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

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Matheus Izvekov (mizvekov)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/94756.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplateDeduction.cpp (+6-7) 
- (modified) clang/test/SemaTemplate/cwg2398.cpp (+16) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplateDeduction.cpp b/clang/lib/Sema/SemaTemplateDeduction.cpp
index 1011db2d2830d..befeb38e1fe5b 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 45e74cce3a98c..f7f69e9d4268a 100644
--- a/clang/test/SemaTemplate/cwg2398.cpp
+++ b/clang/test/SemaTemplate/cwg2398.cpp
@@ -201,3 +201,19 @@ namespace consistency {
     // new-error at -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

``````````

</details>


https://github.com/llvm/llvm-project/pull/94756