //       function is used) to a derived class thereof are enumerated as
+      //       described in 13.3.1.4, and the best one is chosen through
+      //       overload resolution (13.3).
       TryUserDefinedConversion(S, DestType, Kind, Initializer, *this,
                                TopLevelOfInitList);
+    }
     return;
   }
 
   assert(Args.size() >= 1 && "Zero-argument case handled above");
 
   // For HLSL ext vector types we allow list initialization behavior for C++
   // constructor syntax. This is accomplished by converting initialization
   // arguments an InitListExpr late.
   if (S.getLangOpts().HLSL && DestType->isExtVectorType() &&
case SK_OCLZeroOpaqueType: {
     if (!CurInit.get()) return ExprError();
     break;
   }
 
   case SK_ConstructorInitialization:
   case SK_ConstructorInitializationFromList:
   case SK_StdInitializerListConstructorCall:
   case SK_ZeroInitialization:
+  case SK_ParenthesizedListInit:
     break;
   }
 
   // Promote from an unevaluated context to an unevaluated list context in
   // C++11 list-initialization; we need to instantiate entities usable in
   // constant expressions here in order to perform narrowing checks =(
   EnterExpressionEvaluationContext Evaluated(
       S, EnterExpressionEvaluationContext::InitList,
case SK_OCLZeroOpaqueType: {
               Step->Type->isOCLIntelSubgroupAVCType()) &&
              "Wrong type for initialization of OpenCL opaque type.");
 
       CurInit = S.ImpCastExprToType(CurInit.get(), Step->Type,
                                     CK_ZeroToOCLOpaqueType,
                                     CurInit.get()->getValueKind());
       break;
     }
+    case SK_ParenthesizedListInit: {
+      CurInit = false;
+      TryOrBuildParenListInitialization(S, Entity, Kind, Args, *this,
+                                        /*VerifyOnly=*/false, &CurInit);
+      if (CurInit.get() && ResultType)
+        *ResultType = CurInit.get()->getType();
+      break;
+    }
     }
   }
 
   // Check whether the initializer has a shorter lifetime than the initialized
   // entity, and if not, either lifetime-extend or warn as appropriate.
   if (auto *Init = CurInit.get())
     S.checkInitializerLifetime(Entity, Init);
 
OverloadingResult Ovl
       = FailedCandidateSet.BestViableFunction(S, Kind.getLocation(), Best);
     (void)Ovl;
     assert(Ovl == OR_Success && "Inconsistent overload resolution");
     CXXConstructorDecl *CtorDecl = cast<CXXConstructorDecl>(Best->Function);
     S.Diag(CtorDecl->getLocation(),
            diag::note_explicit_ctor_deduction_guide_here) << false;
     break;
   }
+
+  case FK_ParenthesizedListInitFailed:
+    TryOrBuildParenListInitialization(S, Entity, Kind, Args, *this,
+                                      /*VerifyOnly=*/false);
+    break;
   }
 
   PrintInitLocationNote(S, Entity);
   return true;
 }
 
 void InitializationSequence::dump(raw_ostream &OS) const {
   switch (SequenceKind) {
case FailedSequence: {
 
     case FK_ListConstructorOverloadFailed:
       OS << "list constructor overloading failed";
       break;
 
     case FK_ExplicitConstructor:
       OS << "list copy initialization chose explicit constructor";
       break;
+
+    case FK_ParenthesizedListInitFailed:
+      OS << "parenthesized list initialization failed";
+      break;
     }
     OS << '\n';
     return;
   }
 
   case DependentSequence:
     OS << "Dependent sequence\n";
     return;
for (step_iterator S = step_begin(), SEnd = step_end(); S != SEnd; ++S) {
 
     case SK_OCLSamplerInit:
       OS << "OpenCL sampler_t from integer constant";
       break;
 
     case SK_OCLZeroOpaqueType:
       OS << "OpenCL opaque type from zero";
       break;
+    case SK_ParenthesizedListInit:
+      OS << "initialization from a parenthesized list of values";
+      break;
     }
 
     OS << " [" << S->Type << ']';
   }
 
   OS << '\n';
 }
 

clang/lib/Sema/TreeTransform.h

   ///
   /// By default, produces the fallback value for the fold-expression, or
   /// produce an error if there is no fallback value.
   ExprResult RebuildEmptyCXXFoldExpr(SourceLocation EllipsisLoc,
                                      BinaryOperatorKind Operator) {
     return getSema().BuildEmptyCXXFoldExpr(EllipsisLoc, Operator);
   }
 
+  ExprResult RebuildCXXParenListInitExpr(ArrayRef<Expr *> Args, QualType T,
+                                         unsigned NumUserSpecifiedExprs,
+                                         SourceLocation InitLoc,
+                                         SourceLocation LParenLoc,
+                                         SourceLocation RParenLoc) {
+    return CXXParenListInitExpr::Create(getSema().Context, Args, T,
+                                        NumUserSpecifiedExprs, InitLoc,
+                                        LParenLoc, RParenLoc);
+  }
+
   /// Build a new atomic operation expression.
   ///
   /// By default, performs semantic analysis to build the new expression.
   /// Subclasses may override this routine to provide different behavior.
   ExprResult RebuildAtomicExpr(SourceLocation BuiltinLoc, MultiExprArg SubExprs,
                                AtomicExpr::AtomicOp Op,
                                SourceLocation RParenLoc) {
     // Use this for all of the locations, since we don't know the difference
TreeTransform<Derived>::TransformCXXFoldExpr(CXXFoldExpr *E) {
   // then produce a fallback value or error.
   if (Result.isUnset())
     return getDerived().RebuildEmptyCXXFoldExpr(E->getEllipsisLoc(),
                                                 E->getOperator());
 
   return Result;
 }
 
-template<typename Derived>
+template <typename Derived>
 ExprResult
+TreeTransform<Derived>::TransformCXXParenListInitExpr(CXXParenListInitExpr *E) {
+  SmallVector<Expr *, 4> TransformedInits;
+  ArrayRef<Expr *> InitExprs = E->getInitExprs();
+  if (TransformExprs(InitExprs.data(), InitExprs.size(), true,
+                     TransformedInits))
+    return ExprError();
+
+  return getDerived().RebuildCXXParenListInitExpr(
+      TransformedInits, E->getType(), E->getUserSpecifiedInitExprs().size(),
+      E->getInitLoc(), E->getBeginLoc(), E->getEndLoc());
+}
+
+template<typename Derived>
+ExprResult
 TreeTransform<Derived>::TransformCXXStdInitializerListExpr(
     CXXStdInitializerListExpr *E) {
   return getDerived().TransformExpr(E->getSubExpr());
 }
 
 template<typename Derived>
 ExprResult
 TreeTransform<Derived>::TransformObjCStringLiteral(ObjCStringLiteral *E) {

clang/lib/Serialization/ASTReaderStmt.cpp

void ASTStmtReader::VisitCXXFoldExpr(CXXFoldExpr *E) {
   E->RParenLoc = readSourceLocation();
   E->NumExpansions = Record.readInt();
   E->SubExprs[0] = Record.readSubExpr();
   E->SubExprs[1] = Record.readSubExpr();
   E->SubExprs[2] = Record.readSubExpr();
   E->Opcode = (BinaryOperatorKind)Record.readInt();
 }
 
