From df6049fd91c95e341dd80a61e5bd173ce5837131 Mon Sep 17 00:00:00 2001
From: Matheus Izvekov <mizvekov@gmail.com>
Date: Fri, 7 Jun 2024 10:32:12 -0300
Subject: [PATCH] [clang] always use resolved arguments for default argument
 deduction

This fixes a regression introduced with the changes in
https://github.com/llvm/llvm-project/pull/93433 around preservation
of TemplateName sugar in template type deduction.

Since the argument side TST is non-canonical, we have to extract the
arguments from it's canonical type.
This was done for the deduction of the TST arguments, but we missed it
for the default arguments used in the deduction of the TST name.
---
 clang/lib/Sema/SemaTemplateDeduction.cpp | 13 ++++++-------
 clang/test/SemaTemplate/cwg2398.cpp      | 16 ++++++++++++++++
 2 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 1011db2d2830dc..befeb38e1fe5bc 100644
--- a/clang/lib/Sema/SemaTemplateDeduction.cpp
+++ b/clang/lib/Sema/SemaTemplateDeduction.cpp
@@ -712,13 +712,6 @@ DeduceTemplateSpecArguments(Sema &S, TemplateParameterList *TemplateParams,
     if (const auto *TD = TNA.getAsTemplateDecl(); TD && TD->isTypeAlias())
       return TemplateDeductionResult::Success;
 
-    // Perform template argument deduction for the template name.
-    if (auto Result =
-            DeduceTemplateArguments(S, TemplateParams, TNP, TNA, Info,
-                                    SA->template_arguments(), Deduced);
-        Result != TemplateDeductionResult::Success)
-      return Result;
-
     // FIXME: To preserve sugar, the TST needs to carry sugared resolved
     // arguments.
     ArrayRef<TemplateArgument> AResolved =
@@ -726,6 +719,12 @@ DeduceTemplateSpecArguments(Sema &S, TemplateParameterList *TemplateParams,
             ->castAs<TemplateSpecializationType>()
             ->template_arguments();
 
+    // Perform template argument deduction for the template name.
+    if (auto Result = DeduceTemplateArguments(S, TemplateParams, TNP, TNA, Info,
+                                              AResolved, Deduced);
+        Result != TemplateDeductionResult::Success)
+      return Result;
+
     // Perform template argument deduction on each template
     // argument. Ignore any missing/extra arguments, since they could be
     // filled in by default arguments.
diff --git a/clang/test/SemaTemplate/cwg2398.cpp b/clang/test/SemaTemplate/cwg2398.cpp
index 45e74cce3a98c8..f7f69e9d4268ac 100644
--- a/clang/test/SemaTemplate/cwg2398.cpp
+++ b/clang/test/SemaTemplate/cwg2398.cpp
@@ -201,3 +201,19 @@ namespace consistency {
     // new-error@-1 {{ambiguous partial specializations}}
   } // namespace t2
 } // namespace consistency
+
+namespace regression1 {
+  template <typename T, typename Y> struct map {};
+  template <typename T> class foo {};
+
+  template <template <typename...> class MapType, typename Value>
+  Value bar(MapType<int, Value> map);
+
+  template <template <typename...> class MapType, typename Value>
+  Value bar(MapType<int, foo<Value>> map);
+
+  void aux() {
+    map<int, foo<int>> input;
+    bar(input);
+  }
+} // namespace regression1