[clang] Reject 'auto' storage class with type specifier in C++ #166004

osamakader · 2025-11-01T17:57:34Z

Fixes #164273

llvmbot · 2025-11-01T17:58:04Z

@llvm/pr-subscribers-clang

Author: Osama Abdelkader (osamakader)

Changes

Fixes #164273

Full diff: https://github.com/llvm/llvm-project/pull/166004.diff

3 Files Affected:

(modified) clang/include/clang/Basic/DiagnosticParseKinds.td (+2)
(modified) clang/lib/Parse/ParseDecl.cpp (+11-5)
(added) clang/test/Parser/cxx-auto-type-specifier.cpp (+29)

diff --git a/clang/include/clang/Basic/DiagnosticParseKinds.td b/clang/include/clang/Basic/DiagnosticParseKinds.td
index e5e071f43fa75..baf91b107b8c4 100644
--- a/clang/include/clang/Basic/DiagnosticParseKinds.td
+++ b/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">>;
diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 7e4a164e34eda..0044201b459b3 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -4095,11 +4095,17 @@ 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());
+          // In C++ (not C23), 'auto' cannot be combined with a type specifier.
+          if (getLangOpts().CPlusPlus && !getLangOpts().C23) {
+            isInvalid = true;
+            if (!PrevSpec)
+              PrevSpec = "";
+            DiagID = diag::err_auto_type_specifier;
+          } else {
+            // C23 allows 'auto' as storage class with type 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..7a1291b08f7d3
--- /dev/null
+++ b/clang/test/Parser/cxx-auto-type-specifier.cpp
@@ -0,0 +1,29 @@
+// 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 %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 'auto' as a storage class with type specifier is still allowed in C23.
+void g() {
+  auto int x = 1;  // c23-no-diagnostics
+  auto char c = 'a';  // c23-no-diagnostics
+}
+
+// Test that regular 'auto' (type deduction) still works in C++.
+void h() {
+  auto x = 1;      // expected-no-diagnostics
+  auto y = 2.0;   // expected-no-diagnostics
+  auto z = 'c';   // expected-no-diagnostics
+}
+

github-actions · 2025-11-01T19:16:32Z

✅ With the latest revision this PR passed the C/C++ code formatter.

shafik · 2025-11-02T03:02:00Z

I am going to be at the wg21 meeting next week, so I may not have a chance to look at this.

clang/test/CXX/drs/cwg3xx.cpp

Endilll

Changes to C++ DR tests LGTM otherwise.

clang/test/CXX/drs/cwg3xx.cpp

zygoloid · 2025-11-02T18:45:17Z

clang/include/clang/Basic/DiagnosticParseKinds.td

 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++">;


I would drop the " in C++" here. That's not actually true -- auto can be combined with a type specifier in C++98. (And saying " in C++11 onwards" or similar seems unnecessary these days.)

zygoloid · 2025-11-02T18:47:19Z

clang/lib/Parse/ParseDecl.cpp

-              << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+          // In C++ (not C23), 'auto' cannot be combined with a type specifier.
+          // However, OpenCL has its own error handling for this case.
+          if (getLangOpts().CPlusPlus && !getLangOpts().C23 &&


No need to check both CPlusPlus and C23 here; they're mutually exclusive.

zygoloid · 2025-11-02T18:49:40Z

clang/lib/Parse/ParseDecl.cpp

-            Diag(Tok, diag::ext_auto_storage_class)
-              << FixItHint::CreateRemoval(DS.getStorageClassSpecLoc());
+          // In C++ (not C23), 'auto' cannot be combined with a type specifier.
+          // However, OpenCL has its own error handling for this case.


What is the OpenCL handling? If it's the same as the general C++ handling, can we remove the OpenCL handling and enable the new code for OpenCL too?

Without excluding OpenCL, the parser emitted our generic error "'auto' cannot be combined with a type specifier" instead of the OpenCL-specific one.

The test clang/test/SemaOpenCL/storageclass.cl expects:

expected-error{{C++ for OpenCL version 2021 does not support the 'auto' storage class specifier}}

or for OpenCL C:
expected-error-re{{OpenCL C version {{1.2|3.0}} does not support the 'auto' storage class specifier}}

If we don't exclude OpenCL, the parser emits our error and the test fails because it expects the OpenCL-specific message. Excluding OpenCL (!getLangOpts().OpenCLCPlusPlus) lets Sema emit the OpenCL-specific error.

