https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88210
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Even without a fix-it, "has not been declared" would be more helpful than "is
not a template".
Clang is more informative (although wrong about the instantiation needing to
occur in namespace x):
expl.cc:5:23: error: no template named 'blah'; did you mean 'x::blah'?
extern template class blah<int>;
^~~~
x::blah
expl.cc:2:31: note: 'x::blah' declared here
template<typename T> struct blah { };
^
expl.cc:5:23: error: explicit instantiation of 'x::blah' must occur in
namespace 'x'
extern template class blah<int>;
^
expl.cc:2:31: note: explicit instantiation refers here
template<typename T> struct blah { };
^
expl.cc:7:16: error: no template named 'blah'; did you mean 'x::blah'?
template class blah<int>;
^~~~
x::blah
expl.cc:2:31: note: 'x::blah' declared here
template<typename T> struct blah { };
^
expl.cc:7:16: error: explicit instantiation of 'x::blah' must occur in
namespace 'x'
template class blah<int>;
^
expl.cc:2:31: note: explicit instantiation refers here
template<typename T> struct blah { };
^
4 errors generated.