+void ASTStmtReader::VisitCXXParenListInitExpr(CXXParenListInitExpr *E) {
+  VisitExpr(E);
+  unsigned ExpectedNumExprs = Record.readInt();
+  assert(E->NumExprs == ExpectedNumExprs &&
+         "expected number of expressions does not equal the actual number of "
+         "serialized expressions.");
+  E->NumUserSpecifiedExprs = Record.readInt();
+  E->InitLoc = readSourceLocation();
+  E->LParenLoc = readSourceLocation();
+  E->RParenLoc = readSourceLocation();
+  for (unsigned I = 0; I < ExpectedNumExprs; I++)
+    E->getTrailingObjects<Expr *>()[I] = Record.readSubExpr();
+
+  bool HasArrayFiller = Record.readBool();
+  if (HasArrayFiller) {
+    E->setArrayFiller(Record.readSubExpr());
+  }
+  E->updateDependence();
+}
+
 void ASTStmtReader::VisitOpaqueValueExpr(OpaqueValueExpr *E) {
   VisitExpr(E);
   E->SourceExpr = Record.readSubExpr();
   E->OpaqueValueExprBits.Loc = readSourceLocation();
   E->setIsUnique(Record.readInt());
 }
 
 void ASTStmtReader::VisitTypoExpr(TypoExpr *E) {

     case EXPR_MATERIALIZE_TEMPORARY:
       S = new (Context) MaterializeTemporaryExpr(Empty);
       break;
 
     case EXPR_CXX_FOLD:
       S = new (Context) CXXFoldExpr(Empty);
       break;
 
+    case EXPR_CXX_PAREN_LIST_INIT:
+      S = CXXParenListInitExpr::CreateEmpty(
+          Context, /*numExprs=*/Record[ASTStmtReader::NumExprFields], Empty);
+      break;
+
     case EXPR_OPAQUE_VALUE:
       S = new (Context) OpaqueValueExpr(Empty);
       break;
 
     case EXPR_CUDA_KERNEL_CALL:
       S = CUDAKernelCallExpr::CreateEmpty(
           Context, /*NumArgs=*/Record[ASTStmtReader::NumExprFields],
           /*HasFPFeatures=*/Record[ASTStmtReader::NumExprFields + 1], Empty);

clang/lib/Serialization/ASTWriter.cpp

#define RECORD(X) EmitRecordID(X, #X, Stream, Record)
   RECORD(EXPR_CXX_DYNAMIC_CAST);
   RECORD(EXPR_CXX_REINTERPRET_CAST);
   RECORD(EXPR_CXX_CONST_CAST);
   RECORD(EXPR_CXX_ADDRSPACE_CAST);
   RECORD(EXPR_CXX_FUNCTIONAL_CAST);
   RECORD(EXPR_USER_DEFINED_LITERAL);
   RECORD(EXPR_CXX_STD_INITIALIZER_LIST);
   RECORD(EXPR_CXX_BOOL_LITERAL);
+  RECORD(EXPR_CXX_PAREN_LIST_INIT);
   RECORD(EXPR_CXX_NULL_PTR_LITERAL);
   RECORD(EXPR_CXX_TYPEID_EXPR);
   RECORD(EXPR_CXX_TYPEID_TYPE);
   RECORD(EXPR_CXX_THIS);
   RECORD(EXPR_CXX_THROW);
   RECORD(EXPR_CXX_DEFAULT_ARG);
   RECORD(EXPR_CXX_DEFAULT_INIT);
   RECORD(EXPR_CXX_BIND_TEMPORARY);

clang/lib/Serialization/ASTWriterStmt.cpp

void ASTStmtWriter::VisitCXXFoldExpr(CXXFoldExpr *E) {
   Record.push_back(E->NumExpansions);
   Record.AddStmt(E->SubExprs[0]);
   Record.AddStmt(E->SubExprs[1]);
   Record.AddStmt(E->SubExprs[2]);
   Record.push_back(E->Opcode);
   Code = serialization::EXPR_CXX_FOLD;
 }
 
+void ASTStmtWriter::VisitCXXParenListInitExpr(CXXParenListInitExpr *E) {
+  VisitExpr(E);
+  ArrayRef<Expr *> InitExprs = E->getInitExprs();
+  Record.push_back(InitExprs.size());
+  Record.push_back(E->getUserSpecifiedInitExprs().size());
+  Record.AddSourceLocation(E->getInitLoc());
+  Record.AddSourceLocation(E->getBeginLoc());
+  Record.AddSourceLocation(E->getEndLoc());
+  for (Expr *InitExpr : E->getInitExprs())
+    Record.AddStmt(InitExpr);
+  bool HasArrayFiller = E->getArrayFiller();
+  Record.push_back(HasArrayFiller);
+  if (HasArrayFiller)
+    Record.AddStmt(E->getArrayFiller());
+  Code = serialization::EXPR_CXX_PAREN_LIST_INIT;
+}
+
 void ASTStmtWriter::VisitOpaqueValueExpr(OpaqueValueExpr *E) {
   VisitExpr(E);
   Record.AddStmt(E->getSourceExpr());
   Record.AddSourceLocation(E->getLocation());
   Record.push_back(E->isUnique());
   Code = serialization::EXPR_OPAQUE_VALUE;
 }
 

clang/lib/StaticAnalyzer/Core/ExprEngine.cpp

case Stmt::CXXBindTemporaryExprClass: {
       break;
     }
 
     case Stmt::ArrayInitLoopExprClass:
       Bldr.takeNodes(Pred);
       VisitArrayInitLoopExpr(cast<ArrayInitLoopExpr>(S), Pred, Dst);
       Bldr.addNodes(Dst);
       break;
     // Cases not handled yet; but will handle some day.
     case Stmt::DesignatedInitExprClass:
     case Stmt::DesignatedInitUpdateExprClass:
     case Stmt::ArrayInitIndexExprClass:
     case Stmt::ExtVectorElementExprClass:
     case Stmt::ImaginaryLiteralClass:
     case Stmt::ObjCAtCatchStmtClass:
     case Stmt::ObjCAtFinallyStmtClass:
     case Stmt::ObjCAtTryStmtClass:
     case Stmt::ObjCAutoreleasePoolStmtClass:
     case Stmt::ObjCEncodeExprClass:
     case Stmt::ObjCIsaExprClass:
     case Stmt::ObjCProtocolExprClass:
     case Stmt::ObjCSelectorExprClass:
     case Stmt::ParenListExprClass:
     case Stmt::ShuffleVectorExprClass:
     case Stmt::ConvertVectorExprClass:
     case Stmt::VAArgExprClass:
     case Stmt::CUDAKernelCallExprClass:
     case Stmt::OpaqueValueExprClass:
     case Stmt::AsTypeExprClass:
     case Stmt::ConceptSpecializationExprClass:
     case Stmt::CXXRewrittenBinaryOperatorClass:
     case Stmt::RequiresExprClass:
+    case Expr::CXXParenListInitExprClass:
       // Fall through.
 
