[Sema] Error on 'auto' storage class in C++11

[adityak2006]

This patch changes the C-style 'auto' storage class specifier from a warning to an error in C++11 and later modes.

This was previously allowed as an extension for C compatibility, but it's no longer appropriate for modern C++.

Fixes #162473

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[llvmbot]

@llvm/pr-subscribers-clang

Author: Aditya Kolgane (adityak2006)

Changes

This patch changes the C-style 'auto' storage class specifier from a warning to an error in C++11 and later modes.

This was previously allowed as an extension for C compatibility, but it's no longer appropriate for modern C++.

Fixes #162473

---

Full diff: https://github.com/llvm/llvm-project/pull/167162.diff

3 Files Affected:

• (modified) clang/include/clang/Basic/DiagnosticParseKinds.td (+2)
• (modified) clang/lib/Parse/ParseDecl.cpp (+13-5)
• (added) clang/test/Parser/cxx-auto-type-specifier.cpp (+13)

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index aa0ccb0c05101..d900001c8cde0 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/clang/include/clang/Basic/DiagnosticParseKinds.td
@@ -402,6 +402,8 @@ def err_requires_clause_on_declarator_not_declaring_a_function : Error<
   "trailing requires clause can only be used when declaring a function">;
 def err_requires_clause_inside_parens : Error<
   "trailing requires clause should be placed outside parentheses">;
+def err_auto_type_specifier:Error<
+  "'auto' cannot be combined with a type specifier in C++">;
 def ext_auto_storage_class : ExtWarn<
   "'auto' storage class specifier is not permitted in C++11, and will not "
   "be supported in future releases">, InGroup<DiagGroup<"auto-storage-class">>;
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 5fcb659768655..65ed4db468b65 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4095,11 +4095,19 @@ void Parser::ParseDeclarationSpecifiers(
     case tok::kw_auto:
       if (getLangOpts().CPlusPlus11 || getLangOpts().C23) {
         if (isKnownToBeTypeSpecifier(GetLookAheadToken(1))) {
-          isInvalid = DS.SetStorageClassSpec(Actions, DeclSpec::SCS_auto, Loc,
-                                             PrevSpec, DiagID, Policy);
-          if (!isInvalid && !getLangOpts().C23)
-            Diag(Tok, diag::ext_auto_storage_class)
-              << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+          // auto cannot be combined with a type specifier in C++ (except C23).
+          if (getLangOpts().CPlusPlus && !getLangOpts().C23) {
+            if (!PrevSpec) {
+              PrevSpec = "";
+            }
+            isInvalid = true;
+            DiagID = diag::err_auto_type_specifier;
+          } else {
+            // In C23, 'auto' followed by a type specifier is a storage class specifier.
+            isInvalid = DS.SetStorageClassSpec(Actions, DeclSpec::SCS_auto, Loc, PrevSpec, DiagID, Policy);
+          }
+
+
         } else
           isInvalid = DS.SetTypeSpecType(DeclSpec::TST_auto, Loc, PrevSpec,
                                          DiagID, Policy);
diff --git a/clang/test/Parser/cxx-auto-type-specifier.cpp b/clang/test/Parser/cxx-auto-type-specifier.cpp
new file mode 100644
index 0000000000000..d8bce4b7170cf
--- /dev/null
+++ b/clang/test/Parser/cxx-auto-type-specifier.cpp
@@ -0,0 +1,13 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++11 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++14 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++17 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++20 %s
+// RUN: %clang_cc1 -fsyntax-only -verify -std=c++23 %s
+// RUN: %clang_cc1 -fsyntax-only -verify=c23 -std=c23 -x c %s
+
+// Test that 'auto' cannot be combined with a type specifier in C++.
+void f() {
+  auto int x = 1; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+}
+
+// c23-no-diagnostics