Question about existing tests for [dcl.stc]

[Aaron Ballman]

We have a test that's XFAILed for everything:
test\CXX\dcl.dcl\dcl.spec\dcl.stc\p10.cpp

However, I can see no reason why this test should be failed for all
platforms. The only issue I see with the test is that the wording for
a diagnostic may need to be updated.

mutable struct s1 {}; // expected-error{{'mutable' cannot be applied
to non-data members}}

Could be: 'mutable' is not permitted on a declaration of a type

which is what currently fires for that line in the test. However, that
is currently fired as an extension warning, not an error, for C++. Am
I correct in thinking that warning should be upgraded to an error for
C++, the test updated for the new wording, and then the XFAIL can go
away?

~Aaron