[dcl.spec] No restriction against combining `explicit` with `explicit(bool)`

[zygoloid]

Reference (section label): dcl.spec.general, dcl.fct.spec

Link to reflector thread (if any): https://lists.isocpp.org/core/2018/06/4673.php

Issue description:

It's not completely clear whether this is valid, but it is clear that it's nonsensical:

struct X {
  explicit explicit(false) X(int);
};

[dcl.spec]p2 says: "Each decl-specifier shall appear at most once in a complete decl-specifier-seq, except that long may appear twice."

But are explicit and explicit(false) the same decl-specifier? I don't think they are; they're different grammar productions, and they mean different things. I think we'd benefit from some clarification here. (Similarly, are explicit(true) and explicit(false) the same decl-specifier?)

We certainly do intend to allow you to combine (for example) virtual and explicit, so we can't say that this is invalid because each decl-specifier production is only allowed once -- those are both function-specifiers using the same decl-specifier production.

Suggested resolution:

Change in [dcl.spec]p2:

"Each decl-specifier shall appear at most once in a complete decl-specifier-seq, except that long may appear twice. A decl-specifier-seq shall not contain more than one explicit-specifier."

[lprv]

cplusplus/draft#4834 addresses this.

[jensmaurer]

... which was just merged.