[Sema][SVE] Reject atomic sizeless types

It would be difficult to guarantee atomicity for sizeless types,
so the SVE ACLE makes atomic sizeless types invalid.  As it happens,
we already rejected them before the patch, but for the wrong reason:

  error: _Atomic cannot be applied to type 'svint8_t' (aka '__SVInt8_t')
  which is not trivially copyable

The SVE types should be treated as trivially copyable; a later
patch fixes that.

Differential Revision: https://reviews.llvm.org/D75734

Author: rsandifo-arm

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5925,7 +5925,7 @@ def err_func_def_incomplete_result : Error<
   "incomplete result type %0 in function definition">;
 def err_atomic_specifier_bad_type : Error<
   "_Atomic cannot be applied to "
-  "%select{incomplete |array |function |reference |atomic |qualified |}0type "
+  "%select{incomplete |array |function |reference |atomic |qualified |sizeless |}0type "
   "%1 %select{||||||which is not trivially copyable}0">;
 
 // Expressions.

clang/lib/Sema/SemaType.cpp

@@ -8564,9 +8564,11 @@ QualType Sema::BuildAtomicType(QualType T, SourceLocation Loc) {
       DisallowedKind = 4;
     else if (T.hasQualifiers())
       DisallowedKind = 5;
+    else if (T->isSizelessType())
+      DisallowedKind = 6;
     else if (!T.isTriviallyCopyableType(Context))
       // Some other non-trivially-copyable type (probably a C++ class)
-      DisallowedKind = 6;
+      DisallowedKind = 7;
 
     if (DisallowedKind != -1) {
       Diag(Loc, diag::err_atomic_specifier_bad_type) << DisallowedKind << T;

clang/test/Sema/sizeless-1.c

@@ -84,6 +84,7 @@ void func(int sel) {
   const volatile svint8_t const_volatile_int8 = local_int8; // expected-note {{declared const here}}
   const volatile svint8_t uninit_const_volatile_int8;
 
+  _Atomic svint8_t atomic_int8;      // expected-error {{_Atomic cannot be applied to sizeless type 'svint8_t'}}
   __restrict svint8_t restrict_int8; // expected-error {{requires a pointer or reference}}
 
   _Bool test_int8 = init_int8; // expected-error {{initializing '_Bool' with an expression of incompatible type 'svint8_t'}}

clang/test/SemaCXX/sizeless-1.cpp

@@ -98,6 +98,7 @@ void func(int sel) {
   const volatile svint8_t const_volatile_int8 = local_int8; // expected-note {{declared const here}}
   const volatile svint8_t uninit_const_volatile_int8;       // expected-error {{default initialization of an object of const type 'const volatile svint8_t'}}
 
+  _Atomic svint8_t atomic_int8;      // expected-error {{_Atomic cannot be applied to sizeless type 'svint8_t'}}
   __restrict svint8_t restrict_int8; // expected-error {{requires a pointer or reference}}
 
   bool test_int8 = init_int8; // expected-error {{cannot initialize a variable of type 'bool' with an lvalue of type 'svint8_t'}}