[Sema][SVE] Allow ?: to select between SVE types in C

[rsandifo-arm]

Summary

When compiling C, a ?: between two values of the same SVE type
currently gives an error such as:

incompatible operand types ('svint8_t' (aka '__SVInt8_t') and 'svint8_t')

It's supposed to be valid to select between (cv-qualified versions of)
the same SVE type, so this patch adds that case.

These expressions already work for C++ and are tested by
SemaCXX/sizeless-1.cpp.

[efriedma]

Do we want to allow stuff like x ? (svint8_t)0 : (signed char)0

[rsandifo-arm]

In D76693#1939866, @efriedma wrote:

Do we want to allow stuff like x ? (svint8_t)0 : (signed char)0

(svint8_t)0 is invalid as thing stand. The intention is for the types to be opaque and only used via intrinsics, rather than be vector-like.

[efriedma]

LGTM

clang/lib/Sema/SemaExpr.cpp

QualType Sema::CheckConditionalOperands(ExprResult &Cond, ExprResult &LHS,
   // null pointers have been filtered out by this point.
   if (checkPointerIntegerMismatch(*this, LHS, RHS.get(), QuestionLoc,
       /*IsIntFirstExpr=*/true))
     return RHSTy;
   if (checkPointerIntegerMismatch(*this, RHS, LHS.get(), QuestionLoc,
       /*IsIntFirstExpr=*/false))
     return LHSTy;
 
+  // Allow ?: operations in which both operands have the same
+  // built-in sizeless type.
+  if (LHSTy->isSizelessBuiltinType() && LHSTy == RHSTy)
+    return LHSTy;
+
   // Emit a better diagnostic if one of the expressions is a null pointer
   // constant and the other is not a pointer type. In this case, the user most
   // likely forgot to take the address of the other expression.
   if (DiagnoseConditionalForNull(LHS.get(), RHS.get(), QuestionLoc))
     return QualType();
 
   // Otherwise, the operands are not compatible.
   Diag(QuestionLoc, diag::err_typecheck_cond_incompatible_operands)

clang/test/Sema/sizeless-1.c

void func(int sel) {
 
   volatile_int8 = local_int8;
   volatile_int8 = const_int8;
   volatile_int8 = volatile_int8;
   volatile_int8 = const_volatile_int8;
 
   const_volatile_int8 = local_int8; // expected-error {{cannot assign to variable 'const_volatile_int8' with const-qualified type 'const volatile svint8_t'}}
 
+  init_int8 = sel ? init_int8 : local_int8;
+  init_int8 = sel ? init_int8 : const_int8;
+  init_int8 = sel ? volatile_int8 : const_int8;
+  init_int8 = sel ? volatile_int8 : const_volatile_int8;
+
   pass_int8(local_int8);
   pass_int8(local_int16); // expected-error {{passing 'svint16_t' (aka '__SVInt16_t') to parameter of incompatible type 'svint8_t'}}
 
   local_int8 = return_int8();
   local_int16 = return_int8(); // expected-error {{assigning to 'svint16_t' (aka '__SVInt16_t') from incompatible type 'svint8_t'}}
 
   dump(&local_int8);
   dump(&const_int8);