Diagnostics for template keywords showing up inside local classes aren't good

[majnemer]

Bugzilla Link	17585
Version	unspecified
OS	Linux
CC	@DougGregor,@zygoloid

Extended Description

consider:

void foo() {
  template <typename> int i;
}

we issue:

error: expected expression

gcc provides a better error:

error: a template declaration cannot appear at block scope

another case:

void foo() {
  struct S { template <typename> int i; };
}

we issue:

error: member 'i' declared as a template

gcc provides:

invalid declaration of member template in local class

These plain english messages are very nice, we should do something similar.

[zygoloid]

void foo() {
struct S { template int i; };
}

we issue:

error: member 'i' declared as a template

This diagnostic is bad in the non-local-class case too. Something like

non-static data member 'i' cannot be declared as a template

would be better, to suggest that maybe the 'static' keyword is missing.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (991901f3-cc14-4404-b340-165691b62a58)

| | | | --- | --- | | Bugzilla Link | [17585](https://llvm.org/bz17585) | | Version | unspecified | | OS | Linux | | CC | @DougGregor,@zygoloid |

Extended Description

consider:

void foo() {
  template <typename> int i;
}

we issue:

error: expected expression

gcc provides a better error:

error: a template declaration cannot appear at block scope

another case:

void foo() {
  struct S { template <typename> int i; };
}

we issue:

error: member 'i' declared as a template

gcc provides:

invalid declaration of member template in local class

These plain english messages are very nice, we should do something similar.