[clang] Reject 'auto' storage class with type specifier in C++

Previously, clang allowed 'auto int x = 1;' in C++ as an extension
(for C compatibility), emitting only a warning. This was confusing
since 'auto' in C++11+ is a type specifier, not a storage class.

This patch:
- Adds a new error diagnostic 'err_auto_type_specifier'
- Updates the parser to emit an error (instead of warning) when 'auto'
  is used as a storage class with a type specifier in C++ mode
- Preserves C23 behavior where 'auto int' is valid
- Adds comprehensive tests

Fixes #164273

Signed-off-by: Osama Abdelkader <[email protected]>

Author: osamakader

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -399,6 +399,8 @@ 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">>;

clang/lib/Parse/ParseDecl.cpp

@@ -7,12 +7,12 @@
 
 auto int ao; // expected-error {{illegal storage class on file-scoped variable}}
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
 
 auto void af(); // expected-error {{illegal storage class on function}}
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
 
 register int ro; // expected-error {{illegal storage class on file-scoped variable}}
@@ -27,11 +27,11 @@ register void rf(); // expected-error {{illegal storage class on function}}
 struct S {
   auto int ao; // expected-error {{storage class specified for a member declaration}}
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
   auto void af(); // expected-error {{storage class specified for a member declaration}}
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
 
   register int ro; // expected-error {{storage class specified for a member declaration}}
@@ -40,19 +40,19 @@ struct S {
 
 void foo(auto int ap, register int rp) {
 #if __cplusplus >= 201703L
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 // expected-error@-3 {{ISO C++17 does not allow 'register' storage class specifier}}
 #elif __cplusplus >= 201103L
-// expected-warning@-5 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-5 {{'auto' cannot be combined with a type specifier in C++}}
 // expected-warning@-6 {{'register' storage class specifier is deprecated}}
 #endif
   auto int abo;
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
   auto void abf(); // expected-error {{illegal storage class on function}}
 #if __cplusplus >= 201103L // C++11 or later
-// expected-warning@-2 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+// expected-error@-2 {{'auto' cannot be combined with a type specifier in C++}}
 #endif
 
   register int rbo;

clang/test/CXX/dcl.dcl/dcl.spec/dcl.stc/p2.cpp

@@ -56,7 +56,7 @@ namespace p3_example {
   auto x = 5;
   const auto *v = &x, u = 6;
   static auto y = 0.0;
-  auto int r;  // expected-warning {{storage class}} expected-error {{file-scope}}
+  auto int r;  // expected-error {{'auto' cannot be combined with a type specifier in C++}}
 
   static_assert(is_same<decltype(x), int>(), "");
   static_assert(is_same<decltype(v), const int*>(), "");

clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/p3-1y.cpp

@@ -42,7 +42,7 @@ void p3example() {
   static auto y = 0.0;
   // In C++98: 'auto' storage class specifier is redundant and incompatible with C++0x
   // In C++0x: 'auto' storage class specifier is not permitted in C++0x, and will not be supported in future releases
-  auto int r; // expected-warning {{'auto' storage class specifier}}
+  auto int r; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
 
   same<__typeof(x), int> xHasTypeInt;
   same<__typeof(v), const int*> vHasTypeConstIntPtr;

clang/test/CXX/dcl.dcl/dcl.spec/dcl.type/dcl.spec.auto/p3.cpp

@@ -1732,11 +1732,11 @@ namespace cwg395 { // cwg395: 3.0
 namespace cwg396 { // cwg396: 3.0
   void f() {
     auto int a();
-    // since-cxx11-error@-1 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+    // since-cxx11-error@-1 {{'auto' cannot be combined with a type specifier in C++}}
     // expected-error@-2 {{illegal storage class on function}}
     int (i); // #cwg396-i
     auto int (i);
-    // since-cxx11-error@-1 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+    // since-cxx11-error@-1 {{'auto' cannot be combined with a type specifier in C++}}
     // expected-error@-2 {{redefinition of 'i'}}
     //   expected-note@#cwg396-i {{previous definition is here}}
   }

clang/test/CXX/drs/cwg3xx.cpp

@@ -0,0 +1,22 @@
+// 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
+
+// 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++}}
+  auto char c = 'a';    // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+  auto float f = 1.0f;  // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+  auto double d = 1.0;   // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+  auto long l = 1L;     // expected-error {{'auto' cannot be combined with a type specifier in C++}}
+}
+
+// Test that regular 'auto' (type deduction) still works in C++.
+void h() {
+  auto x = 1;
+  auto y = 2.0;
+  auto z = 'c';
+}
+

clang/test/Parser/cxx-auto-type-specifier.cpp

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++11
 // RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++1y
 void f() {
-  auto int a; // expected-warning {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
+  auto int a; // expected-error {{'auto' cannot be combined with a type specifier in C++}}
   int auto b; // expected-error{{cannot combine with previous 'int' declaration specifier}}
 }
 

clang/test/SemaCXX/auto-cxx0x.cpp

@@ -5,8 +5,6 @@ class C {
   auto int errx; // expected-error {{storage class specified for a member declaration}}
 #if __cplusplus <= 199711L
   // expected-warning@-2 {{'auto' storage class specifier is redundant}}
-#else
-  // expected-warning@-4 {{'auto' storage class specifier is not permitted in C++11, and will not be supported in future releases}}
 #endif
   register int erry; // expected-error {{storage class specified for a member declaration}}
   extern int errz; // expected-error {{storage class specified for a member declaration}}