Can we just remove the OpenCL-specific diagnostic from the OpenCL code and emit the diagnostic for both C++ and OpenCL here instead?

cc @svenvh WDYT?

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 llvm#164273 Signed-off-by: Osama Abdelkader <[email protected]>

Sirraide · 2025-11-03T13:25:13Z

clang/include/clang/Basic/DiagnosticParseKinds.td

 def ext_auto_storage_class : ExtWarn<
  "'auto' storage class specifier is not permitted in C++11, and will not "


This pr makes ext_auto_storage_class unused, so we should remove it

AaronBallman · 2025-11-03T14:06:50Z

clang/lib/Parse/ParseDecl.cpp

+            // redundant error (GCC only emits one error for this case: "two or
+            // more data types in declaration").
+            isInvalid = true;
+            PrevSpec = "auto";


Suggested change

PrevSpec = "auto";

PrevSpec = Tok.getIdentifierInfo()->getNameStart();

AaronBallman · 2025-11-03T14:08:42Z

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

+// expected-error@-2 {{'auto' cannot be combined with a type specifier}}
+#else
+// expected-error@-4 {{illegal storage class on file-scoped variable}}


It'd be slightly easier to review this file if we used -verify= and different prefixes instead of preprocessor conditionals, but I don't insist on a change for this PR either unless others have strong feelings.

AaronBallman · 2025-11-03T14:11:43Z

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

 void p3example() {
  auto x = 5;
  const auto *v = &x, u = 6;
  static auto y = 0.0;


This line should also be diagnosed in C++98 mode... this seems to be a preexisting bug: https://godbolt.org/z/3cza618cx (we have a similar bug with register: https://godbolt.org/z/zs5918YY4)

GitHub is too useless to let me put this comment on the correct line, but line 40 has a similar C++98 bug: https://godbolt.org/z/xEozMhe6z because there's no type specifier there, and line 41 does as well for the same reason.

Hmmm, it seems we're treating type deduction as an extension in C++98 mode, but we don't list it in the language extensions page as such and we don't issue pedantic diagnostics for it. @zygoloid do you recall if this was an intentional extension or not? Regardless, because this is existing behavior, nothing needs to change as part of this PR.

AaronBallman · 2025-11-03T14:53:38Z

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

+void f() {
+  auto int x = 1;        // expected-error {{'auto' cannot be combined with a type specifier}}


One more test to add:

template <typename Ty> void g() { auto Ty x; } void test() { g<float>(); }

llvmbot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Nov 1, 2025

osamakader force-pushed the fix-auto-with-type branch from 46b862a to 9bc6045 Compare November 1, 2025 19:14

osamakader requested a review from Endilll as a code owner November 1, 2025 19:14

osamakader force-pushed the fix-auto-with-type branch 8 times, most recently from b0d5dd5 to eb5b66d Compare November 1, 2025 21:54

shafik requested a review from AaronBallman November 2, 2025 03:01

shafik requested review from Sirraide and zygoloid November 2, 2025 03:02

Endilll reviewed Nov 2, 2025

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clang/test/CXX/drs/cwg3xx.cpp Outdated Show resolved Hide resolved

osamakader force-pushed the fix-auto-with-type branch from eb5b66d to 649ca40 Compare November 2, 2025 13:34

Endilll reviewed Nov 2, 2025

View reviewed changes

clang/test/CXX/drs/cwg3xx.cpp Outdated Show resolved Hide resolved

osamakader force-pushed the fix-auto-with-type branch 4 times, most recently from 19c937e to 42ab668 Compare November 2, 2025 15:03

zygoloid reviewed Nov 2, 2025

View reviewed changes

osamakader force-pushed the fix-auto-with-type branch 2 times, most recently from 8473ae6 to 711223c Compare November 2, 2025 21:35

osamakader force-pushed the fix-auto-with-type branch from 711223c to 849e398 Compare November 2, 2025 22:00

Sirraide reviewed Nov 3, 2025

View reviewed changes

AaronBallman reviewed Nov 3, 2025

View reviewed changes

		def ext_auto_storage_class : ExtWarn<
		"'auto' storage class specifier is not permitted in C++11, and will not "

	PrevSpec = "auto";
	PrevSpec = Tok.getIdentifierInfo()->getNameStart();

		void f() {
		auto int x = 1; // expected-error {{'auto' cannot be combined with a type specifier}}

[clang] Reject 'auto' storage class with type specifier in C++ #166004

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[clang] Reject 'auto' storage class with type specifier in C++ #166004

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