     // Cases we intentionally don't evaluate, since they don't need
     // to be explicitly evaluated.
     case Stmt::PredefinedExprClass:
     case Stmt::AddrLabelExprClass:
     case Stmt::AttributedStmtClass:
     case Stmt::IntegerLiteralClass:

clang/test/CXX/temp/temp.decls/temp.variadic/p4.cpp

-// RUN: %clang_cc1 -std=c++11 -fsyntax-only -fexceptions -fcxx-exceptions -verify %s
-// RUN: %clang_cc1 -std=c++2a -fsyntax-only -fexceptions -fcxx-exceptions -verify %s
+// RUN: %clang_cc1 -std=c++11 -fsyntax-only -fexceptions -fcxx-exceptions -verify=expected,pre20 %s
+// RUN: %clang_cc1 -std=c++2a -fsyntax-only -fexceptions -fcxx-exceptions -verify=expected,post20 %s
 
 template<typename... Types> struct tuple;
 template<int I> struct int_c;
 
 template<typename T>
 struct identity {
   typedef T type;
 };

 // In a base-specifier-list (Clause 10); the pattern is a base-specifier.
 template<typename ...Mixins>
 struct HasMixins : public Mixins... { 
   HasMixins();
   HasMixins(const HasMixins&);
   HasMixins(int i);
 };
 
-struct A { }; // expected-note{{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const A' for 1st argument}} \
-// expected-note{{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'A' for 1st argument}} \
-// expected-note{{candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided}}
-struct B { }; // expected-note{{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const B' for 1st argument}} \
-// expected-note{{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'B' for 1st argument}} \
-// expected-note{{candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided}}
+struct A { }; // pre20-note{{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const A' for 1st argument}} \
+// pre20-note{{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'A' for 1st argument}} \
+// pre20-note{{candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided}}
+struct B { }; // pre20-note{{candidate constructor (the implicit copy constructor) not viable: no known conversion from 'int' to 'const B' for 1st argument}} \
+// pre20-note{{candidate constructor (the implicit move constructor) not viable: no known conversion from 'int' to 'B' for 1st argument}} \
+// pre20-note{{candidate constructor (the implicit default constructor) not viable: requires 0 arguments, but 1 was provided}}
 struct C { };
 struct D { };
 
 A *checkA = new HasMixins<A, B, C, D>;
 B *checkB = new HasMixins<A, B, C, D>;
 D *checkD = new HasMixins<A, B, C, D>;
 C *checkC = new HasMixins<A, B, D>; // expected-error{{cannot initialize a variable of type 'C *' with an rvalue of type 'HasMixins<A, B, D> *'}}
 HasMixins<> *checkNone = new HasMixins<>;
 
 template<typename Mixins>
 struct BrokenMixins : public Mixins... { }; // expected-error{{pack expansion does not contain any unexpanded parameter packs}}
 
 // In a mem-initializer-list (12.6.2); the pattern is a mem-initializer.
 template<typename ...Mixins>
 HasMixins<Mixins...>::HasMixins(): Mixins()... { }
 
 template<typename ...Mixins>
 HasMixins<Mixins...>::HasMixins(const HasMixins &other): Mixins(other)... { }
 
 template<typename ...Mixins>
 HasMixins<Mixins...>::HasMixins(int i): Mixins(i)... { }
-// expected-error@-1 {{no matching constructor for initialization of 'A'}}
-// expected-error@-2 {{no matching constructor for initialization of 'B'}}
+// pre20-error@-1 {{no matching constructor for initialization of 'A'}}
+// pre20-error@-2 {{no matching constructor for initialization of 'B'}}
+// post20-error@-3 {{excess elements in struct initializer}}
+// post20-error@-4 {{excess elements in struct initializer}}
 
 void test_has_mixins() {
   HasMixins<A, B> ab;
   HasMixins<A, B> ab2 = ab;
   HasMixins<A, B> ab3(17); // expected-note{{in instantiation of member function 'HasMixins<A, B>::HasMixins' requested here}}
 }
 
