[clang] [Clang] Handle default template arguments for alias CTAD guides (PR #134807)

[Younan Zhang via cfe-commits]

https://github.com/zyn0217 created https://github.com/llvm/llvm-project/pull/134807

It's possible that some deduced template arguments come from default arguments, not just from the return type. So we need to recursively visit the default arguments of the parameter if it's referenced, thereby the template parameter referenced by the defualt arguments could come along to the synthesized deduction guide.

Fixes https://github.com/llvm/llvm-project/issues/133132

>From d93117d955ba3a94c68cba1ec968662ae2eed572 Mon Sep 17 00:00:00 2001
From: Younan Zhang <zyn7109 at gmail.com>
Date: Tue, 8 Apr 2025 16:16:53 +0800
Subject: [PATCH] [Clang] Handle default template arguments for alias CTAD
 guides

It's possible that some deduced template arguments come from default
arguments, not just from the return type. So we need to recursively
visit the default arguments if the parameter is referenced, thereby the
template parameter referenced by the defualt arguments could come along
to the synthesized deduction guide.
---
 clang/docs/ReleaseNotes.rst                   |  1 +
 clang/lib/Sema/SemaTemplateDeductionGuide.cpp | 17 ++++++++
 clang/test/SemaTemplate/deduction-guide.cpp   | 39 +++++++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 5217e04b5e83f..a914657731dba 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -384,6 +384,7 @@ Bug Fixes to C++ Support
 - Clang no longer crashes when trying to unify the types of arrays with
   certain differences in qualifiers (this could happen during template argument
   deduction or when building a ternary operator). (#GH97005)
+- Fixed type alias CTAD issues involving default template arguments. (#GH133132)
 - The initialization kind of elements of structured bindings
   direct-list-initialized from an array is corrected to direct-initialization.
 - Clang no longer crashes when a coroutine is declared ``[[noreturn]]``. (#GH127327)
diff --git a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
index b4863cefc3fb4..30376ca774384 100644
--- a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
+++ b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
@@ -690,6 +690,23 @@ SmallVector<unsigned> TemplateParamsReferencedInTemplateArgumentList(
   SemaRef.MarkUsedTemplateParameters(
       DeducedArgs, TemplateParamsList->getDepth(), ReferencedTemplateParams);
 
+  auto MarkDefaultArgs = [&](auto *Param) {
+    if (!Param || !Param->hasDefaultArgument())
+      return;
+    SemaRef.MarkUsedTemplateParameters(
+        Param->getDefaultArgument().getArgument(),
+        TemplateParamsList->getDepth(), ReferencedTemplateParams);
+  };
+
+  for (unsigned Index = 0; Index < TemplateParamsList->size(); ++Index) {
+    if (!ReferencedTemplateParams[Index])
+      continue;
+    auto *Param = TemplateParamsList->getParam(Index);
+    MarkDefaultArgs(dyn_cast<TemplateTypeParmDecl>(Param));
+    MarkDefaultArgs(dyn_cast<NonTypeTemplateParmDecl>(Param));
+    MarkDefaultArgs(dyn_cast<TemplateTemplateParmDecl>(Param));
+  }
+
   SmallVector<unsigned> Results;
   for (unsigned Index = 0; Index < TemplateParamsList->size(); ++Index) {
     if (ReferencedTemplateParams[Index])
diff --git a/clang/test/SemaTemplate/deduction-guide.cpp b/clang/test/SemaTemplate/deduction-guide.cpp
index 6db132ca37c7e..76b6cb051b7dd 100644
--- a/clang/test/SemaTemplate/deduction-guide.cpp
+++ b/clang/test/SemaTemplate/deduction-guide.cpp
@@ -771,3 +771,42 @@ D d(24);
 // CHECK-NEXT:  `-ParmVarDecl {{.+}} 'U'
 
 } // namespace GH132616_DeductionGuide
+
+namespace GH133132 {
+
+template <class _Ty>
+struct A {};
+
+template <class T = int, class U = T>
+using AA = A<U>;
+
+AA a{};
+
+// CHECK-LABEL: Dumping GH133132::<deduction guide for AA>:
+// CHECK-NEXT:  FunctionTemplateDecl {{.+}} implicit <deduction guide for AA>
+// CHECK-NEXT:  |-TemplateTypeParmDecl {{.+}} class depth 0 index 0 T
+// CHECK-NEXT:  | `-TemplateArgument type 'int'
+// CHECK-NEXT:  |   `-BuiltinType {{.+}} 'int'
+// CHECK-NEXT:  |-TemplateTypeParmDecl {{.+}} class depth 0 index 1 U
+// CHECK-NEXT:  | `-TemplateArgument type 'T':'type-parameter-0-0'
+// CHECK-NEXT:  |   `-TemplateTypeParmType {{.+}} 'T' dependent depth 0 index 0
+// CHECK-NEXT:  |     `-TemplateTypeParm {{.+}} 'T'
+// CHECK-NEXT:  |-TypeTraitExpr {{.+}} 'bool' __is_deducible
+// CHECK-NEXT:  | |-DeducedTemplateSpecializationType {{.+}} 'GH133132::AA' dependent
+// CHECK-NEXT:  | | `-name: 'GH133132::AA'
+// CHECK-NEXT:  | |   `-TypeAliasTemplateDecl {{.+}} AA
+// CHECK-NEXT:  | `-TemplateSpecializationType {{.+}} 'A<U>' dependent
+// CHECK-NEXT:  |   |-name: 'A':'GH133132::A' qualified
+// CHECK-NEXT:  |   | `-ClassTemplateDecl {{.+}} A
+// CHECK-NEXT:  |   `-TemplateArgument type 'U':'type-parameter-0-1'
+// CHECK-NEXT:  |     `-SubstTemplateTypeParmType {{.+}} 'U' sugar dependent class depth 0 index 0 _Ty
+// CHECK-NEXT:  |       |-FunctionTemplate {{.+}} '<deduction guide for A>'
+// CHECK-NEXT:  |       `-TemplateTypeParmType {{.+}} 'U' dependent depth 0 index 1
+// CHECK-NEXT:  |         `-TemplateTypeParm {{.+}} 'U'
+// CHECK-NEXT:  |-CXXDeductionGuideDecl {{.+}} implicit <deduction guide for AA> 'auto () -> A<U>'
+// CHECK-NEXT:  `-CXXDeductionGuideDecl {{.+}} implicit used <deduction guide for AA> 'auto () -> A<int>' implicit_instantiation
+// CHECK-NEXT:    |-TemplateArgument type 'int'
+// CHECK-NEXT:    | `-BuiltinType {{.+}} 'int'
+// CHECK-NEXT:    `-TemplateArgument type 'int'
+// CHECK-NEXT:      `-BuiltinType {{.+}} 'int'
+}