[Clang] Fix ResolveConstructorOverload to not select a conversion function if we are going use copy elision

[shafik]

Summary

ResolveConstructorOverload needs to check properly if we are going to use copy elision we can't use a conversion function.

This fixes:

https://github.com/llvm/llvm-project/issues/39319
https://github.com/llvm/llvm-project/issues/60182
https://github.com/llvm/llvm-project/issues/62157
https://github.com/llvm/llvm-project/issues/64885
https://github.com/llvm/llvm-project/issues/65568

[shafik]

clang/lib/Sema/SemaTemplateInstantiate.cpp

813

I added this fix because the B3 test case I added below although was fixed by my patch it uncovered a new bug that resulted in the compiler locking up.

It looks like the malformed code results in a us seeing a ClassTemplateDecl where we would usually not expect it. If this handled better earlier let me know.

[rsmith]

clang/lib/Sema/SemaTemplateInstantiate.cpp

816

This diagnostic won't include the template arguments that we're using to instantiate, which seems like important information.

Do you know where we're creating the InstantiatingTemplate instance corresponding to this diagnostic? Usually we pass in the declaration whose definition we're instantiating for a TemplateInstantiation record; I wonder if we could do the same for this case.

clang/test/SemaCXX/cxx1z-copy-omission.cpp

175

We should also test that the right function is actually being selected and called. For example:

struct A {
  A();
  A(const A&) = delete;
};
struct B {
  operator A();
} C;
A::A() : A(C) {}

... should be rejected because it calls the deleted copy constructor. (Or you could test this via constant evaluation or look at the generated code, but this is probably the simplest way.)

193

Do you need to omit the { here as part of this test? This will cause problems for people editing the file due to mismatched braces, and makes this test fragile if our error recovery changes. It's best for the test case to be free of errors other than the one(s) being exercised. (Same for missing ; errors below.)

If these errors are necessary to trigger the bug you found, I wonder if there's a problem with our error recovery. (Maybe we create a "bad" InstantiatingTemplate record during error recovery, for example.)

[shafik]

• Fix up A3 test to be less malformed
• Fix other test so we confirm we are attempting to call the right contructor

clang/lib/Sema/SemaTemplateInstantiate.cpp

816

This is happening in Sema::ActOnCallExpr here:

cpp
 // Diagnose uses of the C++20 "ADL-only template-id call" feature in earlier
  // language modes.
  if (auto *ULE = dyn_cast<UnresolvedLookupExpr>(Fn)) {
    if (ULE->hasExplicitTemplateArgs() &&
        ULE->decls_begin() == ULE->decls_end()) {
      Diag(Fn->getExprLoc(), getLangOpts().CPlusPlus20
                                 ? diag::warn_cxx17_compat_adl_only_template_id
                                 : diag::ext_adl_only_template_id)
          << ULE->getName();
    }
  }

if I check:

console
expr CodeSynthesisContexts->rbegin()->Entity

That is indeed the ClassTemplateDecl and we do have MultiExprArg ArgExprs there as well.

The call right before this TemplateInstantiator::RebuildCallExpr

clang/test/SemaCXX/cxx1z-copy-omission.cpp

193

They are not needed to trigger the issue, I removed them. I was torn about how faithful to remain to the original test case.

[shafik]

ping

[shafik]

ping

[shafik]

ping

[dim]

FWIW, this fix works for the test cases I had via https://bugs.freebsd.org/269067 and https://github.com/llvm/llvm-project/issues/60182, but I got the following failure during check-all:

********************
FAIL: Clang :: SemaCXX/cxx1z-copy-omission.cpp (15698 of 67299)
******************** TEST 'Clang :: SemaCXX/cxx1z-copy-omission.cpp' FAILED ********************
Script:
--
: 'RUN: at line 1';   /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang -cc1 -internal-isystem /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/lib/clang/17/include -nostdsysteminc -std=c++1z -verify -Wno-unused /home/dim/src/llvm/llvm-project/clang/test/SemaCXX/cxx1z-copy-omission.cpp
--
Exit Code: 134

