[clang] [clang] CTAD alias: Respecte explicit deduction guides defined after the first use of the alias template. (PR #125478)

[Haojian Wu via cfe-commits]

https://github.com/hokein created https://github.com/llvm/llvm-project/pull/125478

Fixes #103016

This is the last missing piece for the C++20 CTAD alias feature. No release note being added in this PR yet, I will send out a follow-up patch to mark this feature done. 

(Since the release 20 branch is cut, I think we should target on clang21). 

>From 62328c6ab0c9965712aea1d484fca4afa1294acd Mon Sep 17 00:00:00 2001
From: Haojian Wu <hokein.wu at gmail.com>
Date: Mon, 3 Feb 2025 11:55:17 +0100
Subject: [PATCH] [clang] CTAD alias: Respecte explicit deduction guides
 defined after the first use of the alias template.

---
 clang/lib/Sema/SemaTemplateDeductionGuide.cpp | 33 ++++++++++++++-----
 clang/test/SemaCXX/cxx20-ctad-type-alias.cpp  | 13 ++++++--
 2 files changed, 36 insertions(+), 10 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
index 0d079677eecc56..f5884f964d6fbd 100644
--- a/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
+++ b/clang/lib/Sema/SemaTemplateDeductionGuide.cpp
@@ -740,6 +740,24 @@ bool hasDeclaredDeductionGuides(DeclarationName Name, DeclContext *DC) {
   return false;
 }
 
+// Returns all source deduction guides associated with the declared
+// deduction guides that have the specified deduction guide name.
+llvm::DenseSet<const NamedDecl *> getSourceDeductionGuides(DeclarationName Name,
+                                                           DeclContext *DC) {
+  assert(Name.getNameKind() ==
+             DeclarationName::NameKind::CXXDeductionGuideName &&
+         "name must be a deduction guide name");
+  llvm::DenseSet<const NamedDecl *> Result;
+  for (auto *D : DC->lookup(Name)) {
+    if (const auto *FTD = dyn_cast<FunctionTemplateDecl>(D))
+      D = FTD->getTemplatedDecl();
+
+    if (const auto *GD = dyn_cast<CXXDeductionGuideDecl>(D))
+      Result.insert(GD->getSourceDeductionGuide());
+  }
+  return Result;
+}
+
 // Build the associated constraints for the alias deduction guides.
 // C++ [over.match.class.deduct]p3.3:
 //   The associated constraints ([temp.constr.decl]) are the conjunction of the
@@ -1191,13 +1209,10 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
   if (AliasTemplate->isInvalidDecl())
     return;
   auto &Context = SemaRef.Context;
-  // FIXME: if there is an explicit deduction guide after the first use of the
-  // type alias usage, we will not cover this explicit deduction guide. fix this
-  // case.
-  if (hasDeclaredDeductionGuides(
-          Context.DeclarationNames.getCXXDeductionGuideName(AliasTemplate),
-          AliasTemplate->getDeclContext()))
-    return;
+  auto SourceDeductionGuides = getSourceDeductionGuides(
+      Context.DeclarationNames.getCXXDeductionGuideName(AliasTemplate),
+      AliasTemplate->getDeclContext());
+
   auto [Template, AliasRhsTemplateArgs] =
       getRHSTemplateDeclAndArgs(SemaRef, AliasTemplate);
   if (!Template)
@@ -1210,6 +1225,8 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
 
   for (auto *G : Guides) {
     if (auto *DG = dyn_cast<CXXDeductionGuideDecl>(G)) {
+      if (SourceDeductionGuides.contains(DG))
+        continue;
       // The deduction guide is a non-template function decl, we just clone it.
       auto *FunctionType =
           SemaRef.Context.getTrivialTypeSourceInfo(DG->getType());
@@ -1252,7 +1269,7 @@ void DeclareImplicitDeductionGuidesForTypeAlias(
       continue;
     }
     FunctionTemplateDecl *F = dyn_cast<FunctionTemplateDecl>(G);
-    if (!F)
+    if (!F || SourceDeductionGuides.contains(F->getTemplatedDecl()))
       continue;
     // The **aggregate** deduction guides are handled in a different code path
     // (DeclareAggregateDeductionGuideFromInitList), which involves the tricky
diff --git a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
index 2d43e46b9e3d76..6a004769a6d4d6 100644
--- a/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
+++ b/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp
@@ -237,8 +237,17 @@ static_assert(__is_same(decltype(s.t), int));
 // explicit deduction guide.
 Foo(int) -> Foo<X>;
 AFoo s2{i};
-// FIXME: the type should be X because of the above explicit deduction guide.
-static_assert(__is_same(decltype(s2.t), int));
+static_assert(__is_same(decltype(s2.t), X));
+
+
+template<class T>
+using BFoo = AFoo<T>;
+static_assert(__is_same(decltype(BFoo(i).t), X));
+
+
+Foo(double) -> Foo<int>;
+static_assert(__is_same(decltype(AFoo(1.0).t), int));
+static_assert(__is_same(decltype(BFoo(1.0).t), int));
 } // namespace test16
 
 namespace test17 {