[Clang] Implements CTAD for aggregates P1816R0 and P2082R1

[ychen]

format

[cor3ntin]

Hey. Thanks a lot for working on this.
I did a first pass, looking at mostly style issues, looking at conformance will probably take me a lot more time, but i think this looks pretty good overall

clang/include/clang/AST/DeclCXX.h

1951

I'm wondering if the constructor should take a DeductionCandidateKind defaulted to normal here. All the places where it's set seem to be immediately after construction.

1989–1993

I'm not sure how useful these things are, isAggregateDeductionCandidate is only used once

clang/lib/Sema/SemaInit.cpp

35

This does not seems used

310

I think using optional here would make more sense, I guess that's why you included it.
Should it be SmallVectorImpl, such that only the caller has to bake in a size?

1101–1102

Can you had a comment about that case? I'm not sure i understand what scenario we are handling

2185–2187

Maybe using enumerate + a range for loop here would be cleaner?

clang/lib/Sema/SemaOverload.cpp

7265–7266

[ychen]

• Address Corentin's comments

[ychen]

In D139837#3989814, @cor3ntin wrote:

Hey. Thanks a lot for working on this.
I did a first pass, looking at mostly style issues, looking at conformance will probably take me a lot more time, but i think this looks pretty good overall

Thanks for the quick fist-pass review. Appreciate it.

clang/include/clang/AST/DeclCXX.h

1951

That's true indeed. The awkward aspect is that the CXXDeductionGuideDecl::Create call is far from setDeductionCandidateKind; making CXXDeductionGuideDecl::Create takes a DeductionCandidateKind would several other less related functions takes DeductionCandidateKind also.

1989–1993

I meant to make it consistent with isCopyDeductionCandidate which is also used once. Maybe remove both isCopyDeductionCandidate and isAggregateDeductionCandidate?

clang/lib/Sema/SemaInit.cpp

310

I meant to make AggrDeductionCandidateParamTypes a byref semantic like FullyStructuredList. So the InitListChecker populates them as a side-effect. Using optional means the InitListChecker owes the deduced types which should work too but seems weird to me.

1101–1102

Good point. On second thought, I think this is not needed. This function shouldn't accept dependent types.

2185–2187

Using the iterator to test for the last element is easier. The index produced by enumerate would need to produce an iterator nevertheless.

[ychen]

• update cxx_status.html

[cor3ntin]

Sorry for not getting back to you earlier.
It mostly looks reasonable to me.
what do you think @erichkeane ?

clang/lib/Sema/SemaInit.cpp

10805

Maybe add a message here

10815

I think we need to support A{("Hello")}; so we probably need to get rid of parentheses (maybe other implicit nodes?)

clang/lib/Sema/SemaTemplate.cpp

2596–2597

Maybe we should have an asserton !isInvalid then?

clang/lib/Sema/SemaTemplateDeduction.cpp

363

I think you can do that in the for loop as it was before

995

That could always be initialized, to avoid future surprises

[cor3ntin]

clang/include/clang/AST/DeclCXX.h

1989–1993

Yes, i think we might as well

[cor3ntin]

+ @hubert.reinterpretcast In case i missed something conformance wise.

Speaking of conformance, your implementation does not appear to gate the feature on C++20, which it should, so you should add a test that it doesn't compile in c++17 (I'm not sure we could support it as an extension... maybe?)

[cor3ntin]

clang/lib/Sema/SemaInit.cpp

13

Do we actually need that?

[shafik]

clang/lib/Sema/SemaInit.cpp

10674–10678

Minor issues, we need to match the format expected by bugprone-comment

clang/lib/Sema/SemaOverload.cpp

7386–7387

formatting fix again.

clang/lib/Sema/SemaTemplate.cpp

2593

formatting.

[ayzhao]

Thanks for your effort on this!

FYI I landed parenthesized aggregate initialization in D141546, so CTAD for that feature should no longer be blocked.

[ychen]

In D139837#4067208, @ayzhao wrote:

Thanks for your effort on this!

FYI I landed parenthesized aggregate initialization in D141546, so CTAD for that feature should no longer be blocked.

Thanks for letting me know. Considering the existing reviews, I'll create a separate patch for the parenthesized expression-list support.

[cor3ntin]

@ychen Hey! Are you still working on this? Thanks!

[cor3ntin]

@erichkeane @aaron.ballman I think we should determine how much works remains there and if the author is not responsive maybe one of us can push this up the finishing line.

[aaron.ballman]

In D139837#4326501, @cor3ntin wrote:

@erichkeane @aaron.ballman I think we should determine how much works remains there and if the author is not responsive maybe one of us can push this up the finishing line.

That seems like a reasonable approach to me; it'd be good to get this into Clang 17 if possible, and @ychen seems to have been away for ~2 months now, so it seems reasonable to commandeer this review if we need to.

[ychen]

Sorry for the long delay guys. I'll update the patch this weekend.

[ychen]

clang/lib/Sema/SemaInit.cpp

10815

Agreed. Fixed.

clang/lib/Sema/SemaTemplate.cpp

2596–2597

Looked at this a little bit more. It can fail (D46446). So I updated the comments in a few places.

clang/lib/Sema/SemaTemplateDeduction.cpp

363

Done

[ychen]

Address all comments

[ychen]

rebase

[ychen]

• add release notes

[cor3ntin]

I think this is starting to look good, My biggest question is the remaining fixme, how much work would it be to do in this PR?

clang/include/clang/Sema/TemplateDeduction.h

237

Can you specify which c++ version this quote is from?

clang/lib/Sema/SemaTemplate.cpp

2597

Maybe you could do this in this PR?

clang/www/cxx_status.html

1249 ↗

(On Diff #521965)

I think this should be marked a partially supported until we support parenthesized init.

[ychen]

• Fix bitfield on Windows

[ychen]

@cor3ntin Thanks for the review. I've added the parenthesized expression-list support by treating it as braced-init-list during CTAD.

• adjust test check for Windows
• support parenthesized expression-list
• add an InitializationKind BuildingDeductionGuides to help diagnoses

[cor3ntin]

Thanks, It's great to have the complete feature in on PR

clang/include/clang/Sema/Sema.h

9684

Is that comment correct?

clang/lib/Sema/SemaDecl.cpp

12703–12705

clang/lib/Sema/SemaTemplateInstantiate.cpp

1062–1065

This seems unfinished

[aaron.ballman]

Thank you for working on this! Some high-level comments while I was here; I'm still grokking the implementation.

clang/include/clang/AST/DeclBase.h

1691

Best not to give this the same name as a type (I don't care which one changes names).

clang/include/clang/AST/DeclCXX.h

1986

Er, seems a bit odd to cast an 8-bit type to a 64-bit type only to shove it into a 2-bit bit-field. I think DeductionCandidateKind should be an enum class whose underlying type is int and we cast to/from int as needed.

clang/include/clang/Sema/Sema.h

4000

We're up to 12 parameters for this function, five of which are bool parameters... at some point, this probably needs to be refactored.

9369

[aaron.ballman]

clang/include/clang/Sema/Sema.h

9684

Yeah, the comment seems off to me.

clang/lib/Sema/SemaInit.cpp

505–513

We shouldn't force the caller to use the same-sized SmallVector, right?

1034

Can we make this return a const RecordDecl * or does that run into viral const issues?

1035–1036

1037–1038

1443–1445

Be sure to add test coverage for use of VLAs in C++ (we support it as an extension).

10710–10711

10755

clang/lib/Sema/SemaTemplate.cpp

2576

Something is amiss here. Either this should be using dyn_cast or it should not be in an if statement (cast cannot fail; it asserts if it does).

clang/lib/Sema/SemaTemplateInstantiate.cpp

1062–1065

+1

[ychen]

• address existing comments

clang/include/clang/AST/DeclBase.h

1691

I've changed the type name and kept this as is.

clang/include/clang/Sema/Sema.h

4000

Agreed. I will keep my eye on it.

clang/lib/Sema/SemaInit.cpp

505–513

That's right.

1034

"viral const issues". Deep somewhere else needs it non-const.

1443–1445

Array::a2 test case covers this.

clang/lib/Sema/SemaTemplate.cpp

2576

It's the getDefinition() that may be null. I've hoist the cast out to make it obvious.

clang/lib/Sema/SemaTemplateInstantiate.cpp

1062–1065

I meant to keep this a future work since this path is dead until some errors could be thrown out of this context. In the future, if errors happen during building deduction guides, this assertion failure could trigger at build time.

[cor3ntin]

clang/lib/Sema/SemaTemplateInstantiate.cpp

1062–1065

In this case, maybe change the message
assert(false && "unexpected deduction guide in instantiation stack") or something along those lines.

[ychen]

• address comment

[ychen]

clang/lib/Sema/SemaTemplateInstantiate.cpp

1062–1065

Done. I've made it llvm_unreachable to catch the rare chances that users hit this.

[cor3ntin]

I think this looks good but I'll let @aaron.ballman do the final approval.

[ychen]

In D139837#4401134, @cor3ntin wrote:

I think this looks good but I'll let @aaron.ballman do the final approval.

Thanks for the review @cor3ntin.

[shafik]

clang/test/SemaTemplate/aggregate-deduction-candidate.cpp

102

I would also like to see this test:

template <typename T>
struct I {
  using type = T;
};

template <typename T>
struct E {
  typename I<T>::type i;
  T t;
};

E e1 = {1, 2}; // OK, E<int> deduced

Since it is in the paper, this should cover it but it can't hurt.

[ychen]

• add one extra test

[ychen]

clang/test/SemaTemplate/aggregate-deduction-candidate.cpp

102

Yep, just added this test.

[shafik]

clang/include/clang/AST/DeclBase.h

1688–1691

Why remove [C++17] ?

clang/include/clang/AST/DeclCXX.h

1986

It feels a bit weird that we are taking an enum w/ an underlying type of unsigned char casting it to int and then placing it in an unsigned bit-field. I don't have a better suggestion ATM but I wish we had something better.

clang/lib/Sema/SemaDecl.cpp

12713

nitpick but we seem to use Expr* everywhere else.

clang/lib/Sema/SemaInit.cpp

15

I saw your adding headers. How did you figure out which ones were missing?

508

nit

10678

nit,

note: PO` is not a very descriptive name.

10734

10741

10763–10781

Quoted Text

[cor3ntin]

clang/include/clang/AST/DeclBase.h

1688–1691

I don't think this adds any information, I'm happy to see it removed. And arguably in c++17 there was only one kind of DG.

[ychen]

• address comments

clang/include/clang/AST/DeclCXX.h

1986

I've changed it to casting to underlying type of DeductionCandidate so it has one less step of conversion.

clang/lib/Sema/SemaInit.cpp

15

Added headers provide APIs for the new code. I guess many of them are included already indirectly (SmallVector for example). But I thought the best practice is not relying on that, so I added these.

[cor3ntin]

clang/lib/Sema/SemaInit.cpp

15

I think it's easier fore reviewers not to add things are are not needed, I would suggest trying to keep the change to the list of includes to the minimum

[ychen]

• remove extra headers

[cor3ntin]

@ychen You probably want to update the description, i think it's updated (you implemented ctad for parenthesized ctrs)

[ychen]

In D139837#4449327, @cor3ntin wrote:

@ychen You probably want to update the description, i think it's updated (you implemented ctad for parenthesized ctrs)

Thanks for the remainder, Corentin. Hopefully, this will be ready to be landed soon.

[cor3ntin]

In D139837#4449536, @ychen wrote:

In D139837#4449327, @cor3ntin wrote:

@ychen You probably want to update the description, i think it's updated (you implemented ctad for parenthesized ctrs)

Thanks for the remainder, Corentin. Hopefully, this will be ready to be landed soon.

I hope so too. I pinged @aaron.ballman :)

[erichkeane]

Thanks for this! I have no concerns from what I can see, but I see the others are doing a very thorough review, so I'll let them do the approvals.

clang/include/clang/AST/DeclCXX.h

1951

I'm in favor of having this be a part of CXXDeductionGuideDecl::Create and the ctor as a parameter instead as well.

[ychen]

• add a DeductionCandidate parameter

[cor3ntin]

clang/include/clang/AST/DeclBase.h

1689

clang/include/clang/AST/DeclCXX.h

1989–1993

I still think we should remove that

clang/include/clang/Sema/Sema.h

9683

Maybe rename that BuildingDeductionGuidesTag

[ychen]

• address comments

[cor3ntin]

Please give @aaron.ballman a few days to get another look, but otherwise LGTM
Thanks a lot for working on this and your patience during the review.

[aaron.ballman]

LGTM aside from some minor issues, thank you!

clang/docs/ReleaseNotes.rst

113

Paren aggregate init is now supported, so this should be updated.

clang/lib/Sema/SemaInit.cpp

3271–3272

For local style consistency.

10620–10688

clang/lib/Sema/SemaTemplate.cpp

2577–2580

I think this is a bit more clear as to what's happening.

[ychen]

• address comments

clang/docs/ReleaseNotes.rst

113

Thanks for catching this.

[ychen]

Thanks for the review!

[cor3ntin]

Thanks a lot for working on this!
Do you need Aaron or myself to commit this for you? If so, which name / mail do you want us to use?

[ychen]

In D139837#4460664, @cor3ntin wrote:

Thanks a lot for working on this!
Do you need Aaron or myself to commit this for you? If so, which name / mail do you want us to use?

That's okay. I'm doing the rebase and I could commit it later.

[ychen]

• rebase

[cor3ntin]

The changes to clang/www/cxx_status.html got lost during the rebase, can you fix that? Thanks!

[shafik]

So while working on D148474 I realized this PR introduced a new crash bug, see the following code: https://godbolt.org/z/h1EezGjbr

template<typename A3>
class B3 : A3 {
  template<bool = C3<B3>()>
  B3();
}; B3();

which is one of my test cases. I had though it was an interaction between my new code and this but then realized this exists w/o my new code. We can also see from the godbolt above that the crash in this code changed from an assertion to an unreachable. My PR will fix the assertion but I need a good solution to the unreachable bug. I tried changing the unreachable to a break but it reduces the quality of the diagnostic and I don't think it is the correct fix.

[shafik]

Note we have a second crash linked to this PR: https://github.com/llvm/llvm-project/issues/67173#issuecomment-1733647699

clang/docs/ReleaseNotes.rst

- Downgraded use of a reserved identifier in a module export declaration from
   <https://github.com/llvm/llvm-project/issues/61446>`_ and allows easier
   building of standard modules. This diagnostic may be strengthened into an
   error again in the future once there is a less fragile way to mark a module
   as being part of the implementation rather than a user module.
 - Clang now implements `[temp.deduct]p9`. Substitution failures inside lambdas from
   unevaluated contexts will be surfaced as errors. They were previously handled as
   SFINAE.
 - Clang now supports `requires cplusplus20` for module maps.
 - Implemented missing parts of `P2002R1: Consistent comparison operators <https://wg21.link/P2002R1>`_
 - Clang now defines `__cpp_consteval` macro.
+- Implemented `P1816R0: <https://wg21.link/p1816r0>`_ and `P2082R1: <https://wg21.link/p2082r1>`_,
+  which allows CTAD for aggregates.
 