Command Output (stderr):
--
unexpected deduction guide in instantiation stack
UNREACHABLE executed at /home/dim/src/llvm/llvm-project/clang/lib/Sema/SemaTemplateInstantiate.cpp:1067!
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: /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang -cc1 -internal-isystem /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/lib/clang/17/include -nostdsysteminc -std=c++1z -verify -Wno-unused /home/dim/src/llvm/llvm-project/clang/test/SemaCXX/cxx1z-copy-omission.cpp
1.      /home/dim/src/llvm/llvm-project/clang/test/SemaCXX/cxx1z-copy-omission.cpp:198:8: current parser token ';'
2.      /home/dim/src/llvm/llvm-project/clang/test/SemaCXX/cxx1z-copy-omission.cpp:176:1: parsing namespace 'GH39319'
 #0 0x0000000002ca4727 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x2ca4727)
 #1 0x0000000002ca2508 llvm::sys::RunSignalHandlers() (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x2ca2508)
 #2 0x0000000002ca4f00 SignalHandler(int) Signals.cpp:0:0
 #3 0x0000000827a15a3e handle_signal /usr/src/lib/libthr/thread/thr_sig.c:0:3
 #4 0x0000000827a14ff9 thr_sighandler /usr/src/lib/libthr/thread/thr_sig.c:247:1
 #5 0x0000000826e18903 ([vdso]+0x2d3)
 #6 0x000000082dc3f97a __sys_thr_kill /usr/obj/usr/src/lib/libc/thr_kill.S:4:0
 #7 0x000000082dbb8954 __raise /usr/src/lib/libc/gen/raise.c:0:10
 #8 0x000000082dc693e9 abort /usr/src/lib/libc/stdlib/abort.c:73:17
 #9 0x0000000002c23b01 (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x2c23b01)
#10 0x000000000590e986 clang::Sema::PrintInstantiationStack() (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x590e986)
#11 0x000000000504555d clang::Sema::EmitCurrentDiagnostic(unsigned int) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x504555d)
#12 0x00000000050464ea clang::Sema::ImmediateDiagBuilder::~ImmediateDiagBuilder() Sema.cpp:0:0
#13 0x0000000005046668 clang::Sema::SemaDiagnosticBuilder::~SemaDiagnosticBuilder() (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x5046668)
#14 0x00000000053f7c5b clang::Sema::ActOnCallExpr(clang::Scope*, clang::Expr*, clang::SourceLocation, llvm::MutableArrayRef<clang::Expr*>, clang::SourceLocation, clang::Expr*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x53f7c5b)
#15 0x0000000005920009 clang::TreeTransform<(anonymous namespace)::TemplateInstantiator>::TransformCallExpr(clang::CallExpr*) SemaTemplateInstantiate.cpp:0:0
#16 0x000000000591713d clang::Sema::SubstExpr(clang::Expr*, clang::MultiLevelTemplateArgumentList const&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x591713d)
#17 0x00000000059615e5 clang::TemplateDeclInstantiator::VisitNonTypeTemplateParmDecl(clang::NonTypeTemplateParmDecl*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x59615e5)
#18 0x000000000599c9a9 void llvm::function_ref<void ()>::callback_fn<clang::Sema::SubstDecl(clang::Decl*, clang::DeclContext*, clang::MultiLevelTemplateArgumentList const&)::$_0>(long) SemaTemplateInstantiateDecl.cpp:0:0
#19 0x000000000503ec8e clang::Sema::runWithSufficientStackSpace(clang::SourceLocation, llvm::function_ref<void ()>) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x503ec8e)
#20 0x00000000059667a2 clang::Sema::SubstDecl(clang::Decl*, clang::DeclContext*, clang::MultiLevelTemplateArgumentList const&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x59667a2)
#21 0x00000000057c6e6d clang::Sema::DeclareImplicitDeductionGuides(clang::TemplateDecl*, clang::SourceLocation) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x57c6e6d)
#22 0x000000000562bc8d LookupDirect(clang::Sema&, clang::LookupResult&, clang::DeclContext const*) SemaLookup.cpp:0:0
#23 0x0000000005627e5c CppNamespaceLookup(clang::Sema&, clang::LookupResult&, clang::ASTContext&, clang::DeclContext const*, (anonymous namespace)::UnqualUsingDirectiveSet&) SemaLookup.cpp:0:0
#24 0x00000000056274ad clang::Sema::CppLookupName(clang::LookupResult&, clang::Scope*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x56274ad)
