[Sema][SVE] Allow casting SVE types to themselves in C

Casts from an SVE type to itself aren't very useful, but they are
supposed to be valid, and could occur in things like macro expansions.

Such casts already work for C++ and are tested by sizeless-1.cpp.
This patch makes them work for C too.

Differential Revision: https://reviews.llvm.org/D76694

Author: rsandifo-arm

clang/lib/Sema/SemaCast.cpp

@@ -2652,6 +2652,13 @@ void CastOperation::CheckCStyleCast() {
     return;
   }
 
+  // Allow casting a sizeless built-in type to itself.
+  if (DestType->isSizelessBuiltinType() &&
+      Self.Context.hasSameUnqualifiedType(DestType, SrcType)) {
+    Kind = CK_NoOp;
+    return;
+  }
+
   if (!DestType->isScalarType() && !DestType->isVectorType()) {
     const RecordType *DestRecordTy = DestType->getAs<RecordType>();
 

clang/test/Sema/sizeless-1.c

@@ -108,6 +108,8 @@ void func(int sel) {
 
   sel = local_int8; // expected-error {{assigning to 'int' from incompatible type 'svint8_t'}}
 
+  local_int8 = (svint8_t)local_int8;
+  local_int8 = (const svint8_t)local_int8;
   local_int8 = (svint8_t)local_int16; // expected-error {{used type 'svint8_t' (aka '__SVInt8_t') where arithmetic or pointer type is required}}
   local_int8 = (svint8_t)0;           // expected-error {{used type 'svint8_t' (aka '__SVInt8_t') where arithmetic or pointer type is required}}
   sel = (int)local_int8;              // expected-error {{operand of type 'svint8_t' (aka '__SVInt8_t') where arithmetic or pointer type is required}}