release/23.x: [clang] Emit diagnostic for typedef+auto missed case in C++98/C23 (#210141)

clang/lib/Sema/DeclSpec.cpp

@@ -1244,11 +1244,19 @@ void DeclSpec::CheckTypeSpec(Sema &S, const PrintingPolicy &Policy) {
                (S.getLangOpts().CPlusPlus && !S.getLangOpts().CPlusPlus11)) {
       // In C23 or C++98, convert 'auto' to storage class specifier
       if (TypeSpecType == TST_auto) {
-        // "auto int" case: Convert 'auto' to storage class specifier
-        StorageClassSpec = SCS_auto;
-        StorageClassSpecLoc = TSTLoc;
-        TypeSpecType = ConflictingTypeSpecifier;
-        TSTLoc = ConflictingTypeSpecifierLoc;
+        // "auto int" case: Convert 'auto' to storage class specifier.
+        // But typedef + any storage-class-specifier is unconditionally invalid
+        // per [dcl.stc]p1, regardless of C++ version.
+        if (StorageClassSpec == SCS_typedef) {
+          S.Diag(TSTLoc, diag::err_invalid_decl_spec_combination)
+              << "typedef" << FixItHint::CreateRemoval(TSTLoc);
+          TypeSpecType = TST_error;
+        } else {
+          StorageClassSpec = SCS_auto;
+          StorageClassSpecLoc = TSTLoc;
+          TypeSpecType = ConflictingTypeSpecifier;
+          TSTLoc = ConflictingTypeSpecifierLoc;
+        }
         // Clear the conflict tracking
         ConflictingTypeSpecifier = TST_unspecified;
         ConflictingTypeSpecifierLoc = SourceLocation();
@@ -1273,11 +1281,19 @@ void DeclSpec::CheckTypeSpec(Sema &S, const PrintingPolicy &Policy) {
           return;
         }
         // int auto (without constexpr): Convert 'auto' to storage class
-        // specifier. No type conflict error - auto is treated as storage class,
-        // not type specifier.
-        StorageClassSpec = SCS_auto;
-        StorageClassSpecLoc = ConflictingTypeSpecifierLoc;
-        // TypeSpecType already has the correct type (e.g., TST_int)
+        // specifier. But typedef + any storage-class-specifier is
+        // unconditionally invalid per [dcl.stc]p1.
+        if (StorageClassSpec == SCS_typedef) {
+          S.Diag(ConflictingTypeSpecifierLoc,
+                 diag::err_invalid_decl_spec_combination)
+              << "typedef"
+              << FixItHint::CreateRemoval(ConflictingTypeSpecifierLoc);
+          TypeSpecType = TST_error;
+        } else {
+          StorageClassSpec = SCS_auto;
+          StorageClassSpecLoc = ConflictingTypeSpecifierLoc;
+          // TypeSpecType already has the correct type (e.g., TST_int)
+        }
         // Clear the conflict tracking
         ConflictingTypeSpecifier = TST_unspecified;
         ConflictingTypeSpecifierLoc = SourceLocation();

clang/test/SemaCXX/auto-cxx98.cpp

@@ -1,8 +1,16 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++98 -Wc++11-compat 
+// RUN: %clang_cc1 -fsyntax-only -verify %s -std=c++98 -Wc++11-compat
 void f() {
   auto int a; // expected-warning {{'auto' storage class specifier is redundant and incompatible with C++11}}
   int auto b; // expected-warning {{'auto' storage class specifier is redundant and incompatible with C++11}}
   auto c; // expected-warning {{C++11 extension}} expected-error {{requires an initializer}}
   static auto d = 0; // expected-warning {{C++11 extension}}
   auto static e = 0; // expected-warning {{C++11 extension}}
 }
+
+// typedef and auto storage-class-specifier cannot appear in the same
+// decl-specifier-seq ([dcl.stc] p1). This must be diagnosed in C++98 even
+// though 'auto int' (without typedef) is valid there.
+void g() {
+  typedef auto int t1;    // expected-error {{cannot combine with previous 'typedef' declaration specifier}}
+  auto typedef int t2;    // expected-error {{cannot combine with previous 'typedef' declaration specifier}}
+}