[Bug c++/80886] __builtin_constant_p magic has broken at some point
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80886 Jonathan Wakely changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #3 from Jonathan Wakely --- (In reply to steveren from comment #0) > Has this behaviour changed deliberately? Yes, because the standard forbids it. As Andrew said, the __builtin_constant_p part (and what the docs say about it) is irrelevant because the (void *) 0x1000 expression is what gives an error. > As I say, it is contrary to the > Standard, but it would be disappointing to lose such a useful extension. I think it would be better to add support to C++ for "pointer literals", I've been talking with a few people about proposing that for the standard. That would allow you to create constexpr pointers from literal integers, but still disallow arbitrary reinterpret casts between them. Closing as G++ is correct to disallow this.
[Bug c++/80886] __builtin_constant_p magic has broken at some point
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80886
--- Comment #2 from Andrew Pinski ---
This has nothing to do with __builtin_constant_p really.
Just GCC started to rejecting:
constexpr void *phardware {((void *) 0x1000)};
Starting in GCC 7; it was accepted before.
What is interesting is the __builtin_constant_p makes clang accept it but that
is a clang bug.
>This use appears to be documented
I don't think it was ever documented to be accepted this way but I could be
wrong.
[Bug c++/80886] __builtin_constant_p magic has broken at some point
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80886
Jonathan Wakely changed:
What|Removed |Added
Known to work||6.4.0
Version|unknown |7.3.1
Known to fail||7.3.0, 8.0.1
--- Comment #1 from Jonathan Wakely ---
I can't reproduce this with GCC 6, but can with GCC 7:
un.cc:3:55: error: value '4096' of type 'void*' is not a constant expression
constexpr void *phardware {CONST ((void *) 0x1000)};
^
and with trunk:
un.cc:3:39: error: reinterpret_cast from integer to pointer
constexpr void *phardware {CONST ((void *) 0x1000)};
^~~
un.cc:1:45: note: in definition of macro 'CONST'
#define CONST(x) (__builtin_constant_p(x) ? x : x)
^
