cmd/compile/internal/types2: avoid infinite recursion in unification
If the type T inferred for a type parameter P is P itself (or a derived
type containing P), a subsequent unification step leads to infinite
recursion: at each encounter of P with the already inferred type T
(which is or contains P), P stands for that T and the recursive matching
process continues with T, which inevitably contains P again and recursion
never terminates.
This CL introduces a set of masks, one for each type parameter.
When a type parameter is encountered for which a type has already
been inferred, the type parameter is "masked" for the recursive
matching of the inferred type. Masking makes the type parameter
"invisible" such that it will be handled like any other type and
not unpacked further.
Fixes #48619.
For #48656.
Change-Id: Ic1d938322be51fd44323ea14f925303f58b27c97
Reviewed-on: https://go-review.googlesource.com/c/go/+/352832
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org> Reviewed-by: Robert Findley <rfindley@google.com>