[Bug c++/77370] New: missing -Wignored-qualiifiers on a bogus const reference

["msebor at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77370

            Bug ID: 77370
           Summary: missing -Wignored-qualiifiers on a bogus const
                    reference
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC rejects the program below because it tries to initialize the non-const
reference object cr with a temporary, but it doesn't diagnose the declaration
of the reference type CR where it disregards the const qualifier.  In contrast,
Clang diagnoses both.  GCC could be improved by doing the same.

$ (set -x && cat z.C && /build/gcc-trunk-svn/gcc/xgcc -B
/build/gcc-trunk-svn/gcc -Wall -Wextra -Wpedantic z.C;
/build/llvm-trunk/bin/clang -S -Wall z.C)
+ cat z.C
typedef char& R;
typedef const R CR;

char c;
R r = c;
CR cr = 'b';
+ /build/gcc-trunk-svn/gcc/xgcc -B /build/gcc-trunk-svn/gcc -Wall -Wextra
-Wpedantic z.C
z.C:6:9: error: cannot bind non-const lvalue reference of type ‘CR {aka char&}’
to an rvalue of type ‘char’
 CR cr = 'b';
         ^~~
+ /build/llvm-trunk/bin/clang -S -Wall z.C
z.C:2:9: warning: 'const' qualifier on reference type 'R' (aka 'char &') has no
      effect [-Wignored-qualifiers]
typedef const R CR;
        ^~~~~~
z.C:6:4: error: non-const lvalue reference to type 'char' cannot bind to a
      temporary of type 'char'
CR cr = 'b';
   ^    ~~~
1 warning and 1 error generated.