#25 0x000000000562b4d0 clang::Sema::LookupName(clang::LookupResult&, clang::Scope*, bool, bool) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x562b4d0)
#26 0x00000000051f037e clang::Sema::HandleDeclarator(clang::Scope*, clang::Declarator&, llvm::MutableArrayRef<clang::TemplateParameterList*>) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x51f037e)
#27 0x00000000051efc4b clang::Sema::ActOnDeclarator(clang::Scope*, clang::Declarator&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x51efc4b)
#28 0x0000000004f81236 clang::Parser::ParseDeclarationAfterDeclaratorAndAttributes(clang::Declarator&, clang::Parser::ParsedTemplateInfo const&, clang::Parser::ForRangeInit*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f81236)
#29 0x0000000004f7f9f2 clang::Parser::ParseDeclGroup(clang::ParsingDeclSpec&, clang::DeclaratorContext, clang::ParsedAttributes&, clang::SourceLocation*, clang::Parser::ForRangeInit*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f7f9f2)
#30 0x0000000004f11179 clang::Parser::ParseDeclOrFunctionDefInternal(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec&, clang::AccessSpecifier) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f11179)
#31 0x0000000004f10a78 clang::Parser::ParseDeclarationOrFunctionDefinition(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*, clang::AccessSpecifier) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f10a78)
#32 0x0000000004f0f80a clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f0f80a)
#33 0x0000000004f58a37 clang::Parser::ParseInnerNamespace(llvm::SmallVector<clang::Parser::InnerNamespaceInfo, 4u> const&, unsigned int, clang::SourceLocation&, clang::ParsedAttributes&, clang::BalancedDelimiterTracker&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f58a37)
#34 0x0000000004f581f8 clang::Parser::ParseNamespace(clang::DeclaratorContext, clang::SourceLocation&, clang::SourceLocation) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f581f8)
#35 0x0000000004f7e414 clang::Parser::ParseDeclaration(clang::DeclaratorContext, clang::SourceLocation&, clang::ParsedAttributes&, clang::ParsedAttributes&, clang::SourceLocation*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f7e414)
#36 0x0000000004f0f370 clang::Parser::ParseExternalDeclaration(clang::ParsedAttributes&, clang::ParsedAttributes&, clang::ParsingDeclSpec*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f0f370)
#37 0x0000000004f0d3ff clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&, clang::Sema::ModuleImportState&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f0d3ff)
#38 0x0000000004f0794e clang::ParseAST(clang::Sema&, bool, bool) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x4f0794e)
#39 0x0000000003780c33 clang::FrontendAction::Execute() (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x3780c33)
#40 0x0000000003704f2f clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x3704f2f)
#41 0x0000000003853750 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x3853750)
#42 0x0000000001ad1f28 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x1ad1f28)
#43 0x0000000001acd854 ExecuteCC1Tool(llvm::SmallVectorImpl<char const*>&, llvm::ToolContext const&) driver.cpp:0:0
#44 0x0000000001acc6ea clang_main(int, char**, llvm::ToolContext const&) (/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang+0x1acc6ea)
/home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/tools/clang/test/SemaCXX/Output/cxx1z-copy-omission.cpp.script: line 1:   661 Abort trap              (core dumped) /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/bin/clang -cc1 -internal-isystem /home/dim/obj/llvmorg-17-init-17477-g3ab7ef28eebf-freebsd13-amd64-ninja-clang-rel-1/lib/clang/17/include -nostdsysteminc -std=c++1z -verify -Wno-unused /home/dim/src/llvm/llvm-project/clang/test/SemaCXX/cxx1z-copy-omission.cpp

