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

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.

Differential Revision: https://reviews.llvm.org/D76693

Author: rsandifo-arm

clang/lib/Sema/SemaExpr.cpp

@@ -7586,6 +7586,11 @@ QualType Sema::CheckConditionalOperands(ExprResult &Cond, ExprResult &LHS,
       /*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.

clang/test/Sema/sizeless-1.c

@@ -126,6 +126,11 @@ void func(int sel) {
 
   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'}}