[clang] Fix crash on attempt to initialize union with flexible array member

[Fznamznon]

Summary

Due to missing check on union, there was a null expression
added to init list that caused crash later.

Fixes https://github.com/llvm/llvm-project/issues/61746

[aaron.ballman]

The precommit CI failures seem unrelated to these changes.

clang/test/Sema/init.c

168–169 ↗

(On Diff #521595)

I think we might want to move this to its own test file so we can test the various edge cases. I'd like to see a RUN line that tests explicitly in -fms-compatibility mode to demonstrate that we do not issue this diagnostic, and RUN lines for C++ mode as well.

[Fznamznon]

Move the test to a separate file, test C++ and MSVC compatibility mode

[shafik]

LGTM after addressing Aaron's comment

[Fznamznon]

@aaron.ballman , WDYT?

[aaron.ballman]

LGTM with a minor tweak to the test, thanks!

clang/test/Sema/flexible-array-in-union.c

2–17

Slight tweak to get rid of the preprocessor stuff and simplify the test a bit; NFC.

[Fznamznon]

clang/test/Sema/flexible-array-in-union.c

2–17

Neat! Thank you.

clang/docs/ReleaseNotes.rst

- Fix an assertion when instantiating the body of a Class Template Specialization
   template arguments on the containing class. This fixes:
   (`#60778 <https://github.com/llvm/llvm-project/issues/60778>`_).
 - Fix a crash when an enum constant has a dependent-type recovery expression for
   C.
   (`#62446 <https://github.com/llvm/llvm-project/issues/62446>`_).
 - Propagate the value-dependent bit for VAArgExpr. Fixes a crash where a
   __builtin_va_arg call has invalid arguments.
   (`#62711 <https://github.com/llvm/llvm-project/issues/62711>`_).
+- Fix crash on attempt to initialize union with flexible array member.
+  (`#61746 <https://github.com/llvm/llvm-project/issues/61746>`_).
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Bug Fixes to Attribute Support
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - Fixed a bug where attribute annotations on type specifiers (enums, classes,
   structs, unions, and scoped enums) were not properly ignored, resulting in

clang/lib/Sema/SemaInit.cpp

else if (RDecl->isUnion() && isa<CXXRecordDecl>(RDecl) &&
           break;
         }
       }
     } else {
       // The fields beyond ILE->getNumInits() are default initialized, so in
       // order to leave them uninitialized, the ILE is expanded and the extra
       // fields are then filled with NoInitExpr.
       unsigned NumElems = numStructUnionElements(ILE->getType());
-      if (RDecl->hasFlexibleArrayMember())
+      if (!RDecl->isUnion() && RDecl->hasFlexibleArrayMember())
         ++NumElems;
       if (!VerifyOnly && ILE->getNumInits() < NumElems)
         ILE->resizeInits(SemaRef.Context, NumElems);
 
       unsigned Init = 0;
 
       if (auto *CXXRD = dyn_cast<CXXRecordDecl>(RDecl)) {
         for (auto &Base : CXXRD->bases()) {

clang/test/Sema/flexible-array-in-union.c

+// RUN: %clang_cc1 %s -verify=c -fsyntax-only
+// RUN: %clang_cc1 %s -verify -fsyntax-only -x c++
+// RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility
+// RUN: %clang_cc1 %s -verify -fsyntax-only -fms-compatibility -x c++
+
+// The test checks that an attempt to initialize union with flexible array
+// member with an initializer list doesn't crash clang.
+
+
+union { char x[]; } r = {0}; // c-error {{flexible array member 'x' in a union is not allowed}}
+
+// expected-no-diagnostics
+