This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 115d. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
2024-10-26
[Voted into the WP at the March, 2011 meeting as part of paper N3262.]
The status of the following example is not clear:
union U { float f; unsigned long u; };
constexpr U u1 = { 1.0 };
constexpr unsigned long u2 = u1.u;
This might be ill-formed because the aliasing causes undefined behavior, which should make the expression not a constant expression. However, a similar example using a permitted aliasing would presumably be acceptable:
union U {
unsigned char c[sizeof(double)];
double d;
};
constexpr U c1u = { 0x12, 0x34 /* etc. */ };
constexpr double c1 = c1u.d;
One suggestion was that unions should not be considered literal types, but some in the ensuing discussion deemed that excessive. That also would not address similar examples using reinterpret_cast, which is currently also permitted in constant expressions.
Proposed resolution (November, 2010) [SUPERSEDED]:
Change 7.7 [expr.const] paragraph 2 as follows:
an lvalue-to-rvalue conversion...
an lvalue-to-rvalue conversion (7.3.2 [conv.lval]) that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
...
a type conversion from a pointer or pointer-to-member type to a
literal type [Note: a user-defined conversion invokes a function
—end note] a reinterpret_cast
(7.6.1.10 [expr.reinterpret.cast]);
[Note: the proposed resolution of issue 1098 edits this bullet in an incompatible fashion.]