[cfe-dev] _Generic with __attribute__((overloadable))

[Nemanja Ivanovic via cfe-dev]

Rather than opening a bug, I thought I'd check if this is supposed to work
(given that the overloadable attribute is an extension).

Should this compile and have the expected semantics (i.e. call the int
version):

int __attribute__((overloadable)) test (int);
double __attribute__((overloadable)) test (double);
char testc(char);

#define testm(X) \
  _Generic ((X), \
            char : testc, \
            default: test) (X)


int foo() {
  return testm(4);
}

It appears that the _Generic macro triggers overload resolution of some
sort which resolves the call to yet another overloaded function. So we hit
an UNREACHABLE in  Sema::FixOverloadedFunctionReference.

If the argument in the call is replaced with a char value, there is no
crash.

In any case, presumably this crash behaviour should perhaps be improved
with a diagnostic.
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