[clang][Diagnostic] Clarify error message for auto

clang/docs/ReleaseNotes.rst

@@ -398,6 +398,8 @@ Improvements to Clang's diagnostics
 - Clang now emits a diagnostic in case `vector_size` or `ext_vector_type`
   attributes are used with a negative size (#GH165463).
 
+- Improved diagnostic location for templates declared inside local classes.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/lib/Sema/SemaTemplate.cpp

@@ -8454,24 +8454,37 @@ Sema::CheckTemplateDeclScope(Scope *S, TemplateParameterList *TemplateParams) {
   // C++ [temp.class.spec]p6: [P2096]
   //   A partial specialization may be declared in any scope in which the
   //   corresponding primary template may be defined.
+  auto FindTemplateParamsLoc = [](TemplateParameterList *TemplateParams,
+                                  SourceLocation Fallback) {
+    SourceLocation DiagLoc = TemplateParams->getTemplateLoc();
+    if (DiagLoc.isValid())
+      return DiagLoc;
+
+    for (const NamedDecl *Param : *TemplateParams)
+      if (Param && Param->getLocation().isValid())
+        return Param->getLocation();
+
+    return Fallback;
+  };
+
   if (Ctx) {
     if (Ctx->isFileContext())
       return false;
     if (CXXRecordDecl *RD = dyn_cast<CXXRecordDecl>(Ctx)) {
       // C++ [temp.mem]p2:
       //   A local class shall not have member templates.
       if (RD->isLocalClass())
-        return Diag(TemplateParams->getTemplateLoc(),
+        return Diag(FindTemplateParamsLoc(TemplateParams, RD->getLocation()),
                     diag::err_template_inside_local_class)
-          << TemplateParams->getSourceRange();
-      else
-        return false;
+               << TemplateParams->getSourceRange();
+
+      return false;
     }
   }
 
-  return Diag(TemplateParams->getTemplateLoc(),
+  return Diag(FindTemplateParamsLoc(TemplateParams, SourceLocation()),
               diag::err_template_outside_namespace_or_class_scope)
-    << TemplateParams->getSourceRange();
+         << TemplateParams->getSourceRange();
 }
 
 /// Determine what kind of template specialization the given declaration

clang/test/SemaCXX/auto-local-class.cpp

@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 %std_cxx98-14 -fsyntax-only -verify=expected,precxx17 %s
+// RUN: %clang_cc1 %std_cxx17- -fsyntax-only -verify=expected,cxx17 %s
+
+// Check that 'auto' in a local class function parameter
+// produces the correct diagnostic depending on the language standard:
+//  * Before C++17 → "'auto' not allowed in function prototype"
+//  * C++17 and later → "templates cannot be declared inside of a local class"
+
+int main() {
+  struct A {
+    void foo(auto x) {} // precxx17-error {{'auto' not allowed in function prototype}} \
+                        // cxx17-error {{templates cannot be declared inside of a local class}}
+  };
+}

clang/test/SemaCXX/template-local-class.cpp

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+void foo() {
+  struct Local {
+    template <typename T> // expected-error {{member templates are not allowed inside local classes}}
+    void bar();
+  };
+}
+