[Bug c++/58102] New: rejects valid initialization of constexpr object with mutable member

[richard-gccbugzilla at metafoo dot co.uk]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58102

            Bug ID: 58102
           Summary: rejects valid initialization of constexpr object with
                    mutable member
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richard-gccbugzilla at metafoo dot co.uk

Given

  struct S {
    mutable int n;
    constexpr S() : n() {}
  };

GCC correctly accepts:

  constexpr S s {};

but incorrectly rejects:

  constexpr S s = {};

saying:

<stdin>: At global scope:
<stdin>:5:20: error: ‘const S’ cannot be the type of a complete constant
expression because it has mutable sub-objects

Per core issue 1405, both the above cases are valid.