[clang] [clang][SemaCXX] Fix crash when using static_cast to _Atomic types in C++ (PR #207616)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: AZero13 (AZero13)

<details>
<summary>Changes</summary>

This PR fixes a compiler crash (assertion failure) in both the constant evaluator (`ExprConstant.cpp`) and LLVM CodeGen (`CGExprAgg.cpp`) when evaluating a static_cast to an _Atomic type in C++.

When evaluating casts, CastOperation::CastOperation in was unconditionally stripping the _Atomic qualifier from the destination type using getAtomicUnqualifiedType(). This behavior is correct for C23 (under C23 6.5.4p6, where cast expressions yield the non-atomic version of the type), but it is incorrect for C++ where _Atomic(T) is treated as a distinct type wrapper.

Modify CastOperation::CastOperation to only strip the _Atomic qualifier in C mode. In C++ mode, we preserve the _Atomic wrapper, ensuring that InitializationSequence properly evaluates the target type as _Atomic(T) and adds the implicit CK_NonAtomicToAtomic conversion step.

Fixes #<!-- -->207610.

---
Full diff: https://github.com/llvm/llvm-project/pull/207616.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaCast.cpp (+4-1) 
- (modified) clang/test/SemaCXX/atomic-type.cpp (+9) 


``````````diff
diff --git a/clang/lib/Sema/SemaCast.cpp b/clang/lib/Sema/SemaCast.cpp
index 133837623a7a6..bb137a4bf19d7 100644
--- a/clang/lib/Sema/SemaCast.cpp
+++ b/clang/lib/Sema/SemaCast.cpp
@@ -78,7 +78,10 @@ namespace {
       // the qualifier.
       if (!S.Context.getLangOpts().ObjC && !DestType->isRecordType() &&
           !DestType->isArrayType() && !DestType.getPointerAuth()) {
-        DestType = DestType.getAtomicUnqualifiedType();
+        if (S.Context.getLangOpts().CPlusPlus)
+          DestType = DestType.getUnqualifiedType();
+        else
+          DestType = DestType.getAtomicUnqualifiedType();
       }
 
       if (const BuiltinType *placeholder =
diff --git a/clang/test/SemaCXX/atomic-type.cpp b/clang/test/SemaCXX/atomic-type.cpp
index 3200a59bc86a2..b8f8f184da643 100644
--- a/clang/test/SemaCXX/atomic-type.cpp
+++ b/clang/test/SemaCXX/atomic-type.cpp
@@ -111,3 +111,12 @@ namespace non_trivially_copyable {
   _Atomic S s;  // expected-error {{_Atomic cannot be applied to type 'S' which is not trivially copyable}} \
                 // expected-warning {{'_Atomic' is a C11 extension}}
 }
+
+namespace static_cast_crash {
+  struct S { char a; };
+  void static_cast_non_atomic_to_atomic() {
+    _Atomic struct S a; // expected-warning {{'_Atomic' is a C11 extension}}
+    a = static_cast<_Atomic(struct S)>(S()); // expected-warning {{'_Atomic' is a C11 extension}}
+  }
+}
+

``````````

</details>


https://github.com/llvm/llvm-project/pull/207616