__builtin_bit_cast does not allow bitcasting from nullptr_t

[tbaederr]

nullptr_t is not a pointer type, see the static_assert in https://godbolt.org/z/vjq1Tse7W

In this example, the second bitcast works, but the first one does not:

typedef __UINTPTR_TYPE__ uintptr_t;
constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);


typedef decltype(nullptr) nullptr_t;
constexpr decltype(nullptr) N = __builtin_bit_cast(nullptr_t, (uintptr_t)12);
static_assert(N == nullptr);

The output suggests some bogus reason though:

./array.cpp:26:21: error: constexpr variable 'A' must be initialized by a constant expression
   26 | constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);
      |                     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./array.cpp:26:25: note: indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'unsigned long' is invalid
   26 | constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);
      |                         ^

GCC accepts the first one, but not the second. I opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117727 for that.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Timm Baeder (tbaederr)

`nullptr_t` is not a pointer type, see the `static_assert` in https://godbolt.org/z/vjq1Tse7W

In this example, the second bitcast works, but the first one does not:

typedef __UINTPTR_TYPE__ uintptr_t;
constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);


typedef decltype(nullptr) nullptr_t;
constexpr decltype(nullptr) N = __builtin_bit_cast(nullptr_t, (uintptr_t)12);
static_assert(N == nullptr);

The output suggests some bogus reason though:

./array.cpp:26:21: error: constexpr variable 'A' must be initialized by a constant expression
   26 | constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);
      |                     ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./array.cpp:26:25: note: indeterminate value can only initialize an object of type 'unsigned char' or 'std::byte'; 'unsigned long' is invalid
   26 | constexpr uintptr_t A = __builtin_bit_cast(uintptr_t, nullptr);
      |                         ^

GCC accepts the first one, but not the second. I opened https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117727 for that.

[tbaederr]

I guess this is on purpose actually:

llvm-project/clang/lib/AST/ExprConstant.cpp

Lines 7216 to 7218 in a12e79a

	// As a special case, nullptr_t has an indeterminate value.
	if (Ty->isNullPtrType())
	return true;

This line is from eee944e, the first commit introducing __builtin_bit_cast.

[tbaederr]

Paging @frederick-vs-ja for spec knowledge, since this isn't as clear-cut as I thought, according to the gcc bug linked above.

[zygoloid]

In Clang, we treat nullptr_t as having no bits in its value representation. (This choice is not specified by the standard, and is instead up to the implementation -- and part of the ABI, but sadly psABIs don't seem to actually specify it.) For example:

struct X {
  X() {}
  decltype(nullptr) p;
};
X f() { return X(); }

... does not store zero bits to the p field within X, instead leaving it indeterminate. (Some other uses of nullptr do store zero bits; I think that's just bad codegen rather than being intentional.)

Because nullptr_t has no value bits, bit_cast treats its bits as being indeterminate. So I think the behavior observed is correct -- you can bit_cast any value to nullptr_t (because it has no bits in its value representation) but you cannot bit_cast any type with a non-empty value representation from nullptr_t.

[tbaederr]

That makes sense and matches what Jakub said in the gcc bug. Closing this then, seems clang is doing the right thing.