[clang] Emit diagnostic for typedef+auto missed case in C++98/C23 by rahulana-quic · Pull Request #210141 · llvm/llvm-project

rahulana-quic · 2026-07-16T19:00:03Z

CheckTypeSpec() converted 'auto' to a storage-class specifier without
checking whether 'typedef' was already set. [dcl.stc]p1 unconditionally
forbids typedef alongside any storage-class specifier regardless of C++
version.

This change add the check for tydef in the code handling auto.

llvmorg-github-actions · 2026-07-16T19:00:42Z

@llvm/pr-subscribers-clang

Author: Rahul (rahulana-quic)

Changes

CheckTypeSpec() converted 'auto' to a storage-class specifier without
checking whether 'typedef' was already set. [dcl.stc]p1 unconditionally
forbids typedef alongside any storage-class specifier regardless of C++
version.

This change add the check for tydef in the code handling auto.

Full diff: https://github.com/llvm/llvm-project/pull/210141.diff

2 Files Affected:

(modified) clang/lib/Sema/DeclSpec.cpp (+53-22)
(modified) clang/test/SemaCXX/auto-cxx98.cpp (+9-1)

diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 4d20657d5e517..b6764178e89ba 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/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();
@@ -1351,23 +1367,38 @@ void DeclSpec::CheckTypeSpec(Sema &S, const PrintingPolicy &Policy) {
       TypeSpecType = TST_error;
       TypeSpecOwned = false;
     } else if (!S.getLangOpts().CPlusPlus) {
-      // For C, C23, etc., convert 'auto' to storage class specifier
-      // (This is already handled above for C23, but keep for other C dialects)
-      // In C, C23, OpenCL, etc., convert 'auto' to storage class specifier
+      // For pre-C23 C: 'auto' is a storage-class specifier, not a type
+      // specifier. C23 is handled above; this branch covers C17 and earlier.
       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.
+        // typedef + any storage-class-specifier is invalid per [dcl.stc]p1.
+        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();
       } else if (ConflictingTypeSpecifier == TST_auto) {
-        // "int auto" case: Convert 'auto' to storage class specifier
-        StorageClassSpec = SCS_auto;
-        StorageClassSpecLoc = ConflictingTypeSpecifierLoc;
-        // TypeSpecType already has the correct type (e.g., TST_int)
+        // "int auto" case: Convert 'auto' to storage class specifier.
+        // typedef + any storage-class-specifier is 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();
diff --git a/clang/test/SemaCXX/auto-cxx98.cpp b/clang/test/SemaCXX/auto-cxx98.cpp
index 1e28d0635a48d..db2036d462532 100644
--- a/clang/test/SemaCXX/auto-cxx98.cpp
+++ b/clang/test/SemaCXX/auto-cxx98.cpp
@@ -1,4 +1,4 @@
-// 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}}
@@ -6,3 +6,11 @@ void f() {
   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}}
+}

rahulana-quic · 2026-07-16T19:01:01Z

cc: @efriedma-quic

efriedma-quic · 2026-07-16T21:27:55Z

CC @osamakader

Endilll

LGTM

to268

I would like to see a test case for the pre-C23 behavior.

AaronBallman

We should also add a release note to clang/docs/ReleaseNotes.md but that can be done later because this might eventually be something we want to cherry-pick to the 23.x branch.

AaronBallman · 2026-07-17T13:22:22Z

+// 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}}


Aren't we solving the wrong regression?

https://godbolt.org/z/xMrh3rzfv

In 22.x we're diagnosing that you cannot combine auto and int in C++98 mode. This patch restores that behavior, but in C++98, we can combine auto and int (https://godbolt.org/z/K8GWKr7Yj), what we cannot combine is auto and typedef. But we dropped the diagnostic which said 'auto' not allowed in typedef. I think that was the diagnostic we wanted to retain, wasn't it?

In 22.x we're diagnosing that you cannot combine auto and int in C++98 mode. This patch restores that behavior

This patch does not restore that behavior because it does not really modify the auto+int-without-typedef path in any way. I am also confused by what you said about the 22.x behavior since I see here that auto+int (no typedef) works without any issues.

But we dropped the diagnostic which said 'auto' not allowed in typedef

We just emit the diagnostic at a different/earlier point but it is still an equivalent diagnostic which correctly points out the problem:

<stdin>:1:20: error: cannot combine with previous 'typedef' declaration specifier 1 | void g() { typedef auto int t1; } | ^~~~ 1 error generated.

https://godbolt.org/z/xMrh3rzfv

In the current behavior, even though we emit <source>:2:11: error: 'auto' not allowed in typedef we also emit <source>:2:16: error: cannot combine with previous 'auto' declaration specifier which is wrong because like you said, this should be allowed. After the patch, the only diagnostic is what I pointed to above and is correct.

