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>

This is an almost clean port of CL 352832 from types2 to go/types:
The nest files and unify.go where copied verbatim; unify.go was
adjusted with correct package name, a slightly different comment
was restored to what it was. The test files got adjustments for
error position. infer.go got a missing _Todo error code.

For #48619.
For #48656.

Change-Id: Ia1a2d09e8bb37a85032b4b7e7c7a0b08e8c793a5
Reviewed-on: https://go-review.googlesource.com/c/go/+/353029
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

…"fix"

The "fix" (CL 352832) for #48619 was incorrect and broke
the unification algorithm in some cases (e.g., #48695).

This CL reverts the changes made by CL 352832 to unify.go,
and comments out code in corresponding tests.

As a result, #48695 will be fixed, and we will re-open #48619.

Fixes #48695.
For #48619.
For #48656.

Change-Id: I91bc492062dbcc8dae7626f6b33f6dfabf48bcb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/354690
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>

This is a stop gap solution to avoid panics due to stack overflow
during type unification. While this doesn't address the underlying
issues (for which we are still investigating the correct approach),
it prevents a panic during compilation and reports a (possibly not
quite correct) error message.

If the programs are correct in the first place, manually providing
the desired type arguments is a viable work-around, resulting in
code that will continue to work even when the issues here are fixed
satisfactorily.

For #48619.
For #48656.

Change-Id: I13bb14552b38b4170b5a1b820e3172d88ff656ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/377954
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

This is a stop gap solution to avoid panics due to stack overflow
during type unification. While this doesn't address the underlying
issues (for which we are still investigating the correct approach),
it prevents a panic during compilation and reports a (possibly not
quite correct) error message.

If the programs are correct in the first place, manually providing
the desired type arguments is a viable work-around, resulting in
code that will continue to work even when the issues here are fixed
satisfactorily.

For golang#48619.
For golang#48656.

Change-Id: I13bb14552b38b4170b5a1b820e3172d88ff656ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/377954
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>