go/types, types2: consider shared methods when unifying against interfaces
When unifying two types A and B where one or both of them are
interfaces, consider the shared method signatures in unification.
1) If a defined interface (an interface with a type name) is unified
with another (defined) interface, currently they must originate
in the same type declaration (same origin) for unification to
succeed. This is more restrictive than necessary for assignments:
when interfaces are assigned to each other, corresponding methods
must match, but the interfaces don't have to be identical.
In unification, we don't know which direction the assignment is
happening (or if we have an assignment in the first place), but
in any case one interface must implement the other. Thus, we
check that one interface has a subset of the methods of the other
and that corresponding method signatures unify.
The assignment or instantiation may still not be possible but that
will be checked when instantiation and parameter passing is checked.
If two interfaces are compared as part of another type during
unification, the types must be equal. If they are not, unifying
a method subset may still succeed (and possibly produce more type
arguments), but that is ok: again, subsequent instantiation and
assignment will fail if the types are indeed not identical.
2) In a non-interface type is unified with an interface, currently
unification fails. If this unification is a consequence of an
assignment (parameter passing), this is again too restrictive:
the non-interface type must only implement the interface (possibly
among other type set requirements). In any case, all methods of the
interface type must be present in the non-interface type and unify
with the corresponding interface methods. If they don't, unification
will fail either way. If they do, we may infer additional type
arguments. Again, the resulting types may still not be correct but
that will be determined by the instantiation and parameter passing
or assignment checks. If the non-interface type and the interface
type appear as component of another type, unification may now
produce additional type arguments. But that is again ok because the
respective types won't pass instantiation or assignment checks since
they are different types.
This CL introduces a new Config flag, EnableInterfaceInference, to
enable this new behavior. If not set, unification remains unchanged.
To be able to test the flag durign unification, a *Checker is passed
and stored with the unifier.
For #60353.
Fixes #41176.
Fixes #57192.
Change-Id: I6b167a9afa378d0682e9b101d9d86f5777308af7
Reviewed-on: https://go-review.googlesource.com/c/go/+/497015
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