[Bug c++/81717] New: [c++ concepts] - simple "define function if not defined" code doesn't compile, no errors generated

["jordok at wp dot pl" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81717

            Bug ID: 81717
           Summary: [c++ concepts] - simple "define function if not
                    defined" code doesn't compile, no errors generated
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jordok at wp dot pl
  Target Milestone: ---

Created attachment 41925
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=41925&action=edit
Minimal code to reproduce

Simple usage of keyword `requires` (C++ concepts) to define a function only if
it's not already defined causes compilation failure.

Code attached is a minimal example of situation in which compilation fails, but
no error is generated.
g++ --version:  g++.exe (MinGW.org GCC-6.3.0-1) 6.3.0
OS : Windows 7 (x64)
Compiling with command: g++ -Wall -fconcepts source.cpp

Which generates no text output and no files. Are generated.


Checked also here: http://coliru.stacked-crooked.com/a/16466d6c732d9f21
Same source code compiled with:
g++ -std=c++17 -fconcepts -Wall -Wextra main.cpp && ./a.out

main.cpp: In function 'bool foo(A)':
main.cpp:6:12: warning: unused parameter 'a' [-Wunused-parameter]
 bool foo(A a) {
            ^
g++: internal compiler error: Segmentation fault (program cc1plus)
Please submit a full bug report,
with preprocessed source if appropriate.
See <https://gcc.gnu.org/bugs/> for instructions.

In this IDE (Coliru), g++ --version gives:
g++ (GCC) 7.1.0