 C++23 Feature Support
 ^^^^^^^^^^^^^^^^^^^^^
 
 - Implemented `P2036R3: Change scope of lambda trailing-return-type <https://wg21.link/P2036R3>`_
   and `P2579R0 Mitigation strategies for P2036 <https://wg21.link/P2579R0>`_.
   These proposals modify how variables captured in lambdas can appear in trailing return type
   expressions and how their types are deduced therein, in all C++ language versions.

clang/include/clang/AST/DeclBase.h

template<class T> T *find_first() const {
     for (auto *D : *this)
       if (T *Decl = dyn_cast<T>(D))
         return Decl;
 
     return nullptr;
   }
 };
 
+/// Only used by CXXDeductionGuideDecl.
+enum class DeductionCandidate : unsigned char {
+  Normal,
+  Copy,
+  Aggregate,
+};
+
 /// DeclContext - This is used only as base class of specific decl types that
 /// can act as declaration contexts. These decls are (only the top classes
 /// that directly derive from DeclContext are mentioned, not their subclasses):
 ///
 ///   TranslationUnitDecl
 ///   ExternCContext
 ///   NamespaceDecl
 ///   TagDecl
class DeclContext {
   };
 
   /// Number of non-inherited bits in OMPDeclareReductionDeclBitfields.
   enum { NumOMPDeclareReductionDeclBits = 2 };
 
   /// Stores the bits used by FunctionDecl.
   /// If modified NumFunctionDeclBits and the accessor
   /// methods in FunctionDecl and CXXDeductionGuideDecl
-  /// (for IsCopyDeductionCandidate) should be updated appropriately.
+  /// (for DeductionCandidateKind) should be updated appropriately.
   class FunctionDeclBitfields {
     friend class FunctionDecl;
-    /// For IsCopyDeductionCandidate
+    /// For DeductionCandidateKind
     friend class CXXDeductionGuideDecl;
     /// For the bits in DeclContextBitfields.
     uint64_t : NumDeclContextBits;
 
     uint64_t SClass : 3;
     uint64_t IsInline : 1;
     uint64_t IsInlineSpecified : 1;
 
class FunctionDeclBitfields {
     /// Indicates if the function declaration will
     /// have a body, once we're done parsing it.
     uint64_t WillHaveBody : 1;
 
     /// Indicates that this function is a multiversioned
     /// function using attribute 'target'.
     uint64_t IsMultiVersion : 1;
 
-    /// [C++17] Only used by CXXDeductionGuideDecl. Indicates that
-    /// the Deduction Guide is the implicitly generated 'copy
-    /// deduction candidate' (is used during overload resolution).
-    uint64_t IsCopyDeductionCandidate : 1;
+    /// Only used by CXXDeductionGuideDecl. Indicates the kind
+    /// of the Deduction Guide that is implicitly generated
+    /// (used during overload resolution).
+    uint64_t DeductionCandidateKind : 2;
 
     /// Store the ODRHash after first calculation.
     uint64_t HasODRHash : 1;
 
     /// Indicates if the function uses Floating Point Constrained Intrinsics
     uint64_t UsesFPIntrin : 1;
 
     // Indicates this function is a constrained friend, where the constraint

clang/include/clang/AST/DeclCXX.h

 class CXXDeductionGuideDecl : public FunctionDecl {
   void anchor() override;
 
 private:
   CXXDeductionGuideDecl(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
                         ExplicitSpecifier ES,
                         const DeclarationNameInfo &NameInfo, QualType T,
                         TypeSourceInfo *TInfo, SourceLocation EndLocation,
-                        CXXConstructorDecl *Ctor)
+                        CXXConstructorDecl *Ctor, DeductionCandidate Kind)
       : FunctionDecl(CXXDeductionGuide, C, DC, StartLoc, NameInfo, T, TInfo,
                      SC_None, false, false, ConstexprSpecKind::Unspecified),
         Ctor(Ctor), ExplicitSpec(ES) {
     if (EndLocation.isValid())
       setRangeEnd(EndLocation);
-    setIsCopyDeductionCandidate(false);
+    setDeductionCandidateKind(Kind);
   }
 
   CXXConstructorDecl *Ctor;
   ExplicitSpecifier ExplicitSpec;
   void setExplicitSpecifier(ExplicitSpecifier ES) { ExplicitSpec = ES; }
 
 public:
   friend class ASTDeclReader;
   friend class ASTDeclWriter;
 
   static CXXDeductionGuideDecl *
   Create(ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
          ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
          TypeSourceInfo *TInfo, SourceLocation EndLocation,
-         CXXConstructorDecl *Ctor = nullptr);
+         CXXConstructorDecl *Ctor = nullptr,
+         DeductionCandidate Kind = DeductionCandidate::Normal);
 
   static CXXDeductionGuideDecl *CreateDeserialized(ASTContext &C, unsigned ID);
 
   ExplicitSpecifier getExplicitSpecifier() { return ExplicitSpec; }
   const ExplicitSpecifier getExplicitSpecifier() const { return ExplicitSpec; }
 
   /// Return true if the declaration is already resolved to be explicit.
   bool isExplicit() const { return ExplicitSpec.isExplicit(); }
 
   /// Get the template for which this guide performs deduction.
   TemplateDecl *getDeducedTemplate() const {
     return getDeclName().getCXXDeductionGuideTemplate();
   }
 
   /// Get the constructor from which this deduction guide was generated, if
   /// this is an implicit deduction guide.
-  CXXConstructorDecl *getCorrespondingConstructor() const {
-    return Ctor;
-  }
+  CXXConstructorDecl *getCorrespondingConstructor() const { return Ctor; }
 
-  void setIsCopyDeductionCandidate(bool isCDC = true) {
-    FunctionDeclBits.IsCopyDeductionCandidate = isCDC;
+  void setDeductionCandidateKind(DeductionCandidate K) {
+    FunctionDeclBits.DeductionCandidateKind = static_cast<unsigned char>(K);
   }
 
-  bool isCopyDeductionCandidate() const {
-    return FunctionDeclBits.IsCopyDeductionCandidate;
+  DeductionCandidate getDeductionCandidateKind() const {
+    return static_cast<DeductionCandidate>(
+        FunctionDeclBits.DeductionCandidateKind);
   }
 
   // Implement isa/cast/dyncast/etc.
   static bool classof(const Decl *D) { return classofKind(D->getKind()); }
   static bool classofKind(Kind K) { return K == CXXDeductionGuide; }
 };
 
 /// \brief Represents the body of a requires-expression.
 ///

clang/include/clang/Sema/Sema.h

public:
 
   void AddOverloadCandidate(
       FunctionDecl *Function, DeclAccessPair FoundDecl, ArrayRef<Expr *> Args,
       OverloadCandidateSet &CandidateSet, bool SuppressUserConversions = false,
       bool PartialOverloading = false, bool AllowExplicit = true,
       bool AllowExplicitConversion = false,
       ADLCallKind IsADLCandidate = ADLCallKind::NotADL,
       ConversionSequenceList EarlyConversions = std::nullopt,
-      OverloadCandidateParamOrder PO = {});
+      OverloadCandidateParamOrder PO = {},
+      bool AggregateCandidateDeduction = false);
   void AddFunctionCandidates(const UnresolvedSetImpl &Functions,
                       ArrayRef<Expr *> Args,
                       OverloadCandidateSet &CandidateSet,
                       TemplateArgumentListInfo *ExplicitTemplateArgs = nullptr,
                       bool SuppressUserConversions = false,
                       bool PartialOverloading = false,
                       bool FirstArgumentIsBase = false);
   void AddMethodCandidate(DeclAccessPair FoundDecl,
void AddMethodTemplateCandidate(FunctionTemplateDecl *MethodTmpl,
                                   bool PartialOverloading = false,
                                   OverloadCandidateParamOrder PO = {});
   void AddTemplateOverloadCandidate(
       FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
       OverloadCandidateSet &CandidateSet, bool SuppressUserConversions = false,
       bool PartialOverloading = false, bool AllowExplicit = true,
       ADLCallKind IsADLCandidate = ADLCallKind::NotADL,
-      OverloadCandidateParamOrder PO = {});
+      OverloadCandidateParamOrder PO = {},
+      bool AggregateCandidateDeduction = false);
   bool CheckNonDependentConversions(
       FunctionTemplateDecl *FunctionTemplate, ArrayRef<QualType> ParamTypes,
       ArrayRef<Expr *> Args, OverloadCandidateSet &CandidateSet,
       ConversionSequenceList &Conversions, bool SuppressUserConversions,
       CXXRecordDecl *ActingContext = nullptr, QualType ObjectType = QualType(),
       Expr::Classification ObjectClassification = {},
       OverloadCandidateParamOrder PO = {});
   void AddConversionCandidate(
TemplateDeductionResult FinishTemplateArgumentDeduction(
       SmallVectorImpl<OriginalCallArg> const *OriginalCallArgs = nullptr,
       bool PartialOverloading = false,
       llvm::function_ref<bool()> CheckNonDependent = []{ return false; });
 
   TemplateDeductionResult DeduceTemplateArguments(
       FunctionTemplateDecl *FunctionTemplate,
       TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
       FunctionDecl *&Specialization, sema::TemplateDeductionInfo &Info,
-      bool PartialOverloading,
+      bool PartialOverloading, bool AggregateDeductionCandidate,
       llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent);
 
   TemplateDeductionResult
   DeduceTemplateArguments(FunctionTemplateDecl *FunctionTemplate,
                           TemplateArgumentListInfo *ExplicitTemplateArgs,
                           QualType ArgFunctionType,
                           FunctionDecl *&Specialization,
                           sema::TemplateDeductionInfo &Info,

 
   bool CheckIfFunctionSpecializationIsImmediate(FunctionDecl *FD,
                                                 SourceLocation Loc);
 
   /// Declare implicit deduction guides for a class template if we've
   /// not already done so.
   void DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                       SourceLocation Loc);
+  FunctionTemplateDecl *DeclareImplicitDeductionGuideFromInitList(
+      TemplateDecl *Template, MutableArrayRef<QualType> ParamTypes,
+      SourceLocation Loc);
+  llvm::DenseMap<unsigned, CXXDeductionGuideDecl *>
+      AggregateDeductionCandidates;
 
   QualType DeduceTemplateSpecializationFromInitializer(
       TypeSourceInfo *TInfo, const InitializedEntity &Entity,
-      const InitializationKind &Kind, MultiExprArg Init);
+      const InitializationKind &Kind, MultiExprArg Init,
+      ParenListExpr *PL = nullptr);
 
   QualType deduceVarTypeFromInitializer(VarDecl *VDecl, DeclarationName Name,
                                         QualType Type, TypeSourceInfo *TSI,
                                         SourceRange Range, bool DirectInit,
                                         Expr *Init);
 
   TypeLoc getReturnTypeLoc(FunctionDecl *FD) const;
 
enum SynthesisKind {
       /// We are building an implied call from __builtin_dump_struct. The
       /// arguments are in CallArgs.
       BuildingBuiltinDumpStructCall,
 
       /// Added for Template instantiation observation.
       /// Memoization means we are _not_ instantiating a template because
       /// it is already instantiated (but we entered a context where we
       /// would have had to if it was not already instantiated).
-      Memoization
+      Memoization,
+
+      /// We are building deduction guides for a class.
+      BuildingDeductionGuides,
     } Kind;
 
     /// Was the enclosing context a non-instantiation SFINAE context?
     bool SavedInNonInstantiationSFINAEContext;
 
     /// The point of instantiation or synthesis within the source code.
     SourceLocation PointOfInstantiation;
 
InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
                           concepts::NestedRequirement *Req, ConstraintsCheck,
                           SourceRange InstantiationRange = SourceRange());
 
     /// \brief Note that we are checking a requires clause.
     InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
                           const RequiresExpr *E,
                           sema::TemplateDeductionInfo &DeductionInfo,
                           SourceRange InstantiationRange);
+
+    struct BuildingDeductionGuidesTag {};
+    /// \brief Note that we are building deduction guides.
+    InstantiatingTemplate(Sema &SemaRef, SourceLocation PointOfInstantiation,
+                          TemplateDecl *Entity, BuildingDeductionGuidesTag,
+                          SourceRange InstantiationRange = SourceRange());
+
     /// Note that we have finished instantiating this template.
     void Clear();
 
     ~InstantiatingTemplate() { Clear(); }
 
     /// Determines whether we have exceeded the maximum
     /// recursive template instantiations.
     bool isInvalid() const { return Invalid; }

clang/include/clang/Sema/TemplateDeduction.h

public:
 
   /// The index of the function argument that caused a deduction
   /// failure.
   ///
   ///   TDK_DeducedMismatch: this is the index of the argument that had a
   ///   different argument type from its substituted parameter type.
   unsigned CallArgIndex = 0;
 
+  // C++20 [over.match.class.deduct]p5.2:
+  //   During template argument deduction for the aggregate deduction
+  //   candidate, the number of elements in a trailing parameter pack is only
+  //   deduced from the number of remaining function arguments if it is not
+  //   otherwise deduced.
+  bool AggregateDeductionCandidateHasMismatchedArity = false;
+
   /// Information on packs that we're currently expanding.
   ///
   /// FIXME: This should be kept internal to SemaTemplateDeduction.
   SmallVector<DeducedPack *, 8> PendingDeducedPacks;
 
   /// \brief The constraint satisfaction details resulting from the associated
   /// constraints satisfaction tests.
   ConstraintSatisfaction AssociatedConstraintsSatisfaction;

clang/lib/AST/ASTImporter.cpp

CXXConstructorDecl *Ctor =
         importChecked(Err, Guide->getCorrespondingConstructor());
     if (Err)
       return std::move(Err);
     if (GetImportedOrCreateDecl<CXXDeductionGuideDecl>(
             ToFunction, D, Importer.getToContext(), DC, ToInnerLocStart, ESpec,
             NameInfo, T, TInfo, ToEndLoc, Ctor))
       return ToFunction;
     cast<CXXDeductionGuideDecl>(ToFunction)
-        ->setIsCopyDeductionCandidate(Guide->isCopyDeductionCandidate());
+        ->setDeductionCandidateKind(Guide->getDeductionCandidateKind());
   } else {
     if (GetImportedOrCreateDecl(
             ToFunction, D, Importer.getToContext(), DC, ToInnerLocStart,
             NameInfo, T, TInfo, D->getStorageClass(), D->UsesFPIntrin(),
             D->isInlineSpecified(), D->hasWrittenPrototype(),
             D->getConstexprKind(), TrailingRequiresClause))
       return ToFunction;
   }

clang/lib/AST/Decl.cpp

FunctionDecl::FunctionDecl(Kind DK, ASTContext &C, DeclContext *DC,
   FunctionDeclBits.ConstexprKind = static_cast<uint64_t>(ConstexprKind);
   FunctionDeclBits.BodyContainsImmediateEscalatingExpression = false;
   FunctionDeclBits.InstantiationIsPending = false;
   FunctionDeclBits.UsesSEHTry = false;
   FunctionDeclBits.UsesFPIntrin = UsesFPIntrin;
   FunctionDeclBits.HasSkippedBody = false;
   FunctionDeclBits.WillHaveBody = false;
   FunctionDeclBits.IsMultiVersion = false;
-  FunctionDeclBits.IsCopyDeductionCandidate = false;
+  FunctionDeclBits.DeductionCandidateKind =
+      static_cast<unsigned char>(DeductionCandidate::Normal);
   FunctionDeclBits.HasODRHash = false;
   FunctionDeclBits.FriendConstraintRefersToEnclosingTemplate = false;
   if (TrailingRequiresClause)
     setTrailingRequiresClause(TrailingRequiresClause);
 }
 
 void FunctionDecl::getNameForDiagnostic(
     raw_ostream &OS, const PrintingPolicy &Policy, bool Qualified) const {

clang/lib/AST/DeclCXX.cpp

case Decl::Kind::CXXConversion:
     return cast<CXXConversionDecl>(Function)->getExplicitSpecifier();
   case Decl::Kind::CXXDeductionGuide:
     return cast<CXXDeductionGuideDecl>(Function)->getExplicitSpecifier();
   default:
     return {};
   }
 }
 
-CXXDeductionGuideDecl *
-CXXDeductionGuideDecl::Create(ASTContext &C, DeclContext *DC,
-                              SourceLocation StartLoc, ExplicitSpecifier ES,
-                              const DeclarationNameInfo &NameInfo, QualType T,
-                              TypeSourceInfo *TInfo, SourceLocation EndLocation,
-                              CXXConstructorDecl *Ctor) {
+CXXDeductionGuideDecl *CXXDeductionGuideDecl::Create(
+    ASTContext &C, DeclContext *DC, SourceLocation StartLoc,
+    ExplicitSpecifier ES, const DeclarationNameInfo &NameInfo, QualType T,
+    TypeSourceInfo *TInfo, SourceLocation EndLocation, CXXConstructorDecl *Ctor,
+    DeductionCandidate Kind) {
   return new (C, DC) CXXDeductionGuideDecl(C, DC, StartLoc, ES, NameInfo, T,
-                                           TInfo, EndLocation, Ctor);
+                                           TInfo, EndLocation, Ctor, Kind);
 }
 
