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

[Haojian Wu via cfe-commits]

https://github.com/hokein created https://github.com/llvm/llvm-project/pull/75569

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

>From 0f49d91b4a22944216cff8654f9c00f892bb02be Mon Sep 17 00:00:00 2001
From: Haojian Wu <hokein.wu at gmail.com>
Date: Fri, 15 Dec 2023 08:44:57 +0100
Subject: [PATCH] [clang] Fix CTAD not respect default template arguments that
 were added after the definition.

Fixes https://github.com/llvm/llvm-project/issues/69987
---
 clang/docs/ReleaseNotes.rst      |  2 ++
 clang/lib/Sema/SemaTemplate.cpp  | 22 +++++++++++++---------
 clang/test/SemaTemplate/ctad.cpp | 10 ++++++++++
 3 files changed, 25 insertions(+), 9 deletions(-)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index f5ae6bb8925202..be826e6a44bfc7 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -685,6 +685,8 @@ 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. (#69987 <https://github.com/llvm/llvm-project/issues/69987>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaTemplate.cpp b/clang/lib/Sema/SemaTemplate.cpp
index f10abeaba0d451..450a1a1db0ba86 100644
--- a/clang/lib/Sema/SemaTemplate.cpp
+++ b/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,
@@ -2062,7 +2071,7 @@ DeclResult Sema::CheckClassTemplate(
       CheckTemplateParameterList(
           TemplateParams,
           PrevClassTemplate
-              ? PrevClassTemplate->getMostRecentDecl()->getTemplateParameters()
+              ? GetTemplateParameterList(PrevClassTemplate)
               : nullptr,
           (SS.isSet() && SemanticContext && SemanticContext->isRecord() &&
            SemanticContext->isDependentContext())
@@ -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();
 
diff --git a/clang/test/SemaTemplate/ctad.cpp b/clang/test/SemaTemplate/ctad.cpp
index 4d836839d8c346..388ed7d4cced18 100644
--- a/clang/test/SemaTemplate/ctad.cpp
+++ b/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);
+};