 template<typename T>

clang/test/CodeGen/paren-list-agg-init.cpp

+// RUN: %clang_cc1 -std=c++20 %s -emit-llvm -triple x86_64-unknown-linux-gnu -o - | FileCheck %s
+
+// CHECK-DAG: [[STRUCT_A:%.*]] = type { i8, double }
+struct A {
+  char i;
+  double j;
+};
+
+// CHECK-DAG: [[STRUCT_B:%.*]] = type { [[STRUCT_A]], i32 }
+struct B {
+  A a;
+  int b;
+};
+
+// CHECK-DAG: [[STRUCT_C:%.*]] = type <{ [[STRUCT_B]], [[STRUCT_A]], i32, [4 x i8] }>
+struct C : public B, public A {
+  int c;
+};
+
+// CHECK-DAG: [[STRUCT_D:%.*]] = type { [[STRUCT_A]], [[STRUCT_A]], i8, [[STRUCT_A]] }
+struct D {
+  A a;
+  A b = A{2, 2.0};
+  unsigned : 2;
+  A c;
+};
+
+// CHECK-DAG: [[STRUCT_E:%.*]] = type { i32, ptr }
+struct E {
+  int a;
+  const char* fn = __builtin_FUNCTION();
+};
+
+// CHECK-DAG: [[STRUCT_F:%.*]] = type { i8 }
+struct F {
+  F (int i = 1);
+  F (const F &f) = delete;
+  F (F &&f) = default;
+};
+
+// CHECK-DAG: [[STRUCT_G:%.*]] = type <{ i32, [[STRUCT_F]], [3 x i8] }>
+struct G {
+  int a;
+  F f;
+};
+
+// CHECK-DAG: [[UNION_U:%.*]] = type { [[STRUCT_A]] }
+// CHECK-DAG: [[STR:@.*]] = private unnamed_addr constant [6 x i8] {{.*}}foo18{{.*}}, align 1
+union U {
+  unsigned : 1;
+  A a;
+  char b;
+};
+
+// CHECK-DAG: [[A1:@.*a1.*]] = internal constant [[STRUCT_A]] { i8 3, double 2.000000e+00 }, align 8
+constexpr A a1(3.1, 2.0);
+// CHECK-DAG: [[A2:@.*a2.*]] = internal constant [[STRUCT_A]] { i8 99, double 0.000000e+00 }, align 8
+constexpr auto a2 = static_cast<A>('c');
+// CHECK-DAG: [[B1:@.*b1.*]] = internal constant [[STRUCT_B]] { [[STRUCT_A]] { i8 99, double 0.000000e+00 }, i32 0 }, align 8
+constexpr B b1(A('c'));
+// CHECK-DAG: [[C1:@.*c1.*]] = internal constant { [[STRUCT_A]], i32, [4 x i8], i8, double, i32 } { [[STRUCT_A]] { i8 99, double 0.000000e+00 }, i32 0, [4 x i8] undef, i8 3, double 2.000000e+00, i32 0 }, align
+constexpr C c1(b1, a1);
+// CHECK-DAG: [[U1:@.*u1.*]] = internal constant [[UNION_U]] { [[STRUCT_A]] { i8 1, double 1.000000e+00 } }, align 8
+constexpr U u1(A(1, 1));
+// CHECK-DAG: [[D1:@.*d1.*]] = internal constant { [[STRUCT_A]], [[STRUCT_A]], [8 x i8], [[STRUCT_A]] } { [[STRUCT_A]] { i8 2, double 2.000000e+00 }, [[STRUCT_A]] { i8 2, double 2.000000e+00 }, [8 x i8] undef, [[STRUCT_A]] zeroinitializer }, align 8
+constexpr D d1(A(2, 2));
+// CHECK-DAG: [[ARR1:@.*arr1.*]] = internal constant [3 x i32] [i32 1, i32 2, i32 0], align 4
+constexpr int arr1[3](1, 2);
+// CHECK-DAG: [[ARR4:@.*arr4.*]] = internal constant [1 x i32] [i32 1], align 4
+constexpr int arr4[](1);
+// CHECK-DAG: [[ARR5:@.*arr5.*]] = internal constant [2 x i32] [i32 2, i32 0], align 4
+constexpr int arr5[2](2);
+
+// CHECK: define dso_local { i8, double } @{{.*foo1.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[RETVAL:%.*]] = alloca [[STRUCT_A]], align 8
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[RETVAL]], ptr align 8 [[A1]], i64 16, i1 false)
+// CHECK-NEXT: [[TMP_0:%.*]] = load { i8, double }, ptr [[RETVAL]], align 8
+// CHECK-NEXT: ret { i8, double } [[TMP_0]]
+A foo1() {
+  return a1;
+}
+
+// CHECK: define dso_local void @{{.*foo2.*}}(ptr noalias sret([[STRUCT_B]]) align 8 [[AGG_RESULT:%.*]])
+// CHECK-NEXT: entry:
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[AGG_RESULT]], ptr align 8 [[B1]], i64 24, i1 false)
+// CHECK-NEXT: ret void
+B foo2() {
+  return b1;
+}
+
+// CHECK: define dso_local void @{{.*foo3.*}}(ptr noalias sret([[STRUCT_C]]) align 8 [[AGG_RESULT:%.*]])
+// CHECK-NEXT: entry:
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[AGG_RESULT]], ptr align 8 [[C1]], i64 48, i1 false)
+// CHECK-NEXT: ret void
+C foo3() {
+  return c1;
+}
+
+// CHECK: define dso_local void @{{.*foo4.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[C2:%.*]] = alloca [[STRUCT_C:%.*]], align 8
+// CHECK-NEXT: [[REF_TMP:%.*]] = alloca [[STRUCT_B:%.*]], align 8
+// CHECK-NEXT: [[REF_TMP_1:%.*]] = alloca [[STRUCT_A:%.*]], align 8
+// CHECK-NEXT: [[A:%.*]] = getelementptr inbounds [[STRUCT_B]], ptr [[REF_TMP]], i32 0, i32 0
+// CHECK-NEXT: [[I:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT: store i8 1, ptr [[I]], align 8
+// CHECK-NEXT: [[J:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[A]], i32 0, i32 1
+// CHECK-NEXT: store double 1.000000e+00, ptr [[J]], align 8
+// CHECK-NEXT: [[B:%.*]] = getelementptr inbounds [[STRUCT_B]], ptr [[REF_TMP]], i32 0, i32 1
+// CHECK-NEXT: store i32 1, ptr [[B]], align 8
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[C2]], ptr align 8 [[REF_TMP]], i64 24, i1 false)
+// CHECK-NEXT: [[TMP_0:%.*]] = getelementptr inbounds i8, ptr [[C2]], i64 24
+// CHECK-NEXT: [[I2:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[REF_TMP_1]], i32 0, i32 0
+// CHECK-NEXT: store i8 97, ptr [[I2]], align 8
+// CHECK-NEXT: [[J3:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[REF_TMP_1]], i32 0, i32 1
+// CHECK-NEXT: store double 0.000000e+00, ptr [[J3]], align 8
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[TMP_0]], ptr align 8 [[REF_TMP_1]], i64 16, i1 false)
+// CHECK-NEXT: [[C:%.*]] = getelementptr inbounds %struct.C, ptr %c2, i32 0, i32 2
+// CHECK-NEXT: store i32 2, ptr %c, align 8
+// CHECK-NEXT: ret void
+void foo4() {
+  C c2(B(A(1, 1), 1), A('a'), 2);
+}
+
+// CHECK: define dso_local { i64, double } @{{.*foo5.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT [[RETVAL:%.*]] = alloca [[UNION_U]], align 8
+// CHECK-NEXT call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[RETVAL]], ptr align 8 [[U1]], i64 16, i1 false)
+// CHECK-NEXT [[COERCE_DIVE:%.*]] = getelementptr inbounds [[UNION_U]], ptr %retval, i32 0, i32 0
+// CHECK-NEXT [[TMP_0:%.*]] = load { i64, double }, ptr [[COERCE_DIVE]], align 8
+// CHECK-NEXT ret { i64, double } [[TMP_0]]
+U foo5() {
+  return u1;
+}
+
+
+// CHECK: define dso_local { i64, double } @{{.*foo6.*}}(i8 [[A_COERCE_0:%.*]], double [[A_COERCE_1:%.*]])
+// CHECK-NEXT: entry:
+// CHECK-NEXT:   [[RETVAL:%.*]] = alloca [[UNION_U]], align 8
+// CHECK-NEXT:   [[A:%.*]] = alloca [[STRUCT_A]], align 8
+// CHECK-NEXT:   [[TMP_0:%.*]] = getelementptr inbounds { i8, double }, ptr [[A]], i32 0, i32 0
+// CHECK-NEXT:   store i8 [[A_COERCE_0]], ptr [[TMP_0]], align 8
+// CHECK-NEXT:   [[TMP_1:%.*]] = getelementptr inbounds { i8, double }, ptr [[A]], i32 0, i32 1
+// CHECK-NEXT:   store double [[A_COERCE_1]], ptr [[TMP_1]], align 8
+// CHECK-NEXT:   call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[RETVAL]], ptr align 8 [[A]], i64 16, i1 false)
+// CHECK-NEXT:   [[COERCE_DIVE:%.*]] = getelementptr inbounds [[UNION_U]], ptr [[RETVAL]], i32 0, i32 0
+// CHECK-NEXT:   [[TMP_2:%.*]] = load { i64, double }, ptr [[COERCE_DIVE:%.*]], align 8
+// CHECK-NEXT:   ret { i64, double } [[TMP_2]]
+U foo6(A a) {
+  return U(a);
+}
+
+// CHECK: define dso_local void @{{.*foo7.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[D:%.*]] = alloca [[STRUCT_D:%.*]], align 8
+// CHECK-NEXT: [[A:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 0
+// CHECK-NEXT: [[I]] = getelementptr inbounds [[STRUCT_A:%.*]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT: store i8 1, ptr [[I]], align 8
+// CHECK-NEXT: [[J:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[A]], i32 0, i32 1
+// CHECK-NEXT: store double 1.000000e+00, ptr [[J]], align 8
+// CHECK-NEXT: [[B:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 1
+// CHECK-NEXT: [[I1:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[B]], i32 0, i32 0
+// CHECK-NEXT: store i8 11, ptr [[I1]], align 8
+// CHECK-NEXT: [[J2:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[B]], i32 0, i32 1
+// CHECK-NEXT: store double 1.100000e+01, ptr [[J2]], align 8
+// CHECK-NEXT: [[C:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 3
+// CHECK-NEXT: [[I3:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[C]], i32 0, i32 0
