cmd/compile/internal/types2: accept constraint literals with elided interfaces

When collecting type parameters, wrap constraint literals of the
form ~T or A|B into interfaces so the type checker doesn't have
to deal with these type set expressions syntactically anywhere
else but in interfaces (i.e., union types continue to appear
only as embedded elements in interfaces).

Since a type constraint doesn't need to be an interface anymore,
we can remove the respective restriction. Instead, when accessing
the constraint interface via TypeParam.iface, wrap non-interface
constraints at that point and update the constraint so it happens
only once. By computing the types sets of all type parameters at
before the end of type-checking, we ensure that type constraints
are in their final form when accessed through the API.

For #48424.

Change-Id: I3a47a644ad4ab20f91d93ee39fcf3214bb5a81f9
Reviewed-on: https://go-review.googlesource.com/c/go/+/353139
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

0 files changed, 0 insertions, 0 deletions