 CXXDeductionGuideDecl *CXXDeductionGuideDecl::CreateDeserialized(ASTContext &C,
                                                                  unsigned ID) {
   return new (C, ID) CXXDeductionGuideDecl(
       C, nullptr, SourceLocation(), ExplicitSpecifier(), DeclarationNameInfo(),
-      QualType(), nullptr, SourceLocation(), nullptr);
+      QualType(), nullptr, SourceLocation(), nullptr,
+      DeductionCandidate::Normal);
 }
 
 RequiresExprBodyDecl *RequiresExprBodyDecl::Create(
     ASTContext &C, DeclContext *DC, SourceLocation StartLoc) {
   return new (C, DC) RequiresExprBodyDecl(C, DC, StartLoc);
 }
 
 RequiresExprBodyDecl *RequiresExprBodyDecl::CreateDeserialized(ASTContext &C,

clang/lib/Frontend/FrontendActions.cpp

static std::string toString(CodeSynthesisContext::SynthesisKind Kind) {
     case CodeSynthesisContext::NestedRequirementConstraintsCheck:
       return "NestedRequirementConstraintsCheck";
     case CodeSynthesisContext::InitializingStructuredBinding:
       return "InitializingStructuredBinding";
     case CodeSynthesisContext::MarkingClassDllexported:
       return "MarkingClassDllexported";
     case CodeSynthesisContext::BuildingBuiltinDumpStructCall:
       return "BuildingBuiltinDumpStructCall";
+    case CodeSynthesisContext::BuildingDeductionGuides:
+      return "BuildingDeductionGuides";
     }
     return "";
   }
 
   template <bool BeginInstantiation>
   static void displayTemplightEntry(llvm::raw_ostream &Out, const Sema &TheSema,
                                     const CodeSynthesisContext &Inst) {
     std::string YAML;

clang/lib/Sema/SemaDecl.cpp

if (!isa<DeducedTemplateSpecializationType>(Deduced) ||
       return QualType();
     }
   }
 
   ArrayRef<Expr*> DeduceInits;
   if (Init)
     DeduceInits = Init;
 
-  if (DirectInit) {
-    if (auto *PL = dyn_cast_or_null<ParenListExpr>(Init))
-      DeduceInits = PL->exprs();
-  }
+  auto *PL = dyn_cast_if_present<ParenListExpr>(Init);
+  if (DirectInit && PL)
 
   if (isa<DeducedTemplateSpecializationType>(Deduced)) {
     assert(VDecl && "non-auto type for init capture deduction?");
     InitializedEntity Entity = InitializedEntity::InitializeVariable(VDecl);
     InitializationKind Kind = InitializationKind::CreateForInit(
         VDecl->getLocation(), DirectInit, Init);
     // FIXME: Initialization should not be taking a mutable list of inits.
     SmallVector<Expr*, 8> InitsCopy(DeduceInits.begin(), DeduceInits.end());
     return DeduceTemplateSpecializationFromInitializer(TSI, Entity, Kind,
-                                                       InitsCopy);
+                                                       InitsCopy, PL);
   }
 
   if (DirectInit) {
     if (auto *IL = dyn_cast<InitListExpr>(Init))
       DeduceInits = IL->inits();
   }
 
   // Deduction only works if we have exactly one source expression.

clang/lib/Sema/SemaInit.cpp

 //===--- SemaInit.cpp - Semantic Analysis for Initializers ----------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements semantic analysis for initializers.
 //
 //===----------------------------------------------------------------------===//
 
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/DeclObjC.h"
 #include "clang/AST/ExprCXX.h"
 #include "clang/AST/ExprObjC.h"
 #include "clang/AST/ExprOpenMP.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/CharInfo.h"
 #include "clang/Basic/SourceManager.h"
 #include "clang/Basic/TargetInfo.h"
 #include "clang/Sema/Designator.h"
 #include "clang/Sema/EnterExpressionEvaluationContext.h"
 #include "clang/Sema/Initialization.h"
 #include "clang/Sema/Lookup.h"
 #include "clang/Sema/SemaInternal.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/PointerIntPair.h"
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace clang;
 
 //===----------------------------------------------------------------------===//
 // Sema Initialization Checking
 //===----------------------------------------------------------------------===//
 
 /// Check whether T is compatible with a wide character type (wchar_t,
 /// char16_t or char32_t).

 class InitListChecker {
   Sema &SemaRef;
   bool hadError = false;
   bool VerifyOnly; // No diagnostics.
   bool TreatUnavailableAsInvalid; // Used only in VerifyOnly mode.
   bool InOverloadResolution;
   InitListExpr *FullyStructuredList = nullptr;
   NoInitExpr *DummyExpr = nullptr;
+  SmallVectorImpl<QualType> *AggrDeductionCandidateParamTypes = nullptr;
 
   NoInitExpr *getDummyInit() {
     if (!DummyExpr)
       DummyExpr = new (SemaRef.Context) NoInitExpr(SemaRef.Context.VoidTy);
     return DummyExpr;
   }
 
   void CheckImplicitInitList(const InitializedEntity &Entity,
void CheckReferenceType(const InitializedEntity &Entity,
                           InitListExpr *StructuredList,
                           unsigned &StructuredIndex);
   void CheckVectorType(const InitializedEntity &Entity,
                        InitListExpr *IList, QualType DeclType, unsigned &Index,
                        InitListExpr *StructuredList,
                        unsigned &StructuredIndex);
   void CheckStructUnionTypes(const InitializedEntity &Entity,
                              InitListExpr *IList, QualType DeclType,
-                             CXXRecordDecl::base_class_range Bases,
+                             CXXRecordDecl::base_class_const_range Bases,
                              RecordDecl::field_iterator Field,
                              bool SubobjectIsDesignatorContext, unsigned &Index,
                              InitListExpr *StructuredList,
                              unsigned &StructuredIndex,
                              bool TopLevelObject = false);
   void CheckArrayType(const InitializedEntity &Entity,
                       InitListExpr *IList, QualType &DeclType,
                       llvm::APSInt elementIndex,
class InitListChecker {
   void UpdateStructuredListElement(InitListExpr *StructuredList,
                                    unsigned &StructuredIndex,
                                    Expr *expr);
   InitListExpr *createInitListExpr(QualType CurrentObjectType,
                                    SourceRange InitRange,
                                    unsigned ExpectedNumInits);
   int numArrayElements(QualType DeclType);
   int numStructUnionElements(QualType DeclType);
+  static RecordDecl *getRecordDecl(QualType DeclType);
 
   ExprResult PerformEmptyInit(SourceLocation Loc,
                               const InitializedEntity &Entity);
 
   /// Diagnose that OldInit (or part thereof) has been overridden by NewInit.
   void diagnoseInitOverride(Expr *OldInit, SourceRange NewInitRange,
                             bool UnionOverride = false,
                             bool FullyOverwritten = true) {
void FillInEmptyInitializations(const InitializedEntity &Entity,
                                   bool FillWithNoInit = false);
   bool CheckFlexibleArrayInit(const InitializedEntity &Entity,
                               Expr *InitExpr, FieldDecl *Field,
                               bool TopLevelObject);
   void CheckEmptyInitializable(const InitializedEntity &Entity,
                                SourceLocation Loc);
 
 public:
+  InitListChecker(
+      Sema &S, const InitializedEntity &Entity, InitListExpr *IL, QualType &T,
+      bool VerifyOnly, bool TreatUnavailableAsInvalid,
+      bool InOverloadResolution = false,
+      SmallVectorImpl<QualType> *AggrDeductionCandidateParamTypes = nullptr);
   InitListChecker(Sema &S, const InitializedEntity &Entity, InitListExpr *IL,
-                  QualType &T, bool VerifyOnly, bool TreatUnavailableAsInvalid,
-                  bool InOverloadResolution = false);
+                  QualType &T,
+                  SmallVectorImpl<QualType> &AggrDeductionCandidateParamTypes)
+      : InitListChecker(S, Entity, IL, T, /*VerifyOnly=*/true,
+                        /*TreatUnavailableAsInvalid=*/false,
+                        /*InOverloadResolution=*/false,
+                        &AggrDeductionCandidateParamTypes){};
+
   bool HadError() { return hadError; }
 
   // Retrieves the fully-structured initializer list used for
   // semantic analysis and code generation.
   InitListExpr *getFullyStructuredList() const { return FullyStructuredList; }
 };
 
 } // end anonymous namespace
 
else if (!InitExpr && !ILE->hasArrayFiller()) {
                                  RequiresSecondPass, ILE, Init,
                                  /*FillWithNoInit =*/true);
     }
   }
 }
 
 static bool hasAnyDesignatedInits(const InitListExpr *IL) {
   for (const Stmt *Init : *IL)
-    if (Init && isa<DesignatedInitExpr>(Init))
+    if (isa_and_nonnull<DesignatedInitExpr>(Init))
       return true;
   return false;
 }
 
-InitListChecker::InitListChecker(Sema &S, const InitializedEntity &Entity,
-                                 InitListExpr *IL, QualType &T, bool VerifyOnly,
-                                 bool TreatUnavailableAsInvalid,
-                                 bool InOverloadResolution)
+InitListChecker::InitListChecker(
+    Sema &S, const InitializedEntity &Entity, InitListExpr *IL, QualType &T,
+    bool VerifyOnly, bool TreatUnavailableAsInvalid, bool InOverloadResolution,
+    SmallVectorImpl<QualType> *AggrDeductionCandidateParamTypes)
     : SemaRef(S), VerifyOnly(VerifyOnly),
       TreatUnavailableAsInvalid(TreatUnavailableAsInvalid),
-      InOverloadResolution(InOverloadResolution) {
+      InOverloadResolution(InOverloadResolution),
+      AggrDeductionCandidateParamTypes(AggrDeductionCandidateParamTypes) {
   if (!VerifyOnly || hasAnyDesignatedInits(IL)) {
     FullyStructuredList =
         createInitListExpr(T, IL->getSourceRange(), IL->getNumInits());
 
     // FIXME: Check that IL isn't already the semantic form of some other
     // InitListExpr. If it is, we'd create a broken AST.
     if (!VerifyOnly)
       FullyStructuredList->setSyntacticForm(IL);
   }
 
   CheckExplicitInitList(Entity, IL, T, FullyStructuredList,
                         /*TopLevelObject=*/true);
 
-  if (!hadError && FullyStructuredList) {
+  if (!hadError && !AggrDeductionCandidateParamTypes && FullyStructuredList) {
     bool RequiresSecondPass = false;
     FillInEmptyInitializations(Entity, FullyStructuredList, RequiresSecondPass,
                                /*OuterILE=*/nullptr, /*OuterIndex=*/0);
     if (RequiresSecondPass && !hadError)
       FillInEmptyInitializations(Entity, FullyStructuredList,
                                  RequiresSecondPass, nullptr, 0);
   }
   if (hadError && FullyStructuredList)
for (const auto *Field : structDecl->fields())
     if (!Field->isUnnamedBitfield())
       ++InitializableMembers;
 
   if (structDecl->isUnion())
     return std::min(InitializableMembers, 1);
   return InitializableMembers - structDecl->hasFlexibleArrayMember();
 }
 
+RecordDecl *InitListChecker::getRecordDecl(QualType DeclType) {
+  if (const auto *RT = DeclType->getAs<RecordType>())
+    return RT->getDecl();
+  if (const auto *Inject = DeclType->getAs<InjectedClassNameType>())
+    return Inject->getDecl();
+  return nullptr;
+}
+
 /// Determine whether Entity is an entity for which it is idiomatic to elide
 /// the braces in aggregate initialization.
 static bool isIdiomaticBraceElisionEntity(const InitializedEntity &Entity) {
   // Recursive initialization of the one and only field within an aggregate
   // class is considered idiomatic. This case arises in particular for
   // initialization of std::array, where the C++ standard suggests the idiom of
   //
   //   std::array<T, N> arr = {1, 2, 3};
void InitListChecker::CheckImplicitInitList(const InitializedEntity &Entity,
   int maxElements = 0;
 
   if (T->isArrayType())
     maxElements = numArrayElements(T);
   else if (T->isRecordType())
     maxElements = numStructUnionElements(T);
   else if (T->isVectorType())
     maxElements = T->castAs<VectorType>()->getNumElements();
   else
     llvm_unreachable("CheckImplicitInitList(): Illegal type");
 
   if (maxElements == 0) {
     if (!VerifyOnly)
       SemaRef.Diag(ParentIList->getInit(Index)->getBeginLoc(),
                    diag::err_implicit_empty_initializer);
     ++Index;
     hadError = true;
     return;
if (DeclType->isAnyComplexType() && SubobjectIsDesignatorContext) {
     CheckComplexType(Entity, IList, DeclType, Index,
                      StructuredList, StructuredIndex);
   } else if (DeclType->isScalarType()) {
     CheckScalarType(Entity, IList, DeclType, Index,
                     StructuredList, StructuredIndex);
   } else if (DeclType->isVectorType()) {
     CheckVectorType(Entity, IList, DeclType, Index,
                     StructuredList, StructuredIndex);
-  } else if (DeclType->isRecordType()) {
+  } else if (const RecordDecl *RD = getRecordDecl(DeclType)) {
+    auto Bases =
+        CXXRecordDecl::base_class_const_range(CXXRecordDecl::base_class_const_iterator(),
+                                        CXXRecordDecl::base_class_const_iterator());
+    if (DeclType->isRecordType()) {
       assert(DeclType->isAggregateType() &&
              "non-aggregate records should be handed in CheckSubElementType");
-    RecordDecl *RD = DeclType->castAs<RecordType>()->getDecl();
-    auto Bases =
-        CXXRecordDecl::base_class_range(CXXRecordDecl::base_class_iterator(),
-                                        CXXRecordDecl::base_class_iterator());
       if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
         Bases = CXXRD->bases();
+    } else {
+      Bases = cast<CXXRecordDecl>(RD)->bases();
+    }
     CheckStructUnionTypes(Entity, IList, DeclType, Bases, RD->field_begin(),
                           SubobjectIsDesignatorContext, Index, StructuredList,
                           StructuredIndex, TopLevelObject);
   } else if (DeclType->isArrayType()) {
     llvm::APSInt Zero(
                     SemaRef.Context.getTypeSize(SemaRef.Context.getSizeType()),
                     false);
     CheckArrayType(Entity, IList, DeclType, Zero,
if (DeclType->isAnyComplexType() && SubobjectIsDesignatorContext) {
     if (!VerifyOnly)
       SemaRef.Diag(IList->getBeginLoc(), diag::err_init_objc_class) << DeclType;
     hadError = true;
   } else if (DeclType->isOCLIntelSubgroupAVCType() ||
              DeclType->isSizelessBuiltinType()) {
     // Checks for scalar type are sufficient for these types too.
     CheckScalarType(Entity, IList, DeclType, Index, StructuredList,
                     StructuredIndex);
+  } else if (DeclType->isDependentType()) {
+    // C++ [over.match.class.deduct]p1.5:
+    //   brace elision is not considered for any aggregate element that has a
+    //   dependent non-array type or an array type with a value-dependent bound
+    ++Index;
+    assert(AggrDeductionCandidateParamTypes);
+    AggrDeductionCandidateParamTypes->push_back(DeclType);
   } else {
     if (!VerifyOnly)
       SemaRef.Diag(IList->getBeginLoc(), diag::err_illegal_initializer_type)
           << DeclType;
     hadError = true;
   }
 }
 
if (SemaRef.getLangOpts().CPlusPlus || isa<InitListExpr>(expr)) {
     // Vector elements can be initialized from other vectors in which case
     // we need initialization entity with a type of a vector (and not a vector
     // element!) initializing multiple vector elements.
     auto TmpEntity =
         (ElemType->isExtVectorType() && !Entity.getType()->isExtVectorType())
             ? InitializedEntity::InitializeTemporary(ElemType)
             : Entity;
 