+// CHECK-NEXT: store i8 111, ptr [[I3]], align 8
+// CHECK-NEXT: [[J4:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[C]], i32 0, i32 1
+// CHECK-NEXT: store double 1.110000e+02, ptr [[J4]], align 8
+// CHECK-NEXT: ret void
+void foo7() {
+  D d(A(1, 1), A(11, 11), A(111, 111));
+}
+
+// CHECK: dso_local void @{{.*foo8.*}}(ptr noalias sret([[STRUCT_D]]) align 8 [[AGG_RESULT:%.*]])
+// CHECK-NEXT: entry:
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[AGG_RESULT]], ptr align 8 [[D1]], i64 56, i1 false)
+// CHECK-NEXT: ret void
+D foo8() {
+  return d1;
+}
+
+// CHECK: define dso_local void @{{.*foo9.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[D:%.*]] = alloca [[STRUCT_D:%.*]], align 8
+// CHECK-NEXT: [[A:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 0
+// CHECK-NEXT: [[I]] = getelementptr inbounds [[STRUCT_A:%.*]], ptr [[A]], i32 0, i32 0
+// CHECK-NEXT: store i8 1, ptr [[I]], align 8
+// CHECK-NEXT: [[J:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[A]], i32 0, i32 1
+// CHECK-NEXT: store double 1.000000e+00, ptr [[J]], align 8
+// CHECK-NEXT: [[B:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 1
+// CHECK-NEXT: [[I1:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[B]], i32 0, i32 0
+// CHECK-NEXT: store i8 2, ptr [[I1]], align 8
+// CHECK-NEXT: [[J2:%.*]] = getelementptr inbounds [[STRUCT_A]], ptr [[B]], i32 0, i32 1
+// CHECK-NEXT: store double 2.000000e+00, ptr [[J2]], align 8
+// CHECK-NEXT: [[C:%.*]] = getelementptr inbounds [[STRUCT_D]], ptr [[D]], i32 0, i32 3
+// CHECK-NEXT: call void @llvm.memset.p0.i64(ptr align 8 [[C]], i8 0, i64 16, i1 false)
+// CHECK-NEXT: ret void
+void foo9() {
+  D d(A(1, 1));
+}
+
+// CHECK: define dso_local noundef ptr @{{.*foo10.*}}()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret ptr [[ARR1]]
+const int* foo10() {
+  return arr1;
+}
+
+// CHECK: define dso_local void @{{.*foo11.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[A_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[B_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[ARR_2:%.*]] = alloca [4 x i32], align 16
+// CHECK-NEXT: store i32 [[A:%.*]], ptr [[A_ADDR]], align 4
+// CHECK-NEXT: store i32 [[B:%.*]], ptr [[B_ADDR]], align 4
+// CHECK-NEXT: [[ARRINIT_BEGIN:%.*]] = getelementptr inbounds [4 x i32], ptr [[ARR_2]], i64 0, i64 0
+// CHECK-NEXT: [[TMP_0:%.*]] = load i32, ptr [[A_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_0]], ptr [[ARRINIT_BEGIN]], align 4
+// CHECK-NEXT: [[ARRINIT_ELEM:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 1
+// CHECK-NEXT: [[TMP_1:%.*]] = load i32, ptr [[B_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_1]], ptr [[ARRINIT_ELEM]], align 4
+// CHECK-NEXT: [[ARRINIT_START:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_ELEM]], i64 1
+// CHECK-NEXT: [[ARRINIT_END:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 4
+// CHECK-NEXT: br label [[ARRINIT_BODY:%.*]]
+// CHECK: [[ARRINIT_CUR:%.*]] = phi ptr [ [[ARRINIT_START]], %entry ], [ [[ARRINIT_NEXT:%.*]], [[ARRINIT_BODY]] ]
+// CHECK-NEXT: store i32 0, ptr [[ARRINIT_CUR]], align 4
+// CHECK-NEXT: [[ARRINIT_NEXT]] = getelementptr inbounds i32, ptr [[ARRINIT_CUR]], i64 1
+// CHECK-NEXT: [[ARRINIT_DONE:%.*]] = icmp eq ptr [[ARRINIT_NEXT]], [[ARRINIT_END:%.*]]
+// CHECK-NEXT: br i1 [[ARRINIT_DONE]], label [[ARRINIT_END1:%.*]], label [[ARRINIT_BODY]]
+// CHECK: ret void
+void foo11(int a, int b) {
+  int arr2[4](a, b);
+}
+
+// CHECK: define dso_local void @{{.*foo12.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[A_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[B_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[ARR_3:%.*]] = alloca [2 x i32], align 4
+// CHECK-NEXT: store i32 [[A:%.*]], ptr [[A_ADDR]], align 4
+// CHECK-NEXT: store i32 [[B:%.*]], ptr [[B_ADDR]], align 4
+// CHECK-NEXT: [[ARRINIT_BEGIN:%.*]] = getelementptr inbounds [2 x i32], ptr [[ARR_3]], i64 0, i64 0
+// CHECK-NEXT: [[TMP_0:%.*]] = load i32, ptr [[A_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_0]], ptr [[ARRINIT_BEGIN]], align 4
+// CHECK-NEXT: [[ARRINIT_ELEMENT:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 1
+// CHECK-NEXT: [[TMP_1:%.*]] = load i32, ptr [[B_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_1]], ptr [[ARRINIT_ELEMENT]], align 4
+// CHECK-NEXT: ret void
+void foo12(int a, int b) {
+  int arr3[](a, b);
+}
+
+// CHECK: define dso_local { i8, double } @{{.*foo13.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[RETVAL:%.*]] = alloca [[STRUCT_A]], align 8
+// CHECK-NEXT: call void @llvm.memcpy.p0.p0.i64(ptr align 8 [[RETVAL]], ptr align 8 [[A2]], i64 16, i1 false)
+// CHECK-NEXT: [[TMP_0:%.*]] = load { i8, double }, ptr [[RETVAL]], align 8
+// CHECK-NEXT: ret { i8, double } [[TMP_0]]
+A foo13() {
+  return a2;
+}
+
+// CHECK: define dso_local noundef ptr @{{.*foo14.*}}()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret ptr [[ARR4]]
+const int* foo14() {
+  return arr4;
+}
+
+// CHECK: define dso_local noundef ptr @{{.*foo15.*}}()
+// CHECK-NEXT: entry:
+// CHECK-NEXT: ret ptr [[ARR5]]
+const int* foo15() {
+  return arr5;
+}
+
+// CHECK: define dso_local void @{{.*foo16.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[ARR_6:%.*arr6.*]] = alloca ptr, align 8
+// CHECK-NEXT: [[REF_TMP:%.*]] = alloca [1 x i32], align 4
+// CHECK-NEXT: [[ARRINIT_BEGIN:%.*]] = getelementptr inbounds [1 x i32], ptr [[REF_TMP]], i64 0, i64 0
+// CHECK-NEXT: store i32 3, ptr [[ARRINIT_BEGIN]], align 4
+// CHECK-NEXT: store ptr [[REF_TMP]], ptr [[ARR_6]], align 8
+// CHECK-NEXT: ret void
+void foo16() {
+  int (&&arr6)[] = static_cast<int[]>(3);
+}
+
+// CHECK: define dso_local void @{{.*foo17.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[ARR_7:%.*arr7.*]] = alloca ptr, align 8
+// CHECK-NEXT: [[REF_TMP:%.*]] = alloca [2 x i32], align 4
+// CHECK-NEXT: [[ARRINIT_BEGIN:%.*]] = getelementptr inbounds [2 x i32], ptr [[REF_TMP]], i64 0, i64 0
+// CHECK-NEXT: store i32 4, ptr [[ARRINIT_BEGIN]], align 4
+// CHECK-NEXT: [[ARRINIT_START:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 1
+// CHECK-NEXT: [[ARRINIT_END:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 2
+// CHECK-NEXT: br label [[ARRINIT_BODY]]
+// CHECK: [[ARRINIT_CUR:%.*]] = phi ptr [ [[ARRINIT_START]], %entry ], [ [[ARRINIT_NEXT:%.*]], [[ARRINIT_BODY]] ]
+// CHECK-NEXT: store i32 0, ptr [[ARRINIT_CUR]], align 4
+// CHECK-NEXT: [[ARRINIT_NEXT]] = getelementptr inbounds i32, ptr [[ARRINIT_CUR]], i64 1
+// CHECK-NEXT: [[ARRINIT_DONE:%.*]] = icmp eq ptr [[ARRINIT_NEXT]], [[ARRINIT_END:%.*]]
+// CHECK-NEXT: br i1 [[ARRINIT_DONE]], label [[ARRINIT_END1:%.*]], label [[ARRINIT_BODY]]
+// CHECK: store ptr [[REF_TMP]], ptr [[ARR_7]], align 8
+// CHECK: ret void
+void foo17() {
+  int (&&arr7)[2] = static_cast<int[2]>(4);
+}
+
+// CHECK: define dso_local void @{{.*foo18.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[E:%.*e.*]] = alloca [[STRUCT_E]], align 8
+// CHECK-NEXT: [[A:%.*a.*]] = getelementptr inbounds [[STRUCT_E]], ptr [[E]], i32 0, i32 0
+// CHECK-NEXT: store i32 1, ptr [[A]], align 8
+// CHECK-NEXT: [[FN:%.*fn.*]] = getelementptr inbounds [[STRUCT_E]], ptr [[E]], i32 0, i32 1
+// CHECK-NEXT: store ptr [[STR]], ptr [[FN]], align 8
+// CHECK: ret void
+void foo18() {
+  E e(1);
+}
+
+// CHECK: define dso_local void @{{.*foo19.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[G:%.*g.*]] = alloca [[STRUCT_G]], align 4
+// CHECK-NEXT: [[A:%.*a.*]] = getelementptr inbounds [[STRUCT_G]], ptr [[G]], i32 0, i32 0
+// CHECK-NEXT: store i32 2, ptr [[A]], align 4
+// CHECK-NEXT: [[F:%.*f.*]] = getelementptr inbounds [[STRUCT_G]], ptr [[G]], i32 0, i32 1
+// CHECk-NEXT: call void @{{.*F.*}}(ptr noundef nonnull align 1 dereferenceable(1)) [[F]], ie32 noundef 1)
+// CHECK: ret void
+void foo19() {
+  G g(2);
+}

