cmd/compile/internal/types2: disallow aliases for generic types
The existing approach (alias name stands for generic type name)
is an exception: it's the only place where a generic type could
be used without explicit instantiation. The correct solution is
currently under discussion (see proposal issue #46477).
This CL requires that the RHS of an alias type declaration be
an instantiated non-generic type. If #46477 is accepted, the
implementation will require proper representation of alias
types.
Change-Id: Ie85b923213a64f39837e56e38e14757458272b93
Reviewed-on: https://go-review.googlesource.com/c/go/+/346294
Trust: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>