+    if (TmpEntity.getType()->isDependentType()) {
+      // C++ [over.match.class.deduct]p1.5:
+      //   brace elision is not considered for any aggregate element that has a
+      //   dependent non-array type or an array type with a value-dependent
+      //   bound
+      assert(AggrDeductionCandidateParamTypes);
+      if (!isa_and_nonnull<ConstantArrayType>(
+              SemaRef.Context.getAsArrayType(ElemType))) {
+        ++Index;
+        AggrDeductionCandidateParamTypes->push_back(ElemType);
+        return;
+      }
+    } else {
       InitializationSequence Seq(SemaRef, TmpEntity, Kind, expr,
                                  /*TopLevelOfInitList*/ true);
-
       // C++14 [dcl.init.aggr]p13:
       //   If the assignment-expression can initialize a member, the member is
       //   initialized. Otherwise [...] brace elision is assumed
       //
       // Brace elision is never performed if the element is not an
       // assignment-expression.
       if (Seq || isa<InitListExpr>(expr)) {
         if (!VerifyOnly) {
           ExprResult Result = Seq.Perform(SemaRef, TmpEntity, Kind, expr);
           if (Result.isInvalid())
             hadError = true;
 
           UpdateStructuredListElement(StructuredList, StructuredIndex,
                                       Result.getAs<Expr>());
         } else if (!Seq) {
           hadError = true;
         } else if (StructuredList) {
           UpdateStructuredListElement(StructuredList, StructuredIndex,
                                       getDummyInit());
         }
         ++Index;
+        if (AggrDeductionCandidateParamTypes)
+          AggrDeductionCandidateParamTypes->push_back(ElemType);
         return;
       }
+    }
 
     // Fall through for subaggregate initialization
   } else if (ElemType->isScalarType() || ElemType->isAtomicType()) {
     // FIXME: Need to handle atomic aggregate types with implicit init lists.
     return CheckScalarType(Entity, IList, ElemType, Index,
                            StructuredList, StructuredIndex);
   } else if (const ArrayType *arrayType =
                  SemaRef.Context.getAsArrayType(ElemType)) {
else {
     if (ResultExpr != expr && !VerifyOnly) {
       // The type was promoted, update initializer list.
       // FIXME: Why are we updating the syntactic init list?
       IList->setInit(Index, ResultExpr);
     }
   }
   UpdateStructuredListElement(StructuredList, StructuredIndex, ResultExpr);
   ++Index;
+  if (AggrDeductionCandidateParamTypes)
+    AggrDeductionCandidateParamTypes->push_back(DeclType);
 }
 
 void InitListChecker::CheckReferenceType(const InitializedEntity &Entity,
                                          InitListExpr *IList, QualType DeclType,
                                          unsigned &Index,
                                          InitListExpr *StructuredList,
                                          unsigned &StructuredIndex) {
   if (Index >= IList->getNumInits()) {
void InitListChecker::CheckReferenceType(const InitializedEntity &Entity,
 
   expr = Result.getAs<Expr>();
   // FIXME: Why are we updating the syntactic init list?
   if (!VerifyOnly && expr)
     IList->setInit(Index, expr);
 
   UpdateStructuredListElement(StructuredList, StructuredIndex, expr);
   ++Index;
+  if (AggrDeductionCandidateParamTypes)
+    AggrDeductionCandidateParamTypes->push_back(DeclType);
 }
 
 void InitListChecker::CheckVectorType(const InitializedEntity &Entity,
                                       InitListExpr *IList, QualType DeclType,
                                       unsigned &Index,
                                       InitListExpr *StructuredList,
                                       unsigned &StructuredIndex) {
   const VectorType *VT = DeclType->castAs<VectorType>();
if (!isa<InitListExpr>(Init) && Init->getType()->isVectorType()) {
         if (ResultExpr != Init && !VerifyOnly) {
           // The type was promoted, update initializer list.
           // FIXME: Why are we updating the syntactic init list?
           IList->setInit(Index, ResultExpr);
         }
       }
       UpdateStructuredListElement(StructuredList, StructuredIndex, ResultExpr);
       ++Index;
+      if (AggrDeductionCandidateParamTypes)
+        AggrDeductionCandidateParamTypes->push_back(elementType);
       return;
     }
 
     InitializedEntity ElementEntity =
       InitializedEntity::InitializeElement(SemaRef.Context, 0, Entity);
 
     for (unsigned i = 0; i < maxElements; ++i, ++numEltsInit) {
       // Don't attempt to go past the end of the init list
if (IsStringInit(IList->getInit(Index), arrayType, SemaRef.Context) ==
       if (!VerifyOnly)
         CheckStringInit(IList->getInit(Index), DeclType, arrayType, SemaRef);
       if (StructuredList) {
         UpdateStructuredListElement(StructuredList, StructuredIndex,
                                     IList->getInit(Index));
         StructuredList->resizeInits(SemaRef.Context, StructuredIndex);
       }
       ++Index;
+      if (AggrDeductionCandidateParamTypes)
+        AggrDeductionCandidateParamTypes->push_back(DeclType);
       return;
     }
   }
   if (const VariableArrayType *VAT = dyn_cast<VariableArrayType>(arrayType)) {
     // Check for VLAs; in standard C it would be possible to check this
     // earlier, but I don't know where clang accepts VLAs (gcc accepts
     // them in all sorts of strange places).
     bool HasErr = IList->getNumInits() != 0 || SemaRef.getLangOpts().CPlusPlus;
SemaRef.Diag(Field->getLocation(), diag::note_flexible_array_member)
       << Field;
   }
 
   return FlexArrayDiag != diag::ext_flexible_array_init;
 }
 
 void InitListChecker::CheckStructUnionTypes(
     const InitializedEntity &Entity, InitListExpr *IList, QualType DeclType,
-    CXXRecordDecl::base_class_range Bases, RecordDecl::field_iterator Field,
+    CXXRecordDecl::base_class_const_range Bases, RecordDecl::field_iterator Field,
     bool SubobjectIsDesignatorContext, unsigned &Index,
     InitListExpr *StructuredList, unsigned &StructuredIndex,
     bool TopLevelObject) {
-  RecordDecl *structDecl = DeclType->castAs<RecordType>()->getDecl();
+  const RecordDecl *RD = getRecordDecl(DeclType);
 
   // If the record is invalid, some of it's members are invalid. To avoid
   // confusion, we forgo checking the initializer for the entire record.
-  if (structDecl->isInvalidDecl()) {
+  if (RD->isInvalidDecl()) {
     // Assume it was supposed to consume a single initializer.
     ++Index;
     hadError = true;
     return;
   }
 
-  if (DeclType->isUnionType() && IList->getNumInits() == 0) {
-    RecordDecl *RD = DeclType->castAs<RecordType>()->getDecl();
-
+  if (RD->isUnion() && IList->getNumInits() == 0) {
     if (!VerifyOnly)
       for (FieldDecl *FD : RD->fields()) {
         QualType ET = SemaRef.Context.getBaseElementType(FD->getType());
         if (checkDestructorReference(ET, IList->getEndLoc(), SemaRef)) {
           hadError = true;
           return;
         }
       }
for (RecordDecl::field_iterator FieldEnd = RD->field_end();
       }
     }
     return;
   }
 
   bool InitializedSomething = false;
 
   // If we have any base classes, they are initialized prior to the fields.
-  for (auto &Base : Bases) {
+  for (auto I = Bases.begin(), E = Bases.end(); I != E; ++I) {
+    auto &Base = *I;
     Expr *Init = Index < IList->getNumInits() ? IList->getInit(Index) : nullptr;
 
     // Designated inits always initialize fields, so if we see one, all
     // remaining base classes have no explicit initializer.
     if (Init && isa<DesignatedInitExpr>(Init))
       Init = nullptr;
 
+    // C++ [over.match.class.deduct]p1.6:
+    //   each non-trailing aggregate element that is a pack expansion is assumed
+    //   to correspond to no elements of the initializer list, and (1.7) a
+    //   trailing aggregate element that is a pack expansion is assumed to
+    //   correspond to all remaining elements of the initializer list (if any).
+
+    // C++ [over.match.class.deduct]p1.9:
+    //   ... except that additional parameter packs of the form P_j... are
+    //   inserted into the parameter list in their original aggregate element
+    //   position corresponding to each non-trailing aggregate element of
+    //   type P_j that was skipped because it was a parameter pack, and the
+    //   trailing sequence of parameters corresponding to a trailing
+    //   aggregate element that is a pack expansion (if any) is replaced
+    //   by a single parameter of the form T_n....
+    if (AggrDeductionCandidateParamTypes && Base.isPackExpansion()) {
+      AggrDeductionCandidateParamTypes->push_back(
+          SemaRef.Context.getPackExpansionType(Base.getType(), std::nullopt));
+
+      // Trailing pack expansion
+      if (I + 1 == E && RD->field_empty()) {
+        if (Index < IList->getNumInits())
+          Index = IList->getNumInits();
+        return;
+      }
+
+      continue;
+    }
+
     SourceLocation InitLoc = Init ? Init->getBeginLoc() : IList->getEndLoc();
     InitializedEntity BaseEntity = InitializedEntity::InitializeBase(
         SemaRef.Context, &Base, false, &Entity);
     if (Init) {
       CheckSubElementType(BaseEntity, IList, Base.getType(), Index,
                           StructuredList, StructuredIndex);
       InitializedSomething = true;
     } else {
       CheckEmptyInitializable(BaseEntity, InitLoc);
     }
 
     if (!VerifyOnly)
       if (checkDestructorReference(Base.getType(), InitLoc, SemaRef)) {
         hadError = true;
         return;
       }
   }
 
   // If structDecl is a forward declaration, this loop won't do
   // anything except look at designated initializers; That's okay,
   // because an error should get printed out elsewhere. It might be
   // worthwhile to skip over the rest of the initializer, though.
-  RecordDecl *RD = DeclType->castAs<RecordType>()->getDecl();
   RecordDecl::field_iterator FieldEnd = RD->field_end();
   size_t NumRecordDecls = llvm::count_if(RD->decls(), [&](const Decl *D) {
     return isa<FieldDecl>(D) || isa<RecordDecl>(D);
   });
   bool CheckForMissingFields =
     !IList->isIdiomaticZeroInitializer(SemaRef.getLangOpts());
   bool HasDesignatedInit = false;
 
while (Index < IList->getNumInits()) {
     }
 
     if (Field == FieldEnd) {
       // We've run out of fields. We're done.
       break;
     }
 
     // We've already initialized a member of a union. We're done.
-    if (InitializedSomething && DeclType->isUnionType())
+    if (InitializedSomething && RD->isUnion())
       break;
 
     // If we've hit the flexible array member at the end, we're done.
     if (Field->getType()->isIncompleteArrayType())
       break;
 
     if (Field->isUnnamedBitfield()) {
       // Don't initialize unnamed bitfields, e.g. "int : 20;"
while (Index < IList->getNumInits()) {
     }
 
     InitializedEntity MemberEntity =
       InitializedEntity::InitializeMember(*Field, &Entity);
     CheckSubElementType(MemberEntity, IList, Field->getType(), Index,
                         StructuredList, StructuredIndex);
     InitializedSomething = true;
 
-    if (DeclType->isUnionType() && StructuredList) {
+    if (RD->isUnion() && StructuredList) {
       // Initialize the first field within the union.
       StructuredList->setInitializedFieldInUnion(*Field);
     }
 
     ++Field;
   }
 
   // Emit warnings for missing struct field initializers.
   if (!VerifyOnly && InitializedSomething && CheckForMissingFields &&
       Field != FieldEnd && !Field->getType()->isIncompleteArrayType() &&
-      !DeclType->isUnionType()) {
+      !RD->isUnion()) {
     // It is possible we have one or more unnamed bitfields remaining.
     // Find first (if any) named field and emit warning.
     for (RecordDecl::field_iterator it = Field, end = RD->field_end();
          it != end; ++it) {
       if (!it->isUnnamedBitfield() && !it->hasInClassInitializer()) {
         SemaRef.Diag(IList->getSourceRange().getEnd(),
                      diag::warn_missing_field_initializers) << *it;
         break;
       }
     }
   }
 
   // Check that any remaining fields can be value-initialized if we're not
   // building a structured list. (If we are, we'll check this later.)
-  if (!StructuredList && Field != FieldEnd && !DeclType->isUnionType() &&
+  if (!StructuredList && Field != FieldEnd && !RD->isUnion() &&
       !Field->getType()->isIncompleteArrayType()) {
     for (; Field != FieldEnd && !hadError; ++Field) {
       if (!Field->isUnnamedBitfield() && !Field->hasInClassInitializer())
         CheckEmptyInitializable(
             InitializedEntity::InitializeMember(*Field, &Entity),
             IList->getEndLoc());
     }
   }
if (CheckFlexibleArrayInit(Entity, IList->getInit(Index), *Field,
     hadError = true;
     ++Index;
     return;
   }
 
   InitializedEntity MemberEntity =
     InitializedEntity::InitializeMember(*Field, &Entity);
 
-  if (isa<InitListExpr>(IList->getInit(Index)))
+  if (isa<InitListExpr>(IList->getInit(Index)) ||
+      AggrDeductionCandidateParamTypes)
     CheckSubElementType(MemberEntity, IList, Field->getType(), Index,
                         StructuredList, StructuredIndex);
   else
     CheckImplicitInitList(MemberEntity, IList, Field->getType(), Index,
                           StructuredList, StructuredIndex);
 }
 
 /// Expand a field designator that refers to a member of an

 }
 
 namespace {
 
 // Callback to only accept typo corrections that are for field members of
 // the given struct or union.
 class FieldInitializerValidatorCCC final : public CorrectionCandidateCallback {
  public:
-  explicit FieldInitializerValidatorCCC(RecordDecl *RD)
+  explicit FieldInitializerValidatorCCC(const RecordDecl *RD)
       : Record(RD) {}
 
   bool ValidateCandidate(const TypoCorrection &candidate) override {
     FieldDecl *FD = candidate.getCorrectionDeclAs<FieldDecl>();
     return FD && FD->getDeclContext()->getRedeclContext()->Equals(Record);
   }
 
   std::unique_ptr<CorrectionCandidateCallback> clone() override {
     return std::make_unique<FieldInitializerValidatorCCC>(*this);
   }
 
  private:
-  RecordDecl *Record;
+  const RecordDecl *Record;
 };
 
 } // end anonymous namespace
 
 /// Check the well-formedness of a C99 designated initializer.
 ///
 /// Determines whether the designated initializer @p DIE, which
 /// resides at the given @p Index within the initializer list @p
if (DIE->isDirectInit()) {
       if (StructuredList) {
         ExprResult Result = VerifyOnly
                                 ? getDummyInit()
                                 : Seq.Perform(SemaRef, Entity, Kind, Init);
         UpdateStructuredListElement(StructuredList, StructuredIndex,
                                     Result.get());
       }
       ++Index;
+      if (AggrDeductionCandidateParamTypes)
+        AggrDeductionCandidateParamTypes->push_back(CurrentObjectType);
       return !Seq;
     }
 
     // Check the actual initialization for the designated object type.
     bool prevHadError = hadError;
 
     // Temporarily remove the designator expression from the
     // initializer list that the child calls see, so that we don't try
if (D->isFieldDesignator()) {
     //
     //   If a designator has the form
     //
     //      . identifier
     //
     //   then the current object (defined below) shall have
     //   structure or union type and the identifier shall be the
     //   name of a member of that type.
-    const RecordType *RT = CurrentObjectType->getAs<RecordType>();
-    if (!RT) {
+    RecordDecl *RD = getRecordDecl(CurrentObjectType);
+    if (!RD) {
       SourceLocation Loc = D->getDotLoc();
       if (Loc.isInvalid())
         Loc = D->getFieldLoc();
       if (!VerifyOnly)
         SemaRef.Diag(Loc, diag::err_field_designator_non_aggr)
           << SemaRef.getLangOpts().CPlusPlus << CurrentObjectType;
       ++Index;
       return true;
     }
 
     FieldDecl *KnownField = D->getFieldDecl();
     if (!KnownField) {
       const IdentifierInfo *FieldName = D->getFieldName();
-      DeclContext::lookup_result Lookup = RT->getDecl()->lookup(FieldName);
+      DeclContext::lookup_result Lookup = RD->lookup(FieldName);
       for (NamedDecl *ND : Lookup) {
         if (auto *FD = dyn_cast<FieldDecl>(ND)) {
           KnownField = FD;
           break;
         }
         if (auto *IFD = dyn_cast<IndirectFieldDecl>(ND)) {
           // In verify mode, don't modify the original.
           if (VerifyOnly)
if (!KnownField) {
           SemaRef.Diag(Lookup.front()->getLocation(),
                        diag::note_field_designator_found);
           ++Index;
           return true;
         }
 