clang/test/Lexer/cxx-features.cpp

 #endif
 
 #if check(explicit_this_parameter, 0, 0, 0, 0, 0, 0)
 #error "wrong value for __cpp_explicit_this_parameter"
 #endif
 
 // --- C++20 features ---
 
-#if check(aggregate_paren_init, 0, 0, 0, 0, 0, 0)
-// FIXME: 201902 in C++20
+#if check(aggregate_paren_init, 0, 0, 0, 0, 201902, 201902)
 #error "wrong value for __cpp_aggregate_paren_init"
 #endif
 
 #if defined(CHAR8_T) ? check(char8_t, 202207, 202207, 202207, 202207, 202207, 202207) : \
     defined(NO_CHAR8_T) ? check(char8_t, 0, 0, 0, 0, 0, 0) : \
     check(char8_t, 0, 0, 0, 0, 202207, 202207)
 #error "wrong value for __cpp_char8_t"
 #endif

clang/test/PCH/cxx_paren_init.h

+struct S { int i, j; };
+
+constexpr S foo(int i, int j) { return S(i, j); };
+
+void bar(int i, int j) { int arr[4](i, j); };

clang/test/PCH/cxx_paren_init.cpp

+// RUN: %clang_cc1 -x c++ -std=c++20 -triple x86_64-unknown-linux-gnu -emit-pch -o %t %S/cxx_paren_init.h
+// RUN: %clang_cc1 -x c++ -std=c++20 -triple x86_64-unknown-linux-gnu -include-pch %t %s -S -emit-llvm -o - | FileCheck %s
+
+// CHECK-DAG: [[STRUCT_S:%.*]] = type { i32, i32 }
+// CHECK-DAG: @{{.*s.*}} = {{(dso_local )?}}global [[STRUCT_S]] { i32 1, i32 2 }, align 4
+S s = foo(1, 2);
+
+// CHECK: define dso_local void @{{.*bar.*}}
+// CHECK-NEXT: entry:
+// CHECK-NEXT: [[I_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[J_ADDR:%.*]] = alloca i32, align 4
+// CHECK-NEXT: [[ARR:%.*]] = alloca [4 x i32], align 16
+// CHECK-NEXT: store i32 [[A:%.*]], ptr [[I_ADDR]], align 4
+// CHECK-NEXT: store i32 [[B:%.*]], ptr [[J_ADDR]], align 4
+// CHECK-NEXT: [[ARRINIT_BEGIN:%.*]] = getelementptr inbounds [4 x i32], ptr [[ARR]], i64 0, i64 0
+// CHECK-NEXT: [[TMP_0:%.*]] = load i32, ptr [[I_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_0]], ptr [[ARRINIT_BEGIN]], align 4
+// CHECK-NEXT: [[ARRINIT_ELEM:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 1
+// CHECK-NEXT: [[TMP_1:%.*]] = load i32, ptr [[J_ADDR]], align 4
+// CHECK-NEXT: store i32 [[TMP_1]], ptr [[ARRINIT_ELEM]], align 4
+// CHECK-NEXT: [[ARRINIT_START:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_ELEM]], i64 1
+// CHECK-NEXT: [[ARRINIT_END:%.*]] = getelementptr inbounds i32, ptr [[ARRINIT_BEGIN]], i64 4
+// CHECK-NEXT: br label [[ARRINIT_BODY:%.*]]
+// CHECK: [[ARRINIT_CUR:%.*]] = phi ptr [ [[ARRINIT_START]], %entry ], [ [[ARRINIT_NEXT:%.*]], [[ARRINIT_BODY]] ]
+// CHECK-NEXT: store i32 0, ptr [[ARRINIT_CUR]], align 4
+// CHECK-NEXT: [[ARRINIT_NEXT]] = getelementptr inbounds i32, ptr [[ARRINIT_CUR]], i64 1
+// CHECK-NEXT: [[ARRINIT_DONE:%.*]] = icmp eq ptr [[ARRINIT_NEXT]], [[ARRINIT_END:%.*]]
+// CHECK-NEXT: br i1 [[ARRINIT_DONE]], label [[ARRINIT_END1:%.*]], label [[ARRINIT_BODY]]
+// CHECK: ret void

