$ ./cppgl17.sh p252
$ clang++ p252.cpp
p252.cpp:22:1: error: unknown type name 'I'; did you mean 'X::I'?
I b; // error
^
X::I
p252.cpp:8:15: note: 'X::I' declared here
  typedef int I;
              ^
p252.cpp:23:1: error: unknown type name 'Y'; did you mean 'X::Y'?
Y c; // error
^
X::Y
p252.cpp:9:9: note: 'X::Y' declared here
  class Y { /* ... */
        ^
2 errors generated.

$ g++-7 p252.cpp
p252.cpp:22:1: error: 'I' does not name a type
 I b; // error
 ^
p252.cpp:23:1: error: 'Y' does not name a type
 Y c; // error
 ^