[C++4OpenCL] Fix ICE with invalid use of half

[olestrohm]

Summary

Because half is limited to the cl_khr_fp16 extension being enabled, DefaultLvalueConversion can fail when it's not enabled.
Because of this the original assumption that it will never return an error is wrong.

This is fixed by checking for an error and returning early if there is one, instead of assuming that DefaultLvalueConversion is infallible.

Fixes: PR47976

[Anastasia]

LGTM! Thanks

Yeah I think this is the only case where DefaultLvalueConversion exits with an error at present.

clang/lib/Sema/SemaExprCXX.cpp

if (const AtomicType *FromAtomic = FromType->getAs<AtomicType>()) {
                                       From, /*BasePath=*/nullptr, VK_RValue,
                                       FPOptionsOverride());
     }
     break;
 
   case ICK_Lvalue_To_Rvalue: {
     assert(From->getObjectKind() != OK_ObjCProperty);
     ExprResult FromRes = DefaultLvalueConversion(From);
-    assert(!FromRes.isInvalid() && "Can't perform deduced conversion?!");
+    if (FromRes.isInvalid())
+      return ExprError();
+
     From = FromRes.get();
     FromType = From->getType();
     break;
   }
 
   case ICK_Array_To_Pointer:
     FromType = Context.getArrayDecayedType(FromType);
     From = ImpCastExprToType(From, FromType, CK_ArrayToPointerDecay,

clang/test/SemaOpenCLCXX/half.clcpp

+//RUN: %clang_cc1 %s -triple spir -verify -fsyntax-only
+
+#pragma OPENCL EXTENSION cl_khr_fp16 : disable
+
+typedef half half2 __attribute__((ext_vector_type(2)));
+
+half f(half2 h2) { // expected-error{{declaring function return value of type 'half' is not allowed ; did you forget * ?}}
+    return h2.s0; // expected-error{{loading directly from pointer to type '__private half' requires cl_khr_fp16. Use vector data load builtin functions instead}}
+}
+
+#pragma OPENCL EXTENSION cl_khr_fp16 : enable
+
+half f(half2 h2) {
+    return h2.s0;
+}