clang/test/SemaCXX/cxx2a-explicit-bool.cpp

-// RUN: %clang_cc1 -std=c++17 -fsyntax-only %s -verify -Wno-c++2a-extensions
-// RUN: %clang_cc1 -std=c++2a -fsyntax-only %s -verify
+// RUN: %clang_cc1 -std=c++17 -fsyntax-only %s -verify=expected,pre20 -Wno-c++2a-extensions
+// RUN: %clang_cc1 -std=c++2a -fsyntax-only %s -verify=expected,post20
 
 template <bool b, auto val> struct enable_ifv {};
 
 template <auto val> struct enable_ifv<true, val> {
   static constexpr auto value = val;
 };
 
 template <typename T1, typename T2> struct is_same {
   static constexpr bool value = false;
 };
 
 template <typename T> struct is_same<T, T> {
   static constexpr bool value = true;
 };
 
 namespace special_cases
 {
 
 template<int a>
 struct A {
-// expected-note@-1+ {{candidate constructor}}
+// pre20-note@-1+ {{candidate constructor}}
   explicit(1 << a)
 // expected-note@-1 {{negative shift count -1}}
 // expected-error@-2 {{explicit specifier argument is not a constant expression}}
   A(int);
 };
 
 A<-1> a(0);
-// expected-error@-1 {{no matching constructor}}
-// expected-note@-2 {{in instantiation of template class}}
+// pre20-error@-1 {{no matching constructor}}
+// post20-error@-2 {{excess elements in struct initializer}}
+// expected-note@-3 {{in instantiation of template class}}
 
 template<int a>
 struct B {
   explicit(b)
   // expected-error@-1 {{use of undeclared identifier}}
   B(int);
 };
 

clang/test/SemaCXX/paren-list-agg-init.cpp

+// RUN: %clang_cc1 -verify -std=c++20 %s -fsyntax-only
+// RUN: %clang_cc1 -verify=expected,beforecxx20 -Wc++20-extensions -std=c++20 %s -fsyntax-only
+
+struct A { // expected-note 4{{candidate constructor}}
+  char i;
+  double j;
+};
+
+struct B {
+  A a;
+  int b[20];
+  int &&c; // expected-note {{reference member declared here}}
+};
+
+struct C { // expected-note 5{{candidate constructor}}
+  A a;
+  int b[20];
+};
+
+struct D : public C, public A {
+  int a;
+};
+
+struct E { // expected-note 3{{candidate constructor}}
+  struct F {
+    F(int, int);
+  };
+  int a;
+  F f;
+};
+
+int getint(); // expected-note {{declared here}}
+
+struct F {
+  int a;
+  int b = getint(); // expected-note {{non-constexpr function 'getint' cannot be used in a constant expression}}
+};
+
+template <typename T>
+struct G {
+  T t1;
+  T t2;
+};
+
+struct H {
+  virtual void foo() = 0;
+};
+
+struct I : public H { // expected-note 3{{candidate constructor}}
+  int i, j;
+  void foo() override {}
+};
+
+struct J {
+  int a;
+  int b[]; // expected-note {{initialized flexible array member 'b' is here}}
+};
+
+union U {
+  int a;
+  char* b;
+};
+
+template <typename T, char CH>
+void bar() {
+  T t = 0;
+  A a(CH, 1.1); // OK; C++ paren list constructors are supported in semantic tree transformations.
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+}
+
+template <class T, class... Args>
+T Construct(Args... args) {
+  return T(args...); // OK; variadic arguments can be used in paren list initializers.
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+}
+
+void foo() {
+  A a1(1954, 9, 21);
+  // expected-error@-1 {{excess elements in struct initializer}}
+  A a2(2.1);
+  // expected-warning@-1 {{implicit conversion from 'double' to 'char'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  A a3(-1.2, 9.8);
+  // expected-warning@-1 {{implicit conversion from 'double' to 'char'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  A a4 = static_cast<A>(1.1);
+  // expected-warning@-1 {{implicit conversion from 'double' to 'char'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  A a5 = (A)3.1;
+  // expected-warning@-1 {{implicit conversion from 'double' to 'char'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  A a6 = A(8.7);
+  // expected-warning@-1 {{implicit conversion from 'double' to 'char'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+
+  B b1(2022, {7, 8});
+  // expected-error@-1 {{no viable conversion from 'int' to 'A'}}
+  B b2(A(1), {}, 1);
+  // expected-error@-1 {{reference member 'c' binds to a temporary object whose lifetime would be shorter than the lifetime of the constructed object}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  // beforecxx20-warning@-3 {{aggregate initialization of type 'B' from a parenthesized list of values is a C++20 extension}}
+
+  C c(A(1), 1, 2, 3, 4);
+  // expected-error@-1 {{array initializer must be an initializer list}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  D d1(1);
+  // expected-error@-1 {{no viable conversion from 'int' to 'C'}}
+  D d2(C(1));
+  // expected-error@-1 {{no matching conversion for functional-style cast from 'int' to 'C'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'D' from a parenthesized list of values is a C++20 extension}}
+  D d3(C(A(1)), 1);
+  // expected-error@-1 {{no viable conversion from 'int' to 'A'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+  // beforecxx20-warning@-3 {{aggregate initialization of type 'C' from a parenthesized list of values is a C++20 extension}}
+
+  int arr1[](0, 1, 2, A(1));
+  // expected-error@-1 {{no viable conversion from 'A' to 'int'}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'A' from a parenthesized list of values is a C++20 extension}}
+
+  int arr2[2](0, 1, 2);
+  // expected-error@-1 {{excess elements in array initializer}}
+
+  // We should not build paren list initilizations for IK_COPY.
+  int arr3[1] = 1;
+  // expected-error@-1 {{array initializer must be an initializer list}}
+
+  U u1("abcd");
+  // expected-error@-1 {{cannot initialize a member subobject of type 'int' with an lvalue of type 'const char[5]'}}
+  U u2(1, "efgh");
+  // expected-error@-1 {{excess elements in union initializer}}
+
+  E e1(1);
+  // expected-error@-1 {{no matching constructor for initialization of 'E'}}
+
+  constexpr F f1(1);
+  // expected-error@-1 {{constexpr variable 'f1' must be initialized by a constant expression}}
+  // beforecxx20-warning@-2 {{aggregate initialization of type 'const F' from a parenthesized list of values is a C++20 extension}}
+
+  constexpr F f2(1, 1); // OK: f2.b is initialized by a constant expression.
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'const F' from a parenthesized list of values is a C++20 extension}}
+
+  bar<char, 1>();
+
+  G<char> g('b', 'b');
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'G<char>' from a parenthesized list of values is a C++20 extension}}
+
+  A a7 = Construct<A>('i', 2.2);
+  // beforecxx20-note@-1 {{in instantiation of function template specialization 'Construct<A, char, double>' requested here}}
+
+  int arr4[](1, 2);
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'int[2]' from a parenthesized list of values is a C++20 extension}}
+
+  int arr5[2](1, 2);
+  // beforecxx20-warning@-1 {{aggregate initialization of type 'int[2]' from a parenthesized list of values is a C++20 extension}}
+
+  I i(1, 2);
+  // expected-error@-1 {{no matching constructor for initialization of 'I'}}
+
+  J j(1, {2, 3});
+  // expected-error@-1 {{initialization of flexible array member is not allowed}}
+
+  static_assert(__is_trivially_constructible(A, char, double));
+  static_assert(__is_trivially_constructible(A, char, int));
+  static_assert(__is_trivially_constructible(A, char));
+
+  static_assert(__is_trivially_constructible(D, C, A, int));
+  static_assert(__is_trivially_constructible(D, C));
+
+  static_assert(__is_trivially_constructible(int[2], int, int));
+  static_assert(__is_trivially_constructible(int[2], int, double));
+  static_assert(__is_trivially_constructible(int[2], int));
+}