--

********************

Any idea what that might be?

[dim]

In D148474#4490041, @dim wrote:

FWIW, this fix works for the test cases I had via https://bugs.freebsd.org/269067 and https://github.com/llvm/llvm-project/issues/60182, but I got the following failure

This was with llvmorg-17-init-17477-ab7ef28eebf (rG3ab7ef28eebf), by the way.

[shafik]

In D148474#4490041, @dim wrote:

FWIW, this fix works for the test cases I had via https://bugs.freebsd.org/269067 and https://github.com/llvm/llvm-project/issues/60182, but I got the following failure during check-all:

I seem to be seeing the same issue with my build as well. Let me debug it and see if I can figure out what changed since I first worked on this and adjust.

[shafik]

@dim the crash should be fixed by landing: https://github.com/llvm/llvm-project/pull/67373 I need to rebase and test

[shafik]

• Integrate fix from https://github.com/llvm/llvm-project/pull/67373

[shafik]

ping

[shafik]

ping, we have several bugs related to this. So it would be nice to land this fix soon if possible.

[shafik]

ping @dim @rsmith @aaron.ballman

[shafik]

ping @dim @rsmith @aaron.ballman

[dim]

Yes, please. :)

[aaron.ballman]

The changes look reasonable to me (@rsmith had a lot of comments, but I think you addressed them; it would be nice if he could confirm), but should definitely come with a release note. So LGTM modulo those nits and any last-minute comments from Richard.

clang/lib/Sema/SemaTemplateInstantiate.cpp

813

[shafik]

• Adding release note

clang/docs/ReleaseNotes.rst

- Fix a ``clang-17`` regression where a templated friend with constraints is not
   Fixes (`#71595 <https://github.com/llvm/llvm-project/issues/71595>`_)
 - Fix the name of the ifunc symbol emitted for multiversion functions declared with the
   ``target_clones`` attribute. This addresses a linker error that would otherwise occur
   when these functions are referenced from other TUs.
 - Fixes compile error that double colon operator cannot resolve macro with parentheses.
   Fixes (`#64467 <https://github.com/llvm/llvm-project/issues/64467>`_)
 - Clang's ``-Wchar-subscripts`` no longer warns on chars whose values are known non-negative constants.
   Fixes (`#18763 <https://github.com/llvm/llvm-project/issues/18763>`_)
+- Fix crash due to incorrectly allowing conversion functions in copy elision.
+  Fixes (`#39319 <https://github.com/llvm/llvm-project/issues/39319>`_) and
+  (`#60182 <https://github.com/llvm/llvm-project/issues/60182>`_) and
+  (`#62157 <https://github.com/llvm/llvm-project/issues/62157>`_) and
+  (`#64885 <https://github.com/llvm/llvm-project/issues/64885>`_) and
+  (`#65568 <https://github.com/llvm/llvm-project/issues/65568>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to C++ Support

clang/lib/Sema/SemaInit.cpp

static bool hasCopyOrMoveCtorParam(ASTContext &Ctx,
   QualType ParmT =
       Info.Constructor->getParamDecl(0)->getType().getNonReferenceType();
   QualType ClassT =
       Ctx.getRecordType(cast<CXXRecordDecl>(Info.FoundDecl->getDeclContext()));
 
   return Ctx.hasSameUnqualifiedType(ParmT, ClassT);
 }
 
-static OverloadingResult
-ResolveConstructorOverload(Sema &S, SourceLocation DeclLoc,
-                           MultiExprArg Args,
-                           OverloadCandidateSet &CandidateSet,
-                           QualType DestType,
-                           DeclContext::lookup_result Ctors,
-                           OverloadCandidateSet::iterator &Best,
-                           bool CopyInitializing, bool AllowExplicit,
-                           bool OnlyListConstructors, bool IsListInit,
-                           bool SecondStepOfCopyInit = false) {
+static OverloadingResult ResolveConstructorOverload(
+    Sema &S, SourceLocation DeclLoc, MultiExprArg Args,
+    OverloadCandidateSet &CandidateSet, QualType DestType,
+    DeclContext::lookup_result Ctors, OverloadCandidateSet::iterator &Best,
+    bool CopyInitializing, bool AllowExplicit, bool OnlyListConstructors,
+    bool IsListInit, bool RequireActualConstructor,
   CandidateSet.clear(OverloadCandidateSet::CSK_InitByConstructor);
   CandidateSet.setDestAS(DestType.getQualifiers().getAddressSpace());
 
   for (NamedDecl *D : Ctors) {
     auto Info = getConstructorInfo(D);
     if (!Info.Constructor || Info.Constructor->isInvalidDecl())
       continue;
 