We just emit the diagnostic at a different/earlier point but it is still an equivalent diagnostic which correctly points out the problem:

Ah, I think this was my mistake coupled with the diagnostic wording being a bit more ambiguous unless you see the caret. The new behavior is saying cannot combine with the previous typedef specifier, it previously was saying auto specifier.

I think auto not allowed in typedef was more clear, but this is definitely equivalent when I was thinking it wasn't, sorry about that!

CheckTypeSpec() converted 'auto' to a storage-class specifier without checking whether 'typedef' was already set. [dcl.stc]p1 unconditionally forbids typedef alongside any storage-class specifier regardless of C++ version. This change adds the check for tydef in the code handling auto in the C++98/C23 paths.

rahulana-quic · 2026-07-20T05:49:10Z

I would like to see a test case for the pre-C23 behavior.

I jumped the gun a little and added code to handle some extra cases. Pre-C23 was not broken before and that behavior is left unchanged.

AaronBallman

LGTM! No release note needed because I presume this should be backported to 23.x.

AaronBallman · 2026-07-22T13:59:36Z

+// 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}}


We just emit the diagnostic at a different/earlier point but it is still an equivalent diagnostic which correctly points out the problem:

Ah, I think this was my mistake coupled with the diagnostic wording being a bit more ambiguous unless you see the caret. The new behavior is saying cannot combine with the previous typedef specifier, it previously was saying auto specifier.

I think auto not allowed in typedef was more clear, but this is definitely equivalent when I was thinking it wasn't, sorry about that!

rahulana-quic · 2026-07-22T14:31:50Z

@AaronBallman thanks for reviewing. Could you please merge this for me?

AaronBallman · 2026-07-22T14:41:15Z

@AaronBallman thanks for reviewing. Could you please merge this for me?

Sure can! Thank you for the poke. :-)

osamakader · 2026-07-22T15:54:13Z

LGTM too, thanks for the fix!

efriedma-quic · 2026-07-22T17:09:37Z

/cherry-pick 34436db

efriedma-quic · 2026-07-22T18:37:26Z

/cherry-pick 34436db

llvmbot · 2026-07-22T22:13:00Z

/pull-request #211391

…vm#210141) CheckTypeSpec() converted 'auto' to a storage-class specifier without checking whether 'typedef' was already set. [dcl.stc]p1 unconditionally forbids typedef alongside any storage-class specifier regardless of C++ version. This change add the check for tydef in the code handling auto. (cherry picked from commit 34436db)

…vm#210141) CheckTypeSpec() converted 'auto' to a storage-class specifier without checking whether 'typedef' was already set. [dcl.stc]p1 unconditionally forbids typedef alongside any storage-class specifier regardless of C++ version. This change add the check for tydef in the code handling auto.

…/C23 (llvm#210141)" This reverts commit 34436db.

[clang] Add test for typedef+auto diagnostic in C++98/pre-C23 C

62c7c85

llvmorg-github-actions Bot added clang Clang issues not falling into any other category clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Jul 16, 2026

efriedma-quic requested review from Endilll and to268 July 16, 2026 21:27

Endilll reviewed Jul 17, 2026

View reviewed changes

Endilll requested a review from AaronBallman July 17, 2026 04:17

to268 reviewed Jul 17, 2026

View reviewed changes

AaronBallman requested changes Jul 17, 2026

View reviewed changes

rahulana-quic force-pushed the clang-typedef-auto-diagnostic branch from ca4f116 to f27d290 Compare July 20, 2026 05:45

rahulana-quic changed the title ~~[clang] Emit diagnostic for typedef+auto missed case in C++98/pre-C23 C~~ [clang] Emit diagnostic for typedef+auto missed case in C++98/C23 Jul 20, 2026

AaronBallman approved these changes Jul 22, 2026

View reviewed changes

AaronBallman merged commit 34436db into llvm:main Jul 22, 2026
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AaronBallman added this to the LLVM 23.x Release milestone Jul 22, 2026

github-project-automation Bot added this to LLVM Release Status Jul 22, 2026

github-project-automation Bot moved this to Done in LLVM Release Status Jul 22, 2026

to268 added a commit to to268/llvm-project that referenced this pull request Aug 10, 2026

Revert "[clang] Emit diagnostic for typedef+auto missed case in C++98…

a16c6a2

…/C23 (llvm#210141)" This reverts commit 34436db.

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