clang/tools/libclang/CIndex.cpp

public:
   void VisitUnresolvedMemberExpr(const UnresolvedMemberExpr *U);
   void VisitVAArgExpr(const VAArgExpr *E);
   void VisitSizeOfPackExpr(const SizeOfPackExpr *E);
   void VisitPseudoObjectExpr(const PseudoObjectExpr *E);
   void VisitOpaqueValueExpr(const OpaqueValueExpr *E);
   void VisitLambdaExpr(const LambdaExpr *E);
   void VisitConceptSpecializationExpr(const ConceptSpecializationExpr *E);
   void VisitRequiresExpr(const RequiresExpr *E);
+  void VisitCXXParenListInitExpr(const CXXParenListInitExpr *E);
   void VisitOMPExecutableDirective(const OMPExecutableDirective *D);
   void VisitOMPLoopBasedDirective(const OMPLoopBasedDirective *D);
   void VisitOMPLoopDirective(const OMPLoopDirective *D);
   void VisitOMPParallelDirective(const OMPParallelDirective *D);
   void VisitOMPSimdDirective(const OMPSimdDirective *D);
   void
   VisitOMPLoopTransformationDirective(const OMPLoopTransformationDirective *D);
   void VisitOMPTileDirective(const OMPTileDirective *D);
void EnqueueVisitor::VisitConceptSpecializationExpr(
     const ConceptSpecializationExpr *E) {
   WL.push_back(ConceptSpecializationExprVisit(E, Parent));
 }
 void EnqueueVisitor::VisitRequiresExpr(const RequiresExpr *E) {
   WL.push_back(RequiresExprVisit(E, Parent));
   for (ParmVarDecl *VD : E->getLocalParameters())
     AddDecl(VD);
 }
+void EnqueueVisitor::VisitCXXParenListInitExpr(const CXXParenListInitExpr *E) {
+  EnqueueChildren(E);
+}
 void EnqueueVisitor::VisitPseudoObjectExpr(const PseudoObjectExpr *E) {
   // Treat the expression like its syntactic form.
   Visit(E->getSyntacticForm());
 }
 
 void EnqueueVisitor::VisitOMPExecutableDirective(
     const OMPExecutableDirective *D) {
   EnqueueChildren(D);
CXString clang_getCursorKindSpelling(enum CXCursorKind Kind) {
   case CXCursor_ObjCMessageExpr:
     return cxstring::createRef("ObjCMessageExpr");
   case CXCursor_BuiltinBitCastExpr:
     return cxstring::createRef("BuiltinBitCastExpr");
   case CXCursor_ConceptSpecializationExpr:
     return cxstring::createRef("ConceptSpecializationExpr");
   case CXCursor_RequiresExpr:
     return cxstring::createRef("RequiresExpr");
+  case CXCursor_CXXParenListInitExpr:
+    return cxstring::createRef("CXXParenListInitExpr");
   case CXCursor_UnexposedStmt:
     return cxstring::createRef("UnexposedStmt");
   case CXCursor_DeclStmt:
     return cxstring::createRef("DeclStmt");
   case CXCursor_LabelStmt:
     return cxstring::createRef("LabelStmt");
   case CXCursor_CompoundStmt:
     return cxstring::createRef("CompoundStmt");

clang/tools/libclang/CXCursor.cpp

CXCursor cxcursor::MakeCXCursor(const Stmt *S, const Decl *Parent,
   case Stmt::ConceptSpecializationExprClass:
     K = CXCursor_ConceptSpecializationExpr;
     break;
 
   case Stmt::RequiresExprClass:
     K = CXCursor_RequiresExpr;
     break;
 
+  case Stmt::CXXParenListInitExprClass:
+    K = CXCursor_CXXParenListInitExpr;
+    break;
+
   case Stmt::MSDependentExistsStmtClass:
     K = CXCursor_UnexposedStmt;
     break;
   case Stmt::OMPCanonicalLoopClass:
     K = CXCursor_OMPCanonicalLoop;
     break;
   case Stmt::OMPMetaDirectiveClass:
     K = CXCursor_OMPMetaDirective;

clang/www/cxx_status.html

<tr>
       <td rowspan="2" class="full" align="center">Yes</td>
     </tr>
       <tr>
         <td><a href="https://wg21.link/p1139r2">P1139R2</a></td>
       </tr>
     <tr>
       <td rowspan="2">Parenthesized initialization of aggregates</td>
       <td><a href="https://wg21.link/p0960r3">P0960R3</a></td>
-      <td rowspan="2" class="none" align="center">No</td>
+      <td rowspan="2" class="unreleased" align="center">Clang 16</td>
     </tr>
       <tr> <!-- from Belfast -->
         <td><a href="https://wg21.link/p1975r0">P1975R0</a></td>
       </tr>
     <tr>
       <td rowspan="11">Modules</td>
       <td><a href="https://wg21.link/p1103r3">P1103R3</a></td>
       <td class="full" align="center">Clang 15</td>