[Bug c++/72707] New: [4.6/4.7/4.8/4.9/5/6/7 regression] local anonymous union member hides names in the same scope

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72707

            Bug ID: 72707
           Summary: [4.6/4.7/4.8/4.9/5/6/7 regression] local anonymous
                    union member hides names in the same scope
           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: ---

In [class.union], all versions of the C++ standard starting with 1998 require
that:

  The names of the members of an anonymous union shall be distinct from the
names of any other entity in the scope in which the anonymous union is
declared.

The following invalid test case is accepted by G++, implying that G++ fails to
conform to this requirement.  The test case is rejected by Clang 3.8 and EDG
eccp 4.11.

Prior to version 4.6, GCC rejected this test case with

  error: declaration of ‘int x’ shadows a parameter

This error was removed and the regression introduced in r159383.

$ cat xyz.c && /build/gcc-71912/gcc/xgcc -B /build/gcc-71912/gcc -S -Wall
-Wextra -Wpedantic -xc++ xyz.c
void f (double &x)
{
  union { int x; };
  x = 0;
}
xyz.c: In function ‘void f(double&)’:
xyz.c:1:17: warning: unused parameter ‘x’ [-Wunused-parameter]
 void f (double &x)
                 ^