[C11] Allow casting to an _Atomic-qualified type

We were failing to strip off atomic qualification when forming the cast
destination type, but properly stripping off cvr qualification. Now we
accept atomic, qualified, or unqualified destination types.

Note: the semantics of the cast still drop the qualifier, so such a
cast does not result in an atomic rvalue.

Fixes #39596

Author: AaronBallman

clang/docs/ReleaseNotes.rst

@@ -112,6 +112,8 @@ C Language Changes
   added. (`#53562 <https://github.com/llvm/llvm-project/issues/53562>`_)
 - Fixed a bug that prevented initialization of an ``_Atomic``-qualified pointer
   from a null pointer constant.
+- Fixed a bug that prevented casting to an ``_Atomic``-qualified type.
+  (`#39596 <https://github.com/llvm/llvm-project/issues/39596>`_)
 
 C2x Feature Support
 ^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaCast.cpp

@@ -65,9 +65,13 @@ namespace {
       //   If a pr-value initially has the type cv-T, where T is a
       //   cv-unqualified non-class, non-array type, the type of the
       //   expression is adjusted to T prior to any further analysis.
+      // C2x 6.5.4p6:
+      //   Preceding an expression by a parenthesized type name converts the
+      //   value of the expression to the unqualified, non-atomic version of
+      //   the named type.
       if (!S.Context.getLangOpts().ObjC && !DestType->isRecordType() &&
           !DestType->isArrayType()) {
-        DestType = DestType.getUnqualifiedType();
+        DestType = DestType.getAtomicUnqualifiedType();
       }
 
       if (const BuiltinType *placeholder =

clang/test/Sema/atomic-expr.c

@@ -205,3 +205,14 @@ _Atomic(int *) aip3 = &ai; /* expected-warning {{incompatible pointer types init
 // Test the behavior when converting the null pointer constant to an atomic
 // function pointer.
 _Atomic(int (*)(char)) afp = (void *)0;
+
+void func_18(void) {
+  // Ensure we can cast to atomic scalar types.
+  data2 = (_Atomic int)0;
+  (void)(_Atomic(int *))0;
+
+  // But that we correctly reject casts to atomic aggregate types.
+  struct S { int a; } s;
+  struct T { int a; };
+  (void)(_Atomic struct T)s; // expected-error {{used type 'struct T' where arithmetic or pointer type is required}}
+}