CWG3102 [basic.fundamental] Allow expressions of type `void` in a requires-expression

[eisenwave]

Reference (section label): [basic.fundamental] paragraph 15

Issue description

The expression requires { void(); } is ill-formed, but no compiler implements this behavior (https://godbolt.org/z/Eha5vz4oe). Disallowing expression of type cv void here was never motivated and appears to be an oversight.

Furthermore, (void()) is well-formed according to [expr.prim.paren], but is not listed as a bullet, and listing it may provide editorial clarity.

Suggested resolution

Change [basic.fundamental] paragraph 15 as follows:

 A type cv void is an incomplete type that cannot be completed;
 such a type has an empty set of values.
 It is used as the return type for functions that do not return a value.
 An expression of type cv void shall be used only as 
 - an expression statement ([stmt.expr]),
 - the expression in a return statement ([stmt.return])
   for a function with return type cv void
 - an operand of a comma expression ([expr.comma])
+- the operand of a parenthesized expression ([expr.prim.paren])
+- a requirement in a requires expression ([expr.prim.req.general])
 - the second or third operand of ?: ([expr.cond])
 - the operand of a typeid expression ([expr.typeid])
 - the operand of a noexcept operator ([expr.unary.noexcept])
 - the operand of a decltype specifier ([dcl.type.decltype]), or
 - the operand of an explicit conversion to type
   cv void ([expr.type.conv], [expr.static.cast], [expr.cast])

[eisenwave]

On a side note, MSVC allows const_cast<void>(void()) and reinterpret_cast<void>(void()) unlike other compilers, but I think the wording is sufficiently clear that this is not okay.

[t3nsor]

I think the list should also be demoted to a note.

[jensmaurer]

CWG3102

[eisenwave]

@jensmaurer Thank you. I'm pretty sure that adding the reflection operator to the list is wrong though. ^^void is not reflecting on an expression of type void but literally on the type void itself.

[jensmaurer]

... and you can't actually reflect on expressions right now. Removed that addition from CWG3102.