Using global namespace qualifiers with pointer to data members

I'm experiencing a compile error when attempting to use a pointer to data member when including the global namespace qualifier on the struct. I've reduced my code to the following, which works:

namespace foo {
  using sausage = int;
  struct bar { sausage baz; };
}

auto chuckle(foo::bar barry, ::foo::sausage foo::bar::*paul) {
  return barry.*paul;
}

int main() {
  return chuckle(foo::bar{5}, &foo::bar::baz);
}

If I now add the global namespace qualifier to the bar struct in the argument to chuckle:

auto chuckle(foo::bar barry, ::foo::sausage ::foo::bar::*paul) {
                                        //  ^~ added ::
  return barry.*paul;
}

...then it no longer compiles and fails with the following error:

10 : <source>:10:37: error: 'foo::sausage' is not a class, namespace, or enumeration
 auto chuckle(foo::bar barry, ::foo::sausage ::foo::bar::*paul) {
                                     ^~~~~~~
10 : <source>:10:57: error: expected identifier before '*' token
 auto chuckle(foo::bar barry, ::foo::sausage ::foo::bar::*paul) {
                                                         ^
<source>: In function 'auto chuckle(foo::bar, int*)':
11 : <source>:11:17: error: 'paul' cannot be used as a member pointer, since it is of type 'int*'
   return barry.*paul;
                 ^~~~
<source>: In function 'int main()':
15 : <source>:15:45: error: cannot convert 'foo::sausage foo::bar::* {aka int foo::bar::*}' to 'int*' for argument '2' to 'auto chuckle(foo::bar, int*)'
   return chuckle(foo::bar{5}, &foo::bar::baz);
                                             ^

I see this behaviour across MSVC, Clang and GCC.

I've put both examples on Godbolt:

• working example
• failing example

Questions

a) Why does adding the global namespace qualifier cause the compilation to fail?

b) Is there a reason for this in the language specification?