[PATCH] c/66516 - missing diagnostic on taking the address of a builtin function

[Martin Sebor]

Attached is a patch to reject C and C++ constructs that result
in obtaining a pointer (or a reference in C++) to a builtin
function.  These constructs are currently silently accepted by
GCC and, in most cases(*), result in a linker error.  The patch
brings GCC on par with Clang by rejecting all such constructs.

Bootstrapped and tested on x86_64-unknown-linux-gnu.

Okay to commit to trunk?

Martin

[*] Besides rejecting constructs that lead to linker errors
the patch leads to rejecting also some benign constructs (that
are also rejected by Clang).  For example, the program below
currently compiles and links successfully with GCC 5.1 is
rejected with the patch applied.  That's intended (but I'm
open to arguments against it).

$ cat /build/tmp/u.cpp && /build/gcc-66516/gcc/xg++ 
-B/build/gcc-66516/gcc -Wall -c -std=c++11 /build/tmp/u.cpp
static constexpr int (&r)(int) = __builtin_ffs;

int main () { return r (0); }
/build/tmp/u.cpp:1:34: error: invalid initialization of a reference with 
a builtin function
  static constexpr int (&r)(int) = __builtin_ffs;
                                   ^
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