static OverloadingResult ResolveConstructorOverload(
   // from a single expression of class type U, conversion functions of
   // U that convert to the non-reference type cv T are candidates.
   // Explicit conversion functions are only candidates during
   // direct-initialization.
   //
   // Note: SecondStepOfCopyInit is only ever true in this case when
   // evaluating whether to produce a C++98 compatibility warning.
   if (S.getLangOpts().CPlusPlus17 && Args.size() == 1 &&
-      !SecondStepOfCopyInit) {
+      !RequireActualConstructor && !SecondStepOfCopyInit) {
     Expr *Initializer = Args[0];
     auto *SourceRD = Initializer->getType()->getAsCXXRecordDecl();
     if (SourceRD && S.isCompleteType(DeclLoc, Initializer->getType())) {
       const auto &Conversions = SourceRD->getVisibleConversionFunctions();
       for (auto I = Conversions.begin(), E = Conversions.end(); I != E; ++I) {
         NamedDecl *D = *I;
         CXXRecordDecl *ActingDC = cast<CXXRecordDecl>(D->getDeclContext());
         D = D->getUnderlyingDecl();
MultiExprArg UnwrappedArgs =
       ILE ? MultiExprArg(ILE->getInits(), ILE->getNumInits()) : Args;
 
   // The type we're constructing needs to be complete.
   if (!S.isCompleteType(Kind.getLocation(), DestType)) {
     Sequence.setIncompleteTypeFailure(DestType);
     return;
   }
 
+  bool RequireActualConstructor =
+      !(Entity.getKind() != InitializedEntity::EK_Base &&
+        Entity.getKind() != InitializedEntity::EK_Delegating &&
+        Entity.getKind() !=
+            InitializedEntity::EK_LambdaToBlockConversionBlockElement);
+
   // C++17 [dcl.init]p17:
   //     - If the initializer expression is a prvalue and the cv-unqualified
   //       version of the source type is the same class as the class of the
   //       destination, the initializer expression is used to initialize the
   //       destination object.
   // Per DR (no number yet), this does not apply when initializing a base
   // class or delegating to another constructor from a mem-initializer.
   // ObjC++: Lambda captured by the block in the lambda to block conversion
   // should avoid copy elision.
-  if (S.getLangOpts().CPlusPlus17 &&
-      Entity.getKind() != InitializedEntity::EK_Base &&
-      Entity.getKind() != InitializedEntity::EK_Delegating &&
-      Entity.getKind() !=
-          InitializedEntity::EK_LambdaToBlockConversionBlockElement &&
+  if (S.getLangOpts().CPlusPlus17 && !RequireActualConstructor &&
       UnwrappedArgs.size() == 1 && UnwrappedArgs[0]->isPRValue() &&
       S.Context.hasSameUnqualifiedType(UnwrappedArgs[0]->getType(), DestType)) {
     // Convert qualifications if necessary.
     Sequence.AddQualificationConversionStep(DestType, VK_PRValue);
     if (ILE)
       Sequence.RewrapReferenceInitList(DestType, ILE);
     return;
   }
static void TryConstructorInitialization(Sema &S,
   //     constructors of the class T and the argument list consists of the
   //     initializer list as a single argument.
   if (IsListInit) {
     AsInitializerList = true;
 
     // If the initializer list has no elements and T has a default constructor,
     // the first phase is omitted.
     if (!(UnwrappedArgs.empty() && S.LookupDefaultConstructor(DestRecordDecl)))
-      Result = ResolveConstructorOverload(S, Kind.getLocation(), Args,
-                                          CandidateSet, DestType, Ctors, Best,
-                                          CopyInitialization, AllowExplicit,
-                                          /*OnlyListConstructors=*/true,
-                                          IsListInit);
+      Result = ResolveConstructorOverload(
+          S, Kind.getLocation(), Args, CandidateSet, DestType, Ctors, Best,
+          /*OnlyListConstructors=*/true, IsListInit, RequireActualConstructor);
   }
 
   // C++11 [over.match.list]p1:
   //   - If no viable initializer-list constructor is found, overload resolution
   //     is performed again, where the candidate functions are all the
   //     constructors of the class T and the argument list consists of the
   //     elements of the initializer list.
   if (Result == OR_No_Viable_Function) {
     AsInitializerList = false;
-    Result = ResolveConstructorOverload(S, Kind.getLocation(), UnwrappedArgs,
-                                        CandidateSet, DestType, Ctors, Best,
-                                        CopyInitialization, AllowExplicit,
-                                        /*OnlyListConstructors=*/false,
-                                        IsListInit);
+    Result = ResolveConstructorOverload(
+        S, Kind.getLocation(), UnwrappedArgs, CandidateSet, DestType, Ctors,
+        Best, CopyInitialization, AllowExplicit,
+        /*OnlyListConstructors=*/false, IsListInit, RequireActualConstructor);
   }
   if (Result) {
     Sequence.SetOverloadFailure(
         IsListInit ? InitializationSequence::FK_ListConstructorOverloadFailed
                    : InitializationSequence::FK_ConstructorOverloadFailed,
         Result);
 
     if (Result != OR_Deleted)
static ExprResult CopyObject(Sema &S,
   OverloadCandidateSet CandidateSet(Loc, OverloadCandidateSet::CSK_Normal);
   DeclContext::lookup_result Ctors = S.LookupConstructors(Class);
 
