[clang] Allow class with anonymous union member to be const-default-constructible even if a union member has a default member initializer (#95854)

[Rajveer100]

Resolves #95854

-- As per https://eel.is/c++draft/dcl.init#general-8.3

[llvmbot]

@llvm/pr-subscribers-clang

Author: Rajveer Singh Bharadwaj (Rajveer100)

Changes

Resolves #95854

-- As per https://eel.is/c++draft/dcl.init#general-8.3

---

Full diff: https://github.com/llvm/llvm-project/pull/96301.diff

2 Files Affected:

• (modified) clang/include/clang/AST/DeclCXX.h (+2-1)
• (added) clang/test/Sema/debug-95854.cpp (+11)

diff --git a/clang/include/clang/AST/DeclCXX.h b/clang/include/clang/AST/DeclCXX.h
index fb52ac804849d..1d1083a5d6205 100644
--- a/clang/include/clang/AST/DeclCXX.h
+++ b/clang/include/clang/AST/DeclCXX.h
@@ -1392,7 +1392,8 @@ class CXXRecordDecl : public RecordDecl {
   bool allowConstDefaultInit() const {
     return !data().HasUninitializedFields ||
            !(data().HasDefaultedDefaultConstructor ||
-             needsImplicitDefaultConstructor());
+             needsImplicitDefaultConstructor()) ||
+           hasInClassInitializer();
   }
 
   /// Determine whether this class has a destructor which has no
diff --git a/clang/test/Sema/debug-95854.cpp b/clang/test/Sema/debug-95854.cpp
new file mode 100644
index 0000000000000..1fb976558650d
--- /dev/null
+++ b/clang/test/Sema/debug-95854.cpp
@@ -0,0 +1,11 @@
+// RUN: %clang_cc1 -std=c++23 -fsyntax-only -verify %s
+//
+// expected-no-diagnostics
+
+struct A {
+  union {
+    int n = 0;
+    int m;
+  };
+};
+const A a;

[Rajveer100]

@zygoloid

[shafik]

Thank you for this fix. It needs a release note and your summary should have some more details on what the cause of the bug is and how your PR fixes it. efriedma-quic's comment provides a good framework for explaining the cause.

It also looks like your change broke some tests:

Clang :: AST/Interp/records.cpp
Clang :: SemaCXX/cxx0x-cursory-default-delete.cpp

Are these expected breaks? If so then please update these tests

[Rajveer100]

@zygoloid
Let me know if the new changes work well.

[Rajveer100]

@zygoloid
Could you review this?

[Sirraide]

I think the changes generally make sense.

[Rajveer100]

@Sirraide The tests pass, not sure about the failure though.

[shafik]

I don't think the test failures look related to the changes can you push and empty change to see if rerunning the tests comes up clean?

[Rajveer100]

Only windows failed.

PS: Pushed again to resolve merge conflict.

[cor3ntin]

Here are more tests to consider https://godbolt.org/z/M6fj5zTfq
Thanks!

[Rajveer100]

@cor3ntin
CI looks good.

[cor3ntin]

Neither GCC, EDG nor MSVC agree with us https://godbolt.org/z/Wqrhnz63a
The behavior of GCC is a bit weird but I am pretty sure the last two tests should pass
@hubert-reinterpretcast

[Rajveer100]

@shafik
Anything else that's needed?

[cor3ntin]

LGTM, Thanks!
Give a couple of days to @shafik before merging

[Rajveer100]

Thanks for the approval @cor3ntin, could you land this for me?!

[llvm-ci]

LLVM Buildbot has detected a new failure on builder openmp-offload-libc-amdgpu-runtime running on omp-vega20-1 while building clang at step 11 "Add check check-libc-amdgcn-amd-amdhsa".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/73/builds/7389

Here is the relevant piece of the build log for the reference

Step 11 (Add check check-libc-amdgcn-amd-amdhsa) failure: 1200 seconds without output running [b'ninja', b'-j 32', b'check-libc-amdgcn-amd-amdhsa'], attempting to kill
...
[2442/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.strncat_test.__hermetic__.__build__
[2443/2681] Linking CXX executable libc/test/src/locale/libc.test.src.locale.localeconv_test.__hermetic__.__build__
[2444/2681] Linking CXX executable libc/test/src/inttypes/libc.test.src.inttypes.strtoimax_test.__hermetic__.__build__
[2445/2681] Linking CXX executable libc/test/src/inttypes/libc.test.src.inttypes.strtoumax_test.__hermetic__.__build__
[2446/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.memcmp_test.__hermetic__.__build__
[2447/2681] Linking CXX executable libc/test/src/time/libc.test.src.time.clock_test.__hermetic__.__build__
[2448/2681] Linking CXX executable libc/test/src/string/libc.test.src.string.memmove_test.__hermetic__.__build__
[2449/2681] Linking CXX executable libc/test/src/time/libc.test.src.time.clock_gettime_test.__hermetic__.__build__
[2450/2681] Linking CXX executable libc/test/src/stdio/libc.test.src.stdio.asprintf_test.__hermetic__.__build__
[2451/2681] Linking CXX executable libc/test/src/stdio/libc.test.src.stdio.vasprintf_test.__hermetic__.__build__
command timed out: 1200 seconds without output running [b'ninja', b'-j 32', b'check-libc-amdgcn-amd-amdhsa'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1470.210201