         // Name lookup didn't find anything.
         // Determine whether this was a typo for another field name.
-        FieldInitializerValidatorCCC CCC(RT->getDecl());
+        FieldInitializerValidatorCCC CCC(RD);
         if (TypoCorrection Corrected = SemaRef.CorrectTypo(
                 DeclarationNameInfo(FieldName, D->getFieldLoc()),
                 Sema::LookupMemberName, /*Scope=*/nullptr, /*SS=*/nullptr, CCC,
-                Sema::CTK_ErrorRecovery, RT->getDecl())) {
+                Sema::CTK_ErrorRecovery, RD)) {
           SemaRef.diagnoseTypo(
               Corrected,
               SemaRef.PDiag(diag::err_field_designator_unknown_suggest)
                 << FieldName << CurrentObjectType);
           KnownField = Corrected.getCorrectionDeclAs<FieldDecl>();
           hadError = true;
         } else {
           // Typo correction didn't find anything.
           SourceLocation Loc = D->getFieldLoc();
 
           // The loc can be invalid with a "null" designator (i.e. an anonymous
           // union/struct). Do our best to approximate the location.
           if (Loc.isInvalid())
             Loc = IList->getBeginLoc();
 
           SemaRef.Diag(Loc, diag::err_field_designator_unknown)
             << FieldName << CurrentObjectType << DIE->getSourceRange();
           ++Index;
           return true;
         }
       }
     }
 
     unsigned NumBases = 0;
-    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RT->getDecl()))
+    if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RD))
       NumBases = CXXRD->getNumBases();
 
     unsigned FieldIndex = NumBases;
 
-    for (auto *FI : RT->getDecl()->fields()) {
+    for (auto *FI : RD->fields()) {
       if (FI->isUnnamedBitfield())
         continue;
       if (declaresSameEntity(KnownField, FI)) {
         KnownField = FI;
         break;
       }
       ++FieldIndex;
     }
 
     RecordDecl::field_iterator Field =
         RecordDecl::field_iterator(DeclContext::decl_iterator(KnownField));
 
     // All of the fields of a union are located at the same place in
     // the initializer list.
-    if (RT->getDecl()->isUnion()) {
+    if (RD->isUnion()) {
       FieldIndex = 0;
       if (StructuredList) {
         FieldDecl *CurrentField = StructuredList->getInitializedFieldInUnion();
         if (CurrentField && !declaresSameEntity(CurrentField, *Field)) {
           assert(StructuredList->getNumInits() == 1
                  && "A union should never have more than one initializer!");
 
           Expr *ExistingInit = StructuredList->getInit(0);
if (D->isFieldDesignator()) {
     // FIXME: This is the only reordering diagnostic we produce, and it only
     // catches cases where we have a top-level field designator that jumps
     // backwards. This is the only such case that is reachable in an
     // otherwise-valid C++20 program, so is the only case that's required for
     // conformance, but for consistency, we should diagnose all the other
     // cases where a designator takes us backwards too.
     if (IsFirstDesignator && !VerifyOnly && SemaRef.getLangOpts().CPlusPlus &&
         NextField &&
-        (*NextField == RT->getDecl()->field_end() ||
+        (*NextField == RD->field_end() ||
          (*NextField)->getFieldIndex() > Field->getFieldIndex() + 1)) {
       // Find the field that we just initialized.
       FieldDecl *PrevField = nullptr;
-      for (auto FI = RT->getDecl()->field_begin();
-           FI != RT->getDecl()->field_end(); ++FI) {
+      for (auto FI = RD->field_begin(); FI != RD->field_end(); ++FI) {
         if (FI->isUnnamedBitfield())
           continue;
-        if (*NextField != RT->getDecl()->field_end() &&
+        if (*NextField != RD->field_end() &&
             declaresSameEntity(*FI, **NextField))
           break;
         PrevField = *FI;
       }
 
       if (PrevField &&
           PrevField->getFieldIndex() > KnownField->getFieldIndex()) {
         SemaRef.Diag(DIE->getBeginLoc(), diag::ext_designated_init_reordered)
if (IsFirstDesignator) {
       StructuredIndex = FieldIndex;
       return false;
     }
 
     if (!FinishSubobjectInit)
       return false;
 
     // We've already initialized something in the union; we're done.
-    if (RT->getDecl()->isUnion())
+    if (RD->isUnion())
       return hadError;
 
     // Check the remaining fields within this class/struct/union subobject.
     bool prevHadError = hadError;
 
     auto NoBases =
         CXXRecordDecl::base_class_range(CXXRecordDecl::base_class_iterator(),
                                         CXXRecordDecl::base_class_iterator());
if (const ConstantArrayType *CAType = dyn_cast<ConstantArrayType>(AType)) {
       // initializer with many empty entries at the end.
       if (NumElements > ExpectedNumInits)
         NumElements = 0;
     }
   } else if (const VectorType *VType = CurrentObjectType->getAs<VectorType>()) {
     NumElements = VType->getNumElements();
   } else if (CurrentObjectType->isRecordType()) {
     NumElements = numStructUnionElements(CurrentObjectType);
+  } else if (CurrentObjectType->isDependentType()) {
+    NumElements = 1;
   }
 
   Result->reserveInits(SemaRef.Context, NumElements);
 
   return Result;
 }
 
 /// Update the initializer at index @p StructuredIndex within the
 /// structured initializer list to the value @p expr.
 void InitListChecker::UpdateStructuredListElement(InitListExpr *StructuredList,
auto NotSpecialization = [&] (const CXXRecordDecl *Candidate) {
     auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(Candidate);
     return !CTSD || !declaresSameEntity(CTSD->getSpecializedTemplate(), CTD);
   };
   return !(NotSpecialization(RD) && RD->forallBases(NotSpecialization));
 }
 
 QualType Sema::DeduceTemplateSpecializationFromInitializer(
     TypeSourceInfo *TSInfo, const InitializedEntity &Entity,
-    const InitializationKind &Kind, MultiExprArg Inits) {
+    const InitializationKind &Kind, MultiExprArg Inits, ParenListExpr *PL) {
   auto *DeducedTST = dyn_cast<DeducedTemplateSpecializationType>(
       TSInfo->getType()->getContainedDeducedType());
   assert(DeducedTST && "not a deduced template specialization type");
 
   auto TemplateName = DeducedTST->getTemplateName();
   if (TemplateName.isDependent())
     return SubstAutoTypeDependent(TSInfo->getType());
 
QualType Sema::DeduceTemplateSpecializationFromInitializer(
   //   templates are considered to be the constructors of that class type
   //
   // Since we know we're initializing a class type of a type unrelated to that
   // of the initializer, this reduces to something fairly reasonable.
   OverloadCandidateSet Candidates(Kind.getLocation(),
                                   OverloadCandidateSet::CSK_Normal);
   OverloadCandidateSet::iterator Best;
 
-  bool HasAnyDeductionGuide = false;
   bool AllowExplicit = !Kind.isCopyInit() || ListInit;
 
-  auto tryToResolveOverload =
-      [&](bool OnlyListConstructors) -> OverloadingResult {
-    Candidates.clear(OverloadCandidateSet::CSK_Normal);
-    HasAnyDeductionGuide = false;
-
-    for (auto I = Guides.begin(), E = Guides.end(); I != E; ++I) {
-      NamedDecl *D = (*I)->getUnderlyingDecl();
-      if (D->isInvalidDecl())
-        continue;
-
-      auto *TD = dyn_cast<FunctionTemplateDecl>(D);
-      auto *GD = dyn_cast_or_null<CXXDeductionGuideDecl>(
-          TD ? TD->getTemplatedDecl() : dyn_cast<FunctionDecl>(D));
-      if (!GD)
-        continue;
-
-      if (!GD->isImplicit())
-        HasAnyDeductionGuide = true;
-
-      // C++ [over.match.ctor]p1: (non-list copy-initialization from non-class)
-      //   For copy-initialization, the candidate functions are all the
-      //   converting constructors (12.3.1) of that class.
-      // C++ [over.match.copy]p1: (non-list copy-initialization from class)
-      //   The converting constructors of T are candidate functions.
-      if (!AllowExplicit) {
-        // Overload resolution checks whether the deduction guide is declared
-        // explicit for us.
+  // Return true is the candidate is added successfully, false otherwise.
+  auto addDeductionCandidate = [&](FunctionTemplateDecl *TD,
+                                   CXXDeductionGuideDecl *GD,
+                                   DeclAccessPair FoundDecl,
+                                   bool OnlyListConstructors,
+                                   bool AllowAggregateDeductionCandidate) {
 
-        // When looking for a converting constructor, deduction guides that
-        // could never be called with one argument are not interesting to
-        // check or note.
-        if (GD->getMinRequiredArguments() > 1 ||
-            (GD->getNumParams() == 0 && !GD->isVariadic()))
-          continue;
-      }
+        return;
 
-      // C++ [over.match.list]p1.1: (first phase list initialization)
-      //   Initially, the candidate functions are the initializer-list
-      //   constructors of the class T
-      if (OnlyListConstructors && !isInitListConstructor(GD))
-        continue;
+      return;
+
+    if (!AllowAggregateDeductionCandidate &&
+        GD->getDeductionCandidateKind() == DeductionCandidate::Aggregate)
+      return;
 
-      // C++ [over.match.list]p1.2: (second phase list initialization)
-      //   the candidate functions are all the constructors of the class T
-      // C++ [over.match.ctor]p1: (all other cases)
-      //   the candidate functions are all the constructors of the class of
-      //   the object being initialized
 
-      // C++ [over.best.ics]p4:
-      //   When [...] the constructor [...] is a candidate by
-      //    - [over.match.copy] (in all cases)
-      // FIXME: The "second phase of [over.match.list] case can also
-      // theoretically happen here, but it's not clear whether we can
-      // ever have a parameter of the right type.
-      bool SuppressUserConversions = Kind.isCopyInit();
 
-      if (TD)
-        AddTemplateOverloadCandidate(TD, I.getPair(), /*ExplicitArgs*/ nullptr,
-                                     Inits, Candidates, SuppressUserConversions,
-                                     /*PartialOverloading*/ false,
-                                     AllowExplicit);
+    if (TD) {
+      SmallVector<Expr *, 8> TmpInits;
+      for (Expr *E : Inits)
+        if (auto *DI = dyn_cast<DesignatedInitExpr>(E))
+          TmpInits.push_back(DI->getInit());
         else
-        AddOverloadCandidate(GD, I.getPair(), Inits, Candidates,
-                             SuppressUserConversions,
-                             /*PartialOverloading*/ false, AllowExplicit);
+          TmpInits.push_back(E);
+      AddTemplateOverloadCandidate(
+          TD, FoundDecl, /*ExplicitArgs=*/nullptr, TmpInits, Candidates,
+          /*PartialOverloading=*/false, AllowExplicit, ADLCallKind::NotADL,
+          /*PO=*/{}, /*AggregateCandidateDeduction=*/true);
+    } else {
+      AddOverloadCandidate(GD, FoundDecl, Inits, Candidates,
+                           SuppressUserConversions,
+                           /*PartialOverloading=*/false, AllowExplicit);
     }
+  };
+
+  bool FoundDeductionGuide = false;
+
+  auto TryToResolveOverload =
+      [&](bool OnlyListConstructors) -> OverloadingResult {
+    Candidates.clear(OverloadCandidateSet::CSK_Normal);
+    bool HasAnyDeductionGuide = false;
+
+    auto SynthesizeAggrGuide = [&](InitListExpr *ListInit) {
+      auto *RD = cast<CXXRecordDecl>(Template->getTemplatedDecl());
+      if (!(RD->getDefinition() && RD->isAggregate()))
+        return;
+      QualType Ty = Context.getRecordType(RD);
+      SmallVector<QualType, 8> ElementTypes;
+
+      InitListChecker CheckInitList(*this, Entity, ListInit, Ty, ElementTypes);
+      if (!CheckInitList.HadError()) {
+        // C++ [over.match.class.deduct]p1.8:
+        //   if e_i is of array type and x_i is a braced-init-list, T_i is an
+        //   rvalue reference to the declared type of e_i and
+        // C++ [over.match.class.deduct]p1.9:
+        //   if e_i is of array type and x_i is a bstring-literal, T_i is an
+        //   lvalue reference to the const-qualified declared type of e_i and
+        // C++ [over.match.class.deduct]p1.10:
+        //   otherwise, T_i is the declared type of e_i
+        for (int I = 0, E = ListInit->getNumInits();
+             I < E && !isa<PackExpansionType>(ElementTypes[I]); ++I)
+          if (ElementTypes[I]->isArrayType()) {
+            if (isa<InitListExpr>(ListInit->getInit(I)))
+              ElementTypes[I] = Context.getRValueReferenceType(ElementTypes[I]);
+            else if (isa<StringLiteral>(
+                         ListInit->getInit(I)->IgnoreParenImpCasts()))
+              ElementTypes[I] = Context.getLValueReferenceType(ElementTypes[I]);
+          }
+
+        llvm::FoldingSetNodeID ID;
+        ID.AddPointer(Template);
+        for (auto &T : ElementTypes)
+          T.getCanonicalType().Profile(ID);
+        unsigned Hash = ID.ComputeHash();
+        if (AggregateDeductionCandidates.count(Hash) == 0) {
+          if (FunctionTemplateDecl *TD =
+                  DeclareImplicitDeductionGuideFromInitList(
+                      Template, ElementTypes,
+                      TSInfo->getTypeLoc().getEndLoc())) {
+            auto *GD = cast<CXXDeductionGuideDecl>(TD->getTemplatedDecl());
+            GD->setDeductionCandidateKind(DeductionCandidate::Aggregate);
+            AggregateDeductionCandidates[Hash] = GD;
+            addDeductionCandidate(TD, GD, DeclAccessPair::make(TD, AS_public),
+                                  OnlyListConstructors,
+                                  /*AllowAggregateDeductionCandidate=*/true);
+          }
+        } else {
+          CXXDeductionGuideDecl *GD = AggregateDeductionCandidates[Hash];
+          FunctionTemplateDecl *TD = GD->getDescribedFunctionTemplate();
+          assert(TD && "aggregate deduction candidate is function template");
+          addDeductionCandidate(TD, GD, DeclAccessPair::make(TD, AS_public),
+                                OnlyListConstructors,
+                                /*AllowAggregateDeductionCandidate=*/true);
+        }
+        HasAnyDeductionGuide = true;
+      }
+    };
+
+    for (auto I = Guides.begin(), E = Guides.end(); I != E; ++I) {
+      NamedDecl *D = (*I)->getUnderlyingDecl();
+      if (D->isInvalidDecl())
+        continue;
+
+      auto *TD = dyn_cast<FunctionTemplateDecl>(D);
+      auto *GD = dyn_cast_if_present<CXXDeductionGuideDecl>(
+          TD ? TD->getTemplatedDecl() : dyn_cast<FunctionDecl>(D));
+      if (!GD)
+        continue;
+
+      if (!GD->isImplicit())
+        HasAnyDeductionGuide = true;
+
+      addDeductionCandidate(TD, GD, I.getPair(), OnlyListConstructors,
+                            /*AllowAggregateDeductionCandidate=*/false);
+    }
+
+    // C++ [over.match.class.deduct]p1.4:
+    //   if C is defined and its definition satisfies the conditions for an
+    //   aggregate class ([dcl.init.aggr]) with the assumption that any
+    //   dependent base class has no virtual functions and no virtual base
+    //   classes, and the initializer is a non-empty braced-init-list or
+    //   parenthesized expression-list, and there are no deduction-guides for
+    //   C, the set contains an additional function template, called the
+    //   aggregate deduction candidate, defined as follows.
+    if (!HasAnyDeductionGuide) {
+      if (ListInit && ListInit->getNumInits()) {
+        SynthesizeAggrGuide(ListInit);
+      } else if (PL && PL->getNumExprs()) {
+        InitListExpr TempListInit(getASTContext(), PL->getLParenLoc(),
+                                  PL->exprs(), PL->getRParenLoc());
+        SynthesizeAggrGuide(&TempListInit);
+      }
+    }
+
+    FoundDeductionGuide = FoundDeductionGuide || HasAnyDeductionGuide;
+
     return Candidates.BestViableFunction(*this, Kind.getLocation(), Best);
   };
 
