[clang] always use resolved arguments for default argument deduction (#94756)

Author: mizvekov

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -712,20 +712,19 @@ 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 =
         SA->getCanonicalTypeInternal()
             ->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.

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