[Bug c++/91429] New: Conditional explicit not respected with out-of-line definition

[barry.revzin at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91429

            Bug ID: 91429
           Summary: Conditional explicit not respected with out-of-line
                    definition
           Product: gcc
           Version: 9.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: barry.revzin at gmail dot com
  Target Milestone: ---

Reduced from StackOverflow (https://stackoverflow.com/q/57467490/2069064):

using size_t = decltype(sizeof(0));

struct Str {
    template <size_t N>
    explicit(N > 7) Str(char const (&str)[N]);
};

#ifdef OUT_OF_LINE
template <size_t N>
Str::Str(char const(&str)[N]) { }
#endif

Str a = "short";
Str b = "not so short";

As-is, gcc correctly rejects the construction of b (since that constructor is
explicit). But with -DOUT_OF_LINE, gcc accepts.