   OverloadingResult Result = OR_No_Viable_Function;
 
   // C++11 [over.match.list]p1, per DR1467: for list-initialization, first
   // try initializer-list constructors.
   if (ListInit) {
     bool TryListConstructors = true;
 
     // Try list constructors unless the list is empty and the class has one or
     // more default constructors, in which case those constructors win.
     if (!ListInit->getNumInits()) {
       for (NamedDecl *D : Guides) {
         auto *FD = dyn_cast<FunctionDecl>(D->getUnderlyingDecl());
         if (FD && FD->getMinRequiredArguments() == 0) {
           TryListConstructors = false;
           break;
         }
       }
     } else if (ListInit->getNumInits() == 1) {
       // C++ [over.match.class.deduct]:
       //   As an exception, the first phase in [over.match.list] (considering
       //   initializer-list constructors) is omitted if the initializer list
       //   consists of a single expression of type cv U, where U is a
       //   specialization of C or a class derived from a specialization of C.
       Expr *E = ListInit->getInit(0);
       auto *RD = E->getType()->getAsCXXRecordDecl();
       if (!isa<InitListExpr>(E) && RD &&
           isCompleteType(Kind.getLocation(), E->getType()) &&
           isOrIsDerivedFromSpecializationOf(RD, Template))
         TryListConstructors = false;
     }
 
     if (TryListConstructors)
-      Result = tryToResolveOverload(/*OnlyListConstructor*/true);
+      Result = TryToResolveOverload(/*OnlyListConstructor*/true);
     // Then unwrap the initializer list and try again considering all
     // constructors.
     Inits = MultiExprArg(ListInit->getInits(), ListInit->getNumInits());
   }
 
   // If list-initialization fails, or if we're doing any other kind of
   // initialization, we (eventually) consider constructors.
   if (Result == OR_No_Viable_Function)
-    Result = tryToResolveOverload(/*OnlyListConstructor*/false);
+    Result = TryToResolveOverload(/*OnlyListConstructor*/false);
 
   switch (Result) {
   case OR_Ambiguous:
     // FIXME: For list-initialization candidates, it'd usually be better to
     // list why they were not viable when given the initializer list itself as
     // an argument.
     Candidates.NoteCandidates(
         PartialDiagnosticAt(
QualType Sema::DeduceTemplateSpecializationFromInitializer(
   QualType DeducedType =
       SubstAutoType(TSInfo->getType(), Best->Function->getReturnType());
   Diag(TSInfo->getTypeLoc().getBeginLoc(),
        diag::warn_cxx14_compat_class_template_argument_deduction)
       << TSInfo->getTypeLoc().getSourceRange() << 1 << DeducedType;
 
   // Warn if CTAD was used on a type that does not have any user-defined
   // deduction guides.
-  if (!HasAnyDeductionGuide) {
+  if (!FoundDeductionGuide) {
     Diag(TSInfo->getTypeLoc().getBeginLoc(),
          diag::warn_ctad_maybe_unsupported)
         << TemplateName;
     Diag(Template->getLocation(), diag::note_suppress_ctad_maybe_unsupported);
   }
 
   return DeducedType;
 }

clang/lib/Sema/SemaOverload.cpp

 /// \param PartialOverloading true if we are performing "partial" overloading
 /// based on an incomplete set of function arguments. This feature is used by
 /// code completion.
 void Sema::AddOverloadCandidate(
     FunctionDecl *Function, DeclAccessPair FoundDecl, ArrayRef<Expr *> Args,
     OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
     bool PartialOverloading, bool AllowExplicit, bool AllowExplicitConversions,
     ADLCallKind IsADLCandidate, ConversionSequenceList EarlyConversions,
-    OverloadCandidateParamOrder PO) {
+    OverloadCandidateParamOrder PO, bool AggregateCandidateDeduction) {
   const FunctionProtoType *Proto
     = dyn_cast<FunctionProtoType>(Function->getType()->getAs<FunctionType>());
   assert(Proto && "Functions without a prototype cannot be overloaded");
   assert(!Function->getDescribedFunctionTemplate() &&
          "Use AddTemplateOverloadCandidate for function templates");
 
   if (CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(Function)) {
     if (!isa<CXXConstructorDecl>(Method)) {
void Sema::AddOverloadCandidate(
   }
 
   // (C++ 13.3.2p2): A candidate function having more than m parameters
   // is viable only if the (m+1)st parameter has a default argument
   // (8.3.6). For the purposes of overload resolution, the
   // parameter list is truncated on the right, so that there are
   // exactly m parameters.
   unsigned MinRequiredArgs = Function->getMinRequiredArguments();
-  if (Args.size() < MinRequiredArgs && !PartialOverloading) {
+  if (!AggregateCandidateDeduction && Args.size() < MinRequiredArgs &&
+      !PartialOverloading) {
     // Not enough arguments.
     Candidate.Viable = false;
     Candidate.FailureKind = ovl_fail_too_few_arguments;
     return;
   }
 
   // (CUDA B.1): Check for invalid calls between targets.
   if (getLangOpts().CUDA)
void Sema::AddMethodTemplateCandidate(
   //   functions. In such a case, the candidate functions generated from each
   //   function template are combined with the set of non-template candidate
   //   functions.
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   FunctionDecl *Specialization = nullptr;
   ConversionSequenceList Conversions;
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           MethodTmpl, ExplicitTemplateArgs, Args, Specialization, Info,
-          PartialOverloading, [&](ArrayRef<QualType> ParamTypes) {
+          PartialOverloading, /*AggregateDeductionCandidate=*/false,
+          [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 MethodTmpl, ParamTypes, Args, CandidateSet, Conversions,
                 SuppressUserConversions, ActingContext, ObjectType,
                 ObjectClassification, PO);
           })) {
     OverloadCandidate &Candidate =
         CandidateSet.addCandidate(Conversions.size(), Conversions);
     Candidate.FoundDecl = FoundDecl;

 /// Add a C++ function template specialization as a candidate
 /// in the candidate set, using template argument deduction to produce
 /// an appropriate function template specialization.
 void Sema::AddTemplateOverloadCandidate(
     FunctionTemplateDecl *FunctionTemplate, DeclAccessPair FoundDecl,
     TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
     OverloadCandidateSet &CandidateSet, bool SuppressUserConversions,
     bool PartialOverloading, bool AllowExplicit, ADLCallKind IsADLCandidate,
-    OverloadCandidateParamOrder PO) {
+    OverloadCandidateParamOrder PO, bool AggregateCandidateDeduction) {
   if (!CandidateSet.isNewCandidate(FunctionTemplate, PO))
     return;
 
   // If the function template has a non-dependent explicit specification,
   // exclude it now if appropriate; we are not permitted to perform deduction
   // and substitution in this case.
   if (!AllowExplicit && isNonDependentlyExplicit(FunctionTemplate)) {
     OverloadCandidate &Candidate = CandidateSet.addCandidate();
void Sema::AddTemplateOverloadCandidate(
   //   functions. In such a case, the candidate functions generated from each
   //   function template are combined with the set of non-template candidate
   //   functions.
   TemplateDeductionInfo Info(CandidateSet.getLocation());
   FunctionDecl *Specialization = nullptr;
   ConversionSequenceList Conversions;
   if (TemplateDeductionResult Result = DeduceTemplateArguments(
           FunctionTemplate, ExplicitTemplateArgs, Args, Specialization, Info,
-          PartialOverloading, [&](ArrayRef<QualType> ParamTypes) {
+          PartialOverloading, AggregateCandidateDeduction,
+          [&](ArrayRef<QualType> ParamTypes) {
             return CheckNonDependentConversions(
                 FunctionTemplate, ParamTypes, Args, CandidateSet, Conversions,
                 SuppressUserConversions, nullptr, QualType(), {}, PO);
           })) {
     OverloadCandidate &Candidate =
         CandidateSet.addCandidate(Conversions.size(), Conversions);
     Candidate.FoundDecl = FoundDecl;
     Candidate.Function = FunctionTemplate->getTemplatedDecl();
void Sema::AddTemplateOverloadCandidate(
   }
 
   // Add the function template specialization produced by template argument
   // deduction as a candidate.
   assert(Specialization && "Missing function template specialization?");
   AddOverloadCandidate(
       Specialization, FoundDecl, Args, CandidateSet, SuppressUserConversions,
       PartialOverloading, AllowExplicit,
-      /*AllowExplicitConversions*/ false, IsADLCandidate, Conversions, PO);
+      /*AllowExplicitConversions=*/false, IsADLCandidate, Conversions, PO,
+      Info.AggregateDeductionCandidateHasMismatchedArity);
 }
 
 /// Check that implicit conversion sequences can be formed for each argument
 /// whose corresponding parameter has a non-dependent type, per DR1391's
 /// [temp.deduct.call]p10.
 bool Sema::CheckNonDependentConversions(
     FunctionTemplateDecl *FunctionTemplate, ArrayRef<QualType> ParamTypes,
     ArrayRef<Expr *> Args, OverloadCandidateSet &CandidateSet,
// Check C++17 tie-breakers for deduction guides.
     auto *Guide1 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand1.Function);
     auto *Guide2 = dyn_cast_or_null<CXXDeductionGuideDecl>(Cand2.Function);
     if (Guide1 && Guide2) {
       //  -- F1 is generated from a deduction-guide and F2 is not
       if (Guide1->isImplicit() != Guide2->isImplicit())
         return Guide2->isImplicit();
 
       //  -- F1 is the copy deduction candidate(16.3.1.8) and F2 is not
-      if (Guide1->isCopyDeductionCandidate())
+      if (Guide1->getDeductionCandidateKind() == DeductionCandidate::Copy)
         return true;
     }
   }
 
   // Check for enable_if value-based overload resolution.
   if (Cand1.Function && Cand2.Function) {
     Comparison Cmp = compareEnableIfAttrs(S, Cand1.Function, Cand2.Function);
     if (Cmp != Comparison::Equal)

clang/lib/Sema/SemaTemplate.cpp

FunctionTemplateDecl *buildDeductionGuide(
     }
 
     DC->addDecl(GuideTemplate);
     return GuideTemplate;
   }
 };
 }
 
+FunctionTemplateDecl *Sema::DeclareImplicitDeductionGuideFromInitList(
+    TemplateDecl *Template, MutableArrayRef<QualType> ParamTypes,
+    SourceLocation Loc) {
+  if (CXXRecordDecl *DefRecord =
+          cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
+    if (TemplateDecl *DescribedTemplate =
+            DefRecord->getDescribedClassTemplate())
+      Template = DescribedTemplate;
+  }
+
+  DeclContext *DC = Template->getDeclContext();
+  if (DC->isDependentContext())
+    return nullptr;
+
+  ConvertConstructorToDeductionGuideTransform Transform(
+      *this, cast<ClassTemplateDecl>(Template));
+  if (!isCompleteType(Loc, Transform.DeducedType))
+    return nullptr;
+
+  // In case we were expanding a pack when we attempted to declare deduction
+  // guides, turn off pack expansion for everything we're about to do.
+  ArgumentPackSubstitutionIndexRAII SubstIndex(*this,
+                                               /*NewSubstitutionIndex=*/-1);
+  // Create a template instantiation record to track the "instantiation" of
+  // constructors into deduction guides.
+  InstantiatingTemplate BuildingDeductionGuides(
+      *this, Loc, Template,
+      Sema::InstantiatingTemplate::BuildingDeductionGuidesTag{});
+  if (BuildingDeductionGuides.isInvalid())
+    return nullptr;
+
+  return cast<FunctionTemplateDecl>(
+      Transform.buildSimpleDeductionGuide(ParamTypes));
+}
+
 void Sema::DeclareImplicitDeductionGuides(TemplateDecl *Template,
                                           SourceLocation Loc) {
   if (CXXRecordDecl *DefRecord =
           cast<CXXRecordDecl>(Template->getTemplatedDecl())->getDefinition()) {
-    TemplateDecl *DescribedTemplate = DefRecord->getDescribedClassTemplate();
-    Template = DescribedTemplate ? DescribedTemplate : Template;
+    if (TemplateDecl *DescribedTemplate = DefRecord->getDescribedClassTemplate())
+      Template = DescribedTemplate;
   }
 
   DeclContext *DC = Template->getDeclContext();
   if (DC->isDependentContext())
     return;
 
   ConvertConstructorToDeductionGuideTransform Transform(
       *this, cast<ClassTemplateDecl>(Template));
   if (!isCompleteType(Loc, Transform.DeducedType))
     return;
 
   // Check whether we've already declared deduction guides for this template.
   // FIXME: Consider storing a flag on the template to indicate this.
   auto Existing = DC->lookup(Transform.DeductionGuideName);
   for (auto *D : Existing)
     if (D->isImplicit())
       return;
 
   // In case we were expanding a pack when we attempted to declare deduction
   // guides, turn off pack expansion for everything we're about to do.
   ArgumentPackSubstitutionIndexRAII SubstIndex(*this, -1);
   // Create a template instantiation record to track the "instantiation" of
   // constructors into deduction guides.
-  // FIXME: Add a kind for this to give more meaningful diagnostics. But can
-  // this substitution process actually fail?
-  InstantiatingTemplate BuildingDeductionGuides(*this, Loc, Template);
+  InstantiatingTemplate BuildingDeductionGuides(
+      *this, Loc, Template,
+      Sema::InstantiatingTemplate::BuildingDeductionGuidesTag{});
   if (BuildingDeductionGuides.isInvalid())
     return;
 
   // Convert declared constructors into deduction guide templates.
   // FIXME: Skip constructors for which deduction must necessarily fail (those
   // for which some class template parameter without a default argument never
   // appears in a deduced context).
   llvm::SmallPtrSet<NamedDecl *, 8> ProcessedCtors;
if (!AddedAny)
     Transform.buildSimpleDeductionGuide(std::nullopt);
 
   //    -- An additional function template derived as above from a hypothetical
   //    constructor C(C), called the copy deduction candidate.
   cast<CXXDeductionGuideDecl>(
       cast<FunctionTemplateDecl>(
           Transform.buildSimpleDeductionGuide(Transform.DeducedType))
           ->getTemplatedDecl())
-      ->setIsCopyDeductionCandidate();
+      ->setDeductionCandidateKind(DeductionCandidate::Copy);
 }
 
 /// Diagnose the presence of a default template argument on a
 /// template parameter, which is ill-formed in certain contexts.
 ///
 /// \returns true if the default template argument should be dropped.
 static bool DiagnoseDefaultTemplateArgument(Sema &S,
                                             Sema::TemplateParamListContext TPC,

clang/lib/Sema/SemaTemplateDeduction.cpp

 
 /// Verify that the given, deduced template arguments are compatible.
 ///
 /// \returns The deduced template argument, or a NULL template argument if
 /// the deduced template arguments were incompatible.
 static DeducedTemplateArgument
 checkDeducedTemplateArguments(ASTContext &Context,
                               const DeducedTemplateArgument &X,
-                              const DeducedTemplateArgument &Y) {
+                              const DeducedTemplateArgument &Y,
+                              bool AggregateCandidateDeduction = false) {
   // We have no deduction for one or both of the arguments; they're compatible.
   if (X.isNull())
     return Y;
   if (Y.isNull())
     return X;
 
   // If we have two non-type template argument values deduced for the same
   // parameter, they must both match the type of the parameter, and thus must
if (Y.getKind() == TemplateArgument::NullPtr)
           Context.getCommonSugaredType(X.getNullPtrType(), Y.getNullPtrType()),
           true);
 