   OverloadCandidateSet::iterator Best;
   switch (ResolveConstructorOverload(
       S, Loc, CurInitExpr, CandidateSet, T, Ctors, Best,
       /*CopyInitializing=*/false, /*AllowExplicit=*/true,
       /*OnlyListConstructors=*/false, /*IsListInit=*/false,
+      /*RequireActualConstructor=*/false,
       /*SecondStepOfCopyInit=*/true)) {
   case OR_Success:
     break;
 
   case OR_No_Viable_Function:
     CandidateSet.NoteCandidates(
         PartialDiagnosticAt(
             Loc, S.PDiag(IsExtraneousCopy && !S.isSFINAEContext()
DeclContext::lookup_result Ctors =
       S.LookupConstructors(cast<CXXRecordDecl>(Record->getDecl()));
 
   // Perform overload resolution.
   OverloadCandidateSet::iterator Best;
   OverloadingResult OR = ResolveConstructorOverload(
       S, Loc, CurInitExpr, CandidateSet, CurInitExpr->getType(), Ctors, Best,
       /*CopyInitializing=*/false, /*AllowExplicit=*/true,
       /*OnlyListConstructors=*/false, /*IsListInit=*/false,
+      /*RequireActualConstructor=*/false,
       /*SecondStepOfCopyInit=*/true);
 
   PartialDiagnostic Diag = S.PDiag(diag::warn_cxx98_compat_temp_copy)
     << OR << (int)Entity.getKind() << CurInitExpr->getType()
     << CurInitExpr->getSourceRange();
 
   switch (OR) {
   case OR_Success:

clang/lib/Sema/SemaTemplateInstantiate.cpp

case CodeSynthesisContext::TemplateInstantiation: {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_enum_def_here)
           << ED
           << Active->InstantiationRange;
       } else if (FieldDecl *FD = dyn_cast<FieldDecl>(D)) {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_nsdmi_here)
             << FD << Active->InstantiationRange;
+      } else if (ClassTemplateDecl *CTD = dyn_cast<ClassTemplateDecl>(D)) {
+        Diags.Report(Active->PointOfInstantiation,
+                     diag::note_template_class_instantiation_here)
+            << CTD << Active->InstantiationRange;
       } else {
         Diags.Report(Active->PointOfInstantiation,
                      diag::note_template_type_alias_instantiation_here)
           << cast<TypeAliasTemplateDecl>(D)
           << Active->InstantiationRange;
       }
       break;
     }

clang/test/SemaCXX/cxx1z-copy-omission.cpp

 namespace CtorTemplateBeatsNonTemplateConversionFn {
   struct Foo { template <typename Derived> Foo(const Derived &); };
   template <typename Derived> struct Base { operator Foo() const = delete; }; // expected-note {{deleted}}
   struct Derived : Base<Derived> {};
 
   Foo f(Derived d) { return d; } // expected-error {{invokes a deleted function}}
   Foo g(Derived d) { return Foo(d); } // ok, calls constructor
 }
+
+// Make sure we don't consider conversion functions for guaranteed copy elision
+namespace GH39319 {
+struct A {
+  A();
+  A(const A&) = delete; // expected-note {{'A' has been explicitly marked deleted here}}
+};
+struct B {
+  operator A();
+} C;
+A::A() : A(C) {} // expected-error {{call to deleted constructor of}}
+
+struct A2 {
+  struct B2 {
+    operator A2();
+  };
+  A2() : A2(B2()) {}  // expected-error {{call to deleted constructor of}}
+  A2(const A2&) = delete; // expected-note {{'A2' has been explicitly marked deleted here}}
+};
+
+template<typename A3>
+class B3 : A3 {
+  template<bool = C3<B3>()> // expected-warning 2{{use of function template name with no prior declaration in function call with explicit}}
+  B3();
+}; B3(); // expected-error {{deduction guide declaration without trailing return type}} \
+         // expected-note {{while building implicit deduction guide first needed here}}
+}