[clang] Fix CTAD not respect default template arguments that were added after the definition. (#75569)

Fixes https://github.com/llvm/llvm-project/issues/69987

clang/docs/ReleaseNotes.rst

@@ -688,6 +688,9 @@ Bug Fixes in This Version
   (`#62157 <https://github.com/llvm/llvm-project/issues/62157>`_) and
   (`#64885 <https://github.com/llvm/llvm-project/issues/64885>`_) and
   (`#65568 <https://github.com/llvm/llvm-project/issues/65568>`_)
+- Fix an issue where clang doesn't respect detault template arguments that
+  are added in a later redeclaration for CTAD.
+  Fixes (#69987 <https://github.com/llvm/llvm-project/issues/69987>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaTemplate.cpp

@@ -1824,6 +1824,15 @@ static void SetNestedNameSpecifier(Sema &S, TagDecl *T,
     T->setQualifierInfo(SS.getWithLocInContext(S.Context));
 }
 
+// Returns the template parameter list with all default template argument
+// information.
+static TemplateParameterList *GetTemplateParameterList(TemplateDecl *TD) {
+  // Make sure we get the template parameter list from the most
+  // recent declaration, since that is the only one that is guaranteed to
+  // have all the default template argument information.
+  return cast<TemplateDecl>(TD->getMostRecentDecl())->getTemplateParameters();
+}
+
 DeclResult Sema::CheckClassTemplate(
     Scope *S, unsigned TagSpec, TagUseKind TUK, SourceLocation KWLoc,
     CXXScopeSpec &SS, IdentifierInfo *Name, SourceLocation NameLoc,
@@ -2061,13 +2070,13 @@ DeclResult Sema::CheckClassTemplate(
   if (!(TUK == TUK_Friend && CurContext->isDependentContext()) &&
       CheckTemplateParameterList(
           TemplateParams,
-          PrevClassTemplate
-              ? PrevClassTemplate->getMostRecentDecl()->getTemplateParameters()
-              : nullptr,
+          PrevClassTemplate ? GetTemplateParameterList(PrevClassTemplate)
+                            : nullptr,
           (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
            SemanticContext->isDependentContext())
               ? TPC_ClassTemplateMember
-              : TUK == TUK_Friend ? TPC_FriendClassTemplate : TPC_ClassTemplate,
+          : TUK == TUK_Friend ? TPC_FriendClassTemplate
+                              : TPC_ClassTemplate,
           SkipBody))
     Invalid = true;
 
@@ -2298,7 +2307,7 @@ struct ConvertConstructorToDeductionGuideTransform {
     //    -- The template parameters are the template parameters of the class
     //       template followed by the template parameters (including default
     //       template arguments) of the constructor, if any.
-    TemplateParameterList *TemplateParams = Template->getTemplateParameters();
+    TemplateParameterList *TemplateParams = GetTemplateParameterList(Template);
     if (FTD) {
       TemplateParameterList *InnerParams = FTD->getTemplateParameters();
       SmallVector<NamedDecl *, 16> AllParams;
@@ -2424,7 +2433,7 @@ struct ConvertConstructorToDeductionGuideTransform {
       Params.push_back(NewParam);
     }
 
-    return buildDeductionGuide(Template->getTemplateParameters(), nullptr,
+    return buildDeductionGuide(GetTemplateParameterList(Template), nullptr,
                                ExplicitSpecifier(), TSI, Loc, Loc, Loc);
   }
 
@@ -5956,12 +5965,7 @@ bool Sema::CheckTemplateArgumentList(
   // template.
   TemplateArgumentListInfo NewArgs = TemplateArgs;
 
-  // Make sure we get the template parameter list from the most
-  // recent declaration, since that is the only one that is guaranteed to
-  // have all the default template argument information.
-  TemplateParameterList *Params =
-      cast<TemplateDecl>(Template->getMostRecentDecl())
-          ->getTemplateParameters();
+  TemplateParameterList *Params = GetTemplateParameterList(Template);
 
   SourceLocation RAngleLoc = NewArgs.getRAngleLoc();
 

clang/test/SemaTemplate/ctad.cpp

@@ -44,3 +44,13 @@ namespace Access {
   };
   D z = {Z(), {}};
 }
+
+namespace GH69987 {
+template<class> struct X {};
+template<class = void> struct X;
+X x;
+
+template<class T, class B> struct Y { Y(T); };
+template<class T, class B=void> struct Y ;
+Y y(1);
+};