     // All other combinations are incompatible.
     return DeducedTemplateArgument();
 
   case TemplateArgument::Pack: {
     if (Y.getKind() != TemplateArgument::Pack ||
-        X.pack_size() != Y.pack_size())
+        (!AggregateCandidateDeduction && X.pack_size() != Y.pack_size()))
       return DeducedTemplateArgument();
 
     llvm::SmallVector<TemplateArgument, 8> NewPack;
-    for (TemplateArgument::pack_iterator XA = X.pack_begin(),
-                                      XAEnd = X.pack_end(),
-                                         YA = Y.pack_begin();
+    for (TemplateArgument::pack_iterator
+             XA = X.pack_begin(),
+             XAEnd = X.pack_end(), YA = Y.pack_begin(), YAEnd = Y.pack_end();
          XA != XAEnd; ++XA, ++YA) {
+      if (YA != YAEnd) {
         TemplateArgument Merged = checkDeducedTemplateArguments(
             Context, DeducedTemplateArgument(*XA, X.wasDeducedFromArrayBound()),
             DeducedTemplateArgument(*YA, Y.wasDeducedFromArrayBound()));
         if (Merged.isNull() && !(XA->isNull() && YA->isNull()))
           return DeducedTemplateArgument();
         NewPack.push_back(Merged);
+      } else {
+        NewPack.push_back(*XA);
+      }
     }
 
     return DeducedTemplateArgument(
         TemplateArgument::CreatePackCopy(Context, NewPack),
         X.wasDeducedFromArrayBound() && Y.wasDeducedFromArrayBound());
   }
   }
 

 namespace {
 
 /// A scope in which we're performing pack deduction.
 class PackDeductionScope {
 public:
   /// Prepare to deduce the packs named within Pattern.
   PackDeductionScope(Sema &S, TemplateParameterList *TemplateParams,
                      SmallVectorImpl<DeducedTemplateArgument> &Deduced,
-                     TemplateDeductionInfo &Info, TemplateArgument Pattern)
-      : S(S), TemplateParams(TemplateParams), Deduced(Deduced), Info(Info) {
+                     TemplateDeductionInfo &Info, TemplateArgument Pattern,
+                     bool DeducePackIfNotAlreadyDeduced = false)
+      : S(S), TemplateParams(TemplateParams), Deduced(Deduced), Info(Info),
+        DeducePackIfNotAlreadyDeduced(DeducePackIfNotAlreadyDeduced){
     unsigned NumNamedPacks = addPacks(Pattern);
     finishConstruction(NumNamedPacks);
   }
 
   /// Prepare to directly deduce arguments of the parameter with index \p Index.
   PackDeductionScope(Sema &S, TemplateParameterList *TemplateParams,
                      SmallVectorImpl<DeducedTemplateArgument> &Deduced,
                      TemplateDeductionInfo &Info, unsigned Index)
for (auto &Pack : Packs) {
         }
         Loc = &Pack.Outer->DeferredDeduction;
       } else {
         Loc = &Deduced[Pack.Index];
       }
 
       // Check the new pack matches any previous value.
       DeducedTemplateArgument OldPack = *Loc;
-      DeducedTemplateArgument Result =
-          checkDeducedTemplateArguments(S.Context, OldPack, NewPack);
+      DeducedTemplateArgument Result = checkDeducedTemplateArguments(
+          S.Context, OldPack, NewPack, DeducePackIfNotAlreadyDeduced);
+
+      Info.AggregateDeductionCandidateHasMismatchedArity =
+          OldPack.getKind() == TemplateArgument::Pack &&
+          NewPack.getKind() == TemplateArgument::Pack &&
+          OldPack.pack_size() != NewPack.pack_size() && !Result.isNull();
 
       // If we deferred a deduction of this pack, check that one now too.
       if (!Result.isNull() && !Pack.DeferredDeduction.isNull()) {
         OldPack = Result;
         NewPack = Pack.DeferredDeduction;
         Result = checkDeducedTemplateArguments(S.Context, OldPack, NewPack);
       }
 

 
 private:
   Sema &S;
   TemplateParameterList *TemplateParams;
   SmallVectorImpl<DeducedTemplateArgument> &Deduced;
   TemplateDeductionInfo &Info;
   unsigned PackElements = 0;
   bool IsPartiallyExpanded = false;
+  bool DeducePackIfNotAlreadyDeduced = false;
   /// The number of expansions, if we have a fully-expanded pack in this scope.
   std::optional<unsigned> FixedNumExpansions;
 
   SmallVector<DeducedPack, 2> Packs;
 };
 
 } // namespace
 

 /// TDK_NonDependentConversionFailure. The callback is passed the parameter
 /// types (after substituting explicit template arguments).
 ///
 /// \returns the result of template argument deduction.
 Sema::TemplateDeductionResult Sema::DeduceTemplateArguments(
     FunctionTemplateDecl *FunctionTemplate,
     TemplateArgumentListInfo *ExplicitTemplateArgs, ArrayRef<Expr *> Args,
     FunctionDecl *&Specialization, TemplateDeductionInfo &Info,
-    bool PartialOverloading,
+    bool PartialOverloading, bool AggregateDeductionCandidate,
     llvm::function_ref<bool(ArrayRef<QualType>)> CheckNonDependent) {
   if (FunctionTemplate->isInvalidDecl())
     return TDK_Invalid;
 
   FunctionDecl *Function = FunctionTemplate->getTemplatedDecl();
   unsigned NumParams = Function->getNumParams();
 
   unsigned FirstInnerIndex = getFirstInnerIndex(FunctionTemplate);
if (!ParamExpansion) {
 
       ParamTypesForArgChecking.push_back(ParamType);
       if (auto Result = DeduceCallArgument(ParamType, ArgIdx++))
         return Result;
 
       continue;
     }
 
+    bool IsTrailingPack = ParamIdx + 1 == NumParamTypes;
+
     QualType ParamPattern = ParamExpansion->getPattern();
     PackDeductionScope PackScope(*this, TemplateParams, Deduced, Info,
-                                 ParamPattern);
+                                 ParamPattern,
+                                 AggregateDeductionCandidate && IsTrailingPack);
 
     // C++0x [temp.deduct.call]p1:
     //   For a function parameter pack that occurs at the end of the
     //   parameter-declaration-list, the type A of each remaining argument of
     //   the call is compared with the type P of the declarator-id of the
     //   function parameter pack. Each comparison deduces template arguments
     //   for subsequent positions in the template parameter packs expanded by
     //   the function parameter pack. When a function parameter pack appears
     //   in a non-deduced context [not at the end of the list], the type of
     //   that parameter pack is never deduced.
     //
     // FIXME: The above rule allows the size of the parameter pack to change
     // after we skip it (in the non-deduced case). That makes no sense, so
     // we instead notionally deduce the pack against N arguments, where N is
     // the length of the explicitly-specified pack if it's expanded by the
     // parameter pack and 0 otherwise, and we treat each deduction as a
     // non-deduced context.
-    if (ParamIdx + 1 == NumParamTypes || PackScope.hasFixedArity()) {
+    if (IsTrailingPack || PackScope.hasFixedArity()) {
       for (; ArgIdx < Args.size() && PackScope.hasNextElement();
            PackScope.nextPackElement(), ++ArgIdx) {
         ParamTypesForArgChecking.push_back(ParamPattern);
         if (auto Result = DeduceCallArgument(ParamPattern, ArgIdx))
           return Result;
       }
     } else {
       // If the parameter type contains an explicitly-specified pack that we

clang/lib/Sema/SemaTemplateInstantiate.cpp

bool Sema::CodeSynthesisContext::isInstantiationRecord() const {
   case ConstraintSubstitution:
   case ParameterMappingSubstitution:
   case ConstraintNormalization:
   case RewritingOperatorAsSpaceship:
   case InitializingStructuredBinding:
   case MarkingClassDllexported:
   case BuildingBuiltinDumpStructCall:
   case LambdaExpressionSubstitution:
+  case BuildingDeductionGuides:
     return false;
 
   // This function should never be called when Kind's value is Memoization.
   case Memoization:
     break;
   }
 
   llvm_unreachable("Invalid SynthesisKind!");

 Sema::InstantiatingTemplate::InstantiatingTemplate(
     Sema &SemaRef, SourceLocation PointOfInstantiation,
     ParameterMappingSubstitution, NamedDecl *Template,
     SourceRange InstantiationRange)
     : InstantiatingTemplate(
           SemaRef, CodeSynthesisContext::ParameterMappingSubstitution,
           PointOfInstantiation, InstantiationRange, Template) {}
 
+Sema::InstantiatingTemplate::InstantiatingTemplate(
+    Sema &SemaRef, SourceLocation PointOfInstantiation, TemplateDecl *Entity,
+    BuildingDeductionGuidesTag, SourceRange InstantiationRange)
+    : InstantiatingTemplate(
+          SemaRef, CodeSynthesisContext::BuildingDeductionGuides,
+          PointOfInstantiation, InstantiationRange, Entity) {}
+
 
 void Sema::pushCodeSynthesisContext(CodeSynthesisContext Ctx) {
   Ctx.SavedInNonInstantiationSFINAEContext = InNonInstantiationSFINAEContext;
   InNonInstantiationSFINAEContext = false;
 
   CodeSynthesisContexts.push_back(Ctx);
 
   if (!Ctx.isInstantiationRecord())
case CodeSynthesisContext::ConstraintNormalization:
           << cast<NamedDecl>(Active->Entity)->getName()
           << Active->InstantiationRange;
       break;
     case CodeSynthesisContext::ParameterMappingSubstitution:
       Diags.Report(Active->PointOfInstantiation,
                    diag::note_parameter_mapping_substitution_here)
           << Active->InstantiationRange;
       break;
+    case CodeSynthesisContext::BuildingDeductionGuides:
+      llvm_unreachable("unexpected deduction guide in instantiation stack");
     }
   }
 }
 
 std::optional<TemplateDeductionInfo *> Sema::isSFINAEContext() const {
   if (InNonInstantiationSFINAEContext)
     return std::optional<TemplateDeductionInfo *>(nullptr);
 
   bool SawLambdaSubstitution = false;
   for (SmallVectorImpl<CodeSynthesisContext>::const_reverse_iterator
case CodeSynthesisContext::RequirementParameterInstantiation:
       return Active->DeductionInfo;
 
     case CodeSynthesisContext::DeclaringSpecialMember:
     case CodeSynthesisContext::DeclaringImplicitEqualityComparison:
     case CodeSynthesisContext::DefiningSynthesizedFunction:
     case CodeSynthesisContext::InitializingStructuredBinding:
     case CodeSynthesisContext::MarkingClassDllexported:
     case CodeSynthesisContext::BuildingBuiltinDumpStructCall:
+    case CodeSynthesisContext::BuildingDeductionGuides:
       // This happens in a context unrelated to template instantiation, so
       // there is no SFINAE.
       return std::nullopt;
 
     case CodeSynthesisContext::ExceptionSpecEvaluation:
       // FIXME: This should not be treated as a SFINAE context, because
       // we will cache an incorrect exception specification. However, clang
       // bootstrap relies this! See PR31692.

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

Decl *TemplateDeclInstantiator::VisitFunctionDecl(
   if (FunctionRewriteKind != RewriteKind::None)
     adjustForRewrite(FunctionRewriteKind, D, T, TInfo, NameInfo);
 
   FunctionDecl *Function;
   if (auto *DGuide = dyn_cast<CXXDeductionGuideDecl>(D)) {
     Function = CXXDeductionGuideDecl::Create(
         SemaRef.Context, DC, D->getInnerLocStart(),
         InstantiatedExplicitSpecifier, NameInfo, T, TInfo,
-        D->getSourceRange().getEnd());
-    if (DGuide->isCopyDeductionCandidate())
-      cast<CXXDeductionGuideDecl>(Function)->setIsCopyDeductionCandidate();
+        D->getSourceRange().getEnd(), /*Ctor=*/nullptr,
+        DGuide->getDeductionCandidateKind());
     Function->setAccess(D->getAccess());
   } else {
     Function = FunctionDecl::Create(
         SemaRef.Context, DC, D->getInnerLocStart(), NameInfo, T, TInfo,
         D->getCanonicalDecl()->getStorageClass(), D->UsesFPIntrin(),
         D->isInlineSpecified(), D->hasWrittenPrototype(), D->getConstexprKind(),
         TrailingRequiresClause);
     Function->setFriendConstraintRefersToEnclosingTemplate(

clang/lib/Serialization/ASTReaderDecl.cpp

ASTDeclReader::VisitCXXRecordDeclImpl(CXXRecordDecl *D) {
 
   return Redecl;
 }
 
 void ASTDeclReader::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
   D->setExplicitSpecifier(Record.readExplicitSpec());
   D->Ctor = readDeclAs<CXXConstructorDecl>();
   VisitFunctionDecl(D);
-  D->setIsCopyDeductionCandidate(Record.readInt());
+  D->setDeductionCandidateKind(
+      static_cast<DeductionCandidate>(Record.readInt()));
 }
 
 void ASTDeclReader::VisitCXXMethodDecl(CXXMethodDecl *D) {
   VisitFunctionDecl(D);
 
   unsigned NumOverridenMethods = Record.readInt();
   if (D->isCanonicalDecl()) {
     while (NumOverridenMethods--) {

clang/lib/Serialization/ASTWriterDecl.cpp

if (ES.getExpr()) {
     Record.AddStmt(ES.getExpr());
   }
 }
 
 void ASTDeclWriter::VisitCXXDeductionGuideDecl(CXXDeductionGuideDecl *D) {
   addExplicitSpecifier(D->getExplicitSpecifier(), Record);
   Record.AddDeclRef(D->Ctor);
   VisitFunctionDecl(D);
-  Record.push_back(D->isCopyDeductionCandidate());
+  Record.push_back(static_cast<unsigned char>(D->getDeductionCandidateKind()));
   Code = serialization::DECL_CXX_DEDUCTION_GUIDE;
 }
 
 void ASTDeclWriter::VisitObjCMethodDecl(ObjCMethodDecl *D) {
   static_assert(DeclContext::NumObjCMethodDeclBits == 24,
                 "You need to update the serializer after you change the "
                 "ObjCMethodDeclBits");
 

