[Voted into the WP at the March, 2011 meeting as part of paper N3262.]

Proposed resolution (November, 2010) [SUPERSEDED]: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.]

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.