clang/test/SemaTemplate/aggregate-deduction-candidate.cpp

+// RUN: %clang_cc1 -std=c++20 -verify -ast-dump -ast-dump-decl-types -ast-dump-filter "deduction guide" %s | FileCheck %s --strict-whitespace
+
+namespace Basic {
+  template<class T> struct A {
+    T x;
+    T y;
+  };
+
+  A a1 = {3.0, 4.0};
+  A a2 = {.x = 3.0, .y = 4.0};
+
+  A a3(3.0, 4.0);
+
+  // CHECK-LABEL: Dumping Basic::<deduction guide for A>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for A>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced class depth 0 index 0 T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for A> 'auto (T, T) -> A<T>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'T'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for A> 'auto (double, double) -> Basic::A<double>'
+  // CHECK:   |-TemplateArgument type 'double'
+  // CHECK:   | `-BuiltinType {{.*}} 'double'
+  // CHECK:   |-ParmVarDecl {{.*}} 'double':'double'
+  // CHECK:   `-ParmVarDecl {{.*}} 'double':'double'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T, T) -> A<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'A<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'A'
+  // CHECK: |-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK: | `-TemplateTypeParm {{.*}} 'T'
+  // CHECK: `-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:   `-TemplateTypeParm {{.*}} 'T'
+
+  template <typename T> struct S { // expected-note 2 {{candidate}}
+    T x;
+    T y;
+  };
+
+  template <typename T> struct C { // expected-note 10 {{candidate}}
+    S<T> s;
+    T t;
+  };
+
+  template <typename T> struct D { // expected-note 6 {{candidate}}
+    S<int> s;
+    T t;
+  };
+
+  C c1 = {1, 2}; // expected-error {{no viable}}
+  C c2 = {1, 2, 3}; // expected-error {{no viable}}
+  C c3 = {{1u, 2u}, 3};
+
+  C c4(1, 2);    // expected-error {{no viable}}
+  C c5(1, 2, 3); // expected-error {{no viable}}
+  C c6({1u, 2u}, 3);
+
+  D d1 = {1, 2}; // expected-error {{no viable}}
+  D d2 = {1, 2, 3};
+
+  D d3(1, 2); // expected-error {{no viable}}
+  // CTAD succeed but brace elision is not allowed for parenthesized aggregate init. 
+  D d4(1, 2, 3); // expected-error {{no viable}}
+
+  // CHECK-LABEL: Dumping Basic::<deduction guide for C>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for C>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for C> 'auto (S<T>, T) -> C<T>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'S<T>':'S<T>'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for C> 'auto (S<int>, int) -> Basic::C<int>'
+  // CHECK:   |-TemplateArgument type 'int'
+  // CHECK:   | `-BuiltinType {{.*}} 'int'
+  // CHECK:   |-ParmVarDecl {{.*}} 'S<int>':'Basic::S<int>'
+  // CHECK:   `-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK: FunctionProtoType {{.*}} 'auto (S<T>, T) -> C<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'C<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'C'
+  // CHECK: |-ElaboratedType {{.*}} 'S<T>' sugar dependent
+  // CHECK: | `-TemplateSpecializationType {{.*}} 'S<T>' dependent S
+  // CHECK: |   `-TemplateArgument type 'T'
+  // CHECK: |     `-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK: |       `-TemplateTypeParm {{.*}} 'T'
+  // CHECK: `-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:   `-TemplateTypeParm {{.*}} 'T'
+
+  // CHECK-LABEL: Dumping Basic::<deduction guide for D>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for D>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for D> 'auto (int, int) -> D<T>'
+  // CHECK:   |-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK:   `-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK: FunctionProtoType {{.*}} 'auto (int, int) -> D<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'D<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'D'
+  // CHECK: |-SubstTemplateTypeParmType {{.*}} 'int' sugar typename depth 0 index 0 T
+  // CHECK: | |-ClassTemplateSpecialization {{.*}} 'S'
+  // CHECK: | `-BuiltinType {{.*}} 'int'
+  // CHECK: `-SubstTemplateTypeParmType {{.*}} 'int' sugar typename depth 0 index 0 T
+  // CHECK:   |-ClassTemplateSpecialization {{.*}} 'S'
+  // CHECK:   `-BuiltinType {{.*}} 'int'
+
+  template <typename T> struct E {
+    T t;
+    decltype(t) t2;
+  };
+
+  E e1 = {1, 2};
+
+  E e2(1, 2);
+
+  // CHECK-LABEL: Dumping Basic::<deduction guide for E>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for E>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for E> 'auto (T, decltype(t)) -> E<T>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'T'
+  // CHECK: | `-ParmVarDecl {{.*}} 'decltype(t)'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for E> 'auto (int, decltype(t)) -> Basic::E<int>'
+  // CHECK:   |-TemplateArgument type 'int'
+  // CHECK:   | `-BuiltinType {{.*}} 'int'
+  // CHECK:   |-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK:   `-ParmVarDecl {{.*}} 'decltype(t)':'int'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T, decltype(t)) -> E<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'E<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'E'
+  // CHECK: |-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK: | `-TemplateTypeParm {{.*}} 'T'
+  // CHECK: `-DecltypeType {{.*}} 'decltype(t)' dependent
+  // CHECK:   `-DeclRefExpr {{.*}} 'T' lvalue Field {{.*}} 't' 'T' non_odr_use_unevaluated
+
+  template <typename T>
+  struct I {
+    using type = T;
+  };
+
+  template <typename T>
+  struct F {
+    typename I<T>::type i;
+    T t;
+  };
+
+  F f1 = {1, 2};
+
+  // CHECK-LABEL: Dumping Basic::<deduction guide for F>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for F>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for F> 'auto (typename I<T>::type, T) -> F<T>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'typename I<T>::type'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for F> 'auto (typename I<int>::type, int) -> Basic::F<int>'
+  // CHECK:   |-TemplateArgument type 'int'
+  // CHECK:   | `-BuiltinType {{.*}} 'int'
+  // CHECK:   |-ParmVarDecl {{.*}} 'typename I<int>::type':'int'
+  // CHECK:   `-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK: FunctionProtoType {{.*}} 'auto (typename I<T>::type, T) -> F<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'F<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'F'
+  // CHECK: |-DependentNameType {{.*}} 'typename I<T>::type' dependent
+  // CHECK: `-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:   `-TemplateTypeParm {{.*}} 'T'
+}
+
+namespace Array {
+  typedef __SIZE_TYPE__ size_t;
+  template <typename T, size_t N> struct A { // expected-note 2 {{candidate}}
+    T array[N];
+  };
+
+  A a1 = {{1, 2, 3}};
+  A a2 = {1, 2, 3}; // expected-error {{no viable}}
+  A a3 = {"meow"};
+  A a4 = {("meow")};
+
+  A a5({1, 2, 3});
+  A a6("meow");
+  A a7(("meow"));
+
+  // CHECK-LABEL: Dumping Array::<deduction guide for A>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for A>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-NonTypeTemplateParmDecl {{.*}} 'size_t':'unsigned long{{( long)?}}' depth 0 index 1 N
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for A> 'auto (T (&&)[N]) -> A<T, N>'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T (&&)[N]'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for A> 'auto (int (&&)[3]) -> Array::A<int, 3>'
+  // CHECK:   |-TemplateArgument type 'int'
+  // CHECK:   | `-BuiltinType {{.*}} 'int'
+  // CHECK:   |-TemplateArgument integral 3
+  // CHECK:   `-ParmVarDecl {{.*}} 'int (&&)[3]'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T (&&)[N]) -> A<T, N>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'A<T, N>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'A'
+  // CHECK: `-RValueReferenceType {{.*}} 'T (&&)[N]' dependent
+  // CHECK:   `-DependentSizedArrayType {{.*}} 'T[N]' dependent
+  // CHECK:     |-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:     | `-TemplateTypeParm {{.*}} 'T'
+  // CHECK:     `-DeclRefExpr {{.*}} 'size_t':'unsigned long{{( long)?}}' NonTypeTemplateParm {{.*}} 'N' 'size_t':'unsigned long{{( long)?}}'
+
+  // CHECK: Dumping Array::<deduction guide for A>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for A>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-NonTypeTemplateParmDecl {{.*}} 'size_t':'unsigned long{{( long)?}}' depth 0 index 1 N
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for A> 'auto (T (&)[N]) -> A<T, N>'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T (&)[N]'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for A> 'auto (const char (&)[5]) -> Array::A<const char, 5>'
+  // CHECK:   |-TemplateArgument type 'const char'
+  // CHECK:   | `-QualType {{.*}} 'const char' const
+  // CHECK:   |   `-BuiltinType {{.*}} 'char'
+  // CHECK:   |-TemplateArgument integral 5
+  // CHECK:   `-ParmVarDecl {{.*}} 'const char (&)[5]'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T (&)[N]) -> A<T, N>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'A<T, N>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'A'
+  // CHECK: `-LValueReferenceType {{.*}} 'T (&)[N]' dependent
+  // CHECK:   `-DependentSizedArrayType {{.*}} 'T[N]' dependent
+  // CHECK:     |-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:     | `-TemplateTypeParm {{.*}} 'T'
+  // CHECK:     `-DeclRefExpr {{.*}} 'size_t':'unsigned long{{( long)?}}' NonTypeTemplateParm {{.*}} 'N' 'size_t':'unsigned long{{( long)?}}'
+}
+
+namespace BraceElision {
+  template <typename T> struct A {
+    T array[2];
+  };
+
+  A a1 = {0, 1};
+
+  // CTAD succeed but brace elision is not allowed for parenthesized aggregate init. 
+  A a2(0, 1); // expected-error {{array initializer must be an initializer list}}
+
+  // CHECK-LABEL: Dumping BraceElision::<deduction guide for A>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for A>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for A> 'auto (T, T) -> A<T>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'T'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for A> 'auto (int, int) -> BraceElision::A<int>'
+  // CHECK:   |-TemplateArgument type 'int'
+  // CHECK:   | `-BuiltinType {{.*}} 'int'
+  // CHECK:   |-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK:   `-ParmVarDecl {{.*}} 'int':'int'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T, T) -> A<T>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'A<T>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'A'
+  // CHECK: |-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK: | `-TemplateTypeParm {{.*}} 'T'
+  // CHECK: `-TemplateTypeParmType {{.*}} 'T' dependent depth 0 index 0
+  // CHECK:   `-TemplateTypeParm {{.*}} 'T'
+}
+
+namespace TrailingPack {
+  template<typename... T> struct A : T... {
+  };
+
+  A a1 = {
+    []{ return 1; },
+    []{ return 2; }
+  };
+
+  A a2(
+    []{ return 1; },
+    []{ return 2; }
+  );
+
+  // CHECK-LABEL: Dumping TrailingPack::<deduction guide for A>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for A>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 ... T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for A> 'auto (T...) -> A<T...>'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T...' pack
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for A> 
+  // CHECK-SAME: 'auto (TrailingPack::(lambda at {{.*}}), TrailingPack::(lambda at {{.*}})) -> 
+  // CHECK-SAME:     TrailingPack::A<TrailingPack::(lambda at {{.*}}), TrailingPack::(lambda at {{.*}})>'
+  // CHECK: |-TemplateArgument pack
+  // CHECK: | |-TemplateArgument type 'TrailingPack::(lambda at {{.*}})'
+  // CHECK: | | `-RecordType {{.*}} 'TrailingPack::(lambda at {{.*}})'
+  // CHECK: | |   `-CXXRecord {{.*}} ''
+  // CHECK: | `-TemplateArgument type 'TrailingPack::(lambda at {{.*}})'
+  // CHECK: |   `-RecordType {{.*}} 'TrailingPack::(lambda at {{.*}})'
+  // CHECK: |     `-CXXRecord {{.*}} ''
+  // CHECK: |-ParmVarDecl {{.*}} 'TrailingPack::(lambda at {{.*}})':'TrailingPack::(lambda at {{.*}})'
+  // CHECK: `-ParmVarDecl {{.*}} 'TrailingPack::(lambda at {{.*}})':'TrailingPack::(lambda at {{.*}})'
+  // CHECK: FunctionProtoType {{.*}} 'auto (T...) -> A<T...>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'A<T...>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'A'
+  // CHECK: `-PackExpansionType {{.*}} 'T...' dependent
+  // CHECK:   `-TemplateTypeParmType {{.*}} 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
+  // CHECK:     `-TemplateTypeParm {{.*}} 'T'
+}
+
+namespace NonTrailingPack {
+  template<typename... T> struct A : T... { // expected-note 4 {{candidate}}
+    int a;
+  };
+
+  A a1 = { // expected-error {{no viable}}
+    []{ return 1; },
+    []{ return 2; }
+  };
+
+  A a2( // expected-error {{no viable}}
+    []{ return 1; },
+    []{ return 2; }
+  );
+}
+
+namespace DeduceArity {
+  template <typename... T> struct Types {};
+  template <typename... T> struct F : Types<T...>, T... {}; // expected-note 12 {{candidate}}
+
+  struct X {};
+  struct Y {};
+  struct Z {};
+  struct W { operator Y(); };
+
+  F f1 = {Types<X, Y, Z>{}, {}, {}};
+  F f2 = {Types<X, Y, Z>{}, X{}, Y{}};
+  F f3 = {Types<X, Y, Z>{}, X{}, W{}}; // expected-error {{no viable}}
+  F f4 = {Types<X>{}, {}, {}}; // expected-error {{no viable}}
+
+  F f5(Types<X, Y, Z>{}, {}, {});
+  F f6(Types<X, Y, Z>{}, X{}, Y{});
+  F f7(Types<X, Y, Z>{}, X{}, W{}); // expected-error {{no viable}}
+  F f8(Types<X>{}, {}, {}); // expected-error {{no viable}}
+
+  // CHECK-LABEL: Dumping DeduceArity::<deduction guide for F>:
+  // CHECK: FunctionTemplateDecl {{.*}} implicit <deduction guide for F>
+  // CHECK: |-TemplateTypeParmDecl {{.*}} referenced typename depth 0 index 0 ... T
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for F> 'auto (Types<T...>, T...) -> F<T...>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'Types<T...>':'Types<T...>'
+  // CHECK: | `-ParmVarDecl {{.*}} 'T...' pack
+  // CHECK: |-CXXDeductionGuideDecl {{.*}} implicit used <deduction guide for F> 
+  // CHECK-SAME: 'auto (Types<X, Y, Z>, DeduceArity::X, DeduceArity::Y, DeduceArity::Z) -> 
+  // CHECK-SAME:     DeduceArity::F<DeduceArity::X, DeduceArity::Y, DeduceArity::Z>'
+  // CHECK: | |-TemplateArgument pack
+  // CHECK: | | |-TemplateArgument type 'DeduceArity::X'
+  // CHECK: | | | `-RecordType {{.*}} 'DeduceArity::X'
+  // CHECK: | | |   `-CXXRecord {{.*}} 'X'
+  // CHECK: | | |-TemplateArgument type 'DeduceArity::Y'
+  // CHECK: | | | `-RecordType {{.*}} 'DeduceArity::Y'
+  // CHECK: | | |   `-CXXRecord {{.*}} 'Y'
+  // CHECK: | | `-TemplateArgument type 'DeduceArity::Z'
+  // CHECK: | |   `-RecordType {{.*}} 'DeduceArity::Z'
+  // CHECK: | |     `-CXXRecord {{.*}} 'Z'
+  // CHECK: | |-ParmVarDecl {{.*}} 'Types<X, Y, Z>':'DeduceArity::Types<DeduceArity::X, DeduceArity::Y, DeduceArity::Z>'
+  // CHECK: | |-ParmVarDecl {{.*}} 'DeduceArity::X':'DeduceArity::X'
+  // CHECK: | |-ParmVarDecl {{.*}} 'DeduceArity::Y':'DeduceArity::Y'
+  // CHECK: | `-ParmVarDecl {{.*}} 'DeduceArity::Z':'DeduceArity::Z'
+  // CHECK: `-CXXDeductionGuideDecl {{.*}} implicit <deduction guide for F> 'auto (Types<X>, DeduceArity::X) -> DeduceArity::F<DeduceArity::X>'
+  // CHECK:   |-TemplateArgument pack
+  // CHECK:   | `-TemplateArgument type 'DeduceArity::X'
+  // CHECK:   |   `-RecordType {{.*}} 'DeduceArity::X'
+  // CHECK:   |     `-CXXRecord {{.*}} 'X'
+  // CHECK:   |-ParmVarDecl {{.*}} 'Types<X>':'DeduceArity::Types<DeduceArity::X>'
+  // CHECK:   `-ParmVarDecl {{.*}} 'DeduceArity::X':'DeduceArity::X'
+  // CHECK: FunctionProtoType {{.*}} 'auto (Types<T...>, T...) -> F<T...>' dependent trailing_return cdecl
+  // CHECK: |-InjectedClassNameType {{.*}} 'F<T...>' dependent
+  // CHECK: | `-CXXRecord {{.*}} 'F'
+  // CHECK: |-ElaboratedType {{.*}} 'Types<T...>' sugar dependent
+  // CHECK: | `-TemplateSpecializationType {{.*}} 'Types<T...>' dependent Types
+  // CHECK: |   `-TemplateArgument type 'T...'
+  // CHECK: |     `-PackExpansionType {{.*}} 'T...' dependent
+  // CHECK: |       `-TemplateTypeParmType {{.*}} 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
+  // CHECK: |         `-TemplateTypeParm {{.*}} 'T'
+  // CHECK: `-PackExpansionType {{.*}} 'T...' dependent
+  // CHECK:   `-TemplateTypeParmType {{.*}} 'T' dependent contains_unexpanded_pack depth 0 index 0 pack
+  // CHECK:     `-TemplateTypeParm {{.*}} 'T'
+}

clang/unittests/AST/ASTImporterTest.cpp

Decl *FromTU = getTuDecl(
       A a{(int)0};
       )",
       Lang_CXX17, "input.cc");
   auto *FromD = FirstDeclMatcher<CXXDeductionGuideDecl>().match(
       FromTU,
       cxxDeductionGuideDecl(hasParameter(0, hasType(asString("A<T>")))));
   auto *ToD = Import(FromD, Lang_CXX17);
   ASSERT_TRUE(ToD);
-  EXPECT_TRUE(ToD->isCopyDeductionCandidate());
+  EXPECT_EQ(ToD->getDeductionCandidateKind(), DeductionCandidate::Copy);
   // Check that the deduced class template is also imported.
   EXPECT_TRUE(findFromTU(FromD)->Importer->GetAlreadyImportedOrNull(
       FromD->getDeducedTemplate()));
 }
 
 TEST_P(ImportFunctions, CTADUserDefinedExplicit) {
   Decl *FromTU = getTuDecl(
       R"(