…isely

The new description matches the implementation (CL 370774).

Also, in the section on type constraints, use "defines" instead of
"determines" because the constraint interface defines the type set
which is precisely the set of acceptable type arguments.

For #49482.

Change-Id: I6f30f49100e8ba8bec0a0f1b450f88cae54312eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/372874
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

This is a port of CL 370774 to go/parser and go/printer. It is adjusted
for the slightly different factoring of parameter list parsing and
printing in go/parser and go/printer.

For #49482

Change-Id: I1c5b1facddbfcb7f7b2be356c817fc7e608223f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/385575
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Add tests that verify consistent behavior of go/types and types2 with
respect to potentially ambiguous type parameter lists.

For #49482

Change-Id: I3386d4fa3eb91f2a8ea0987372ca40a6962de886
Reviewed-on: https://go-review.googlesource.com/c/go/+/385756
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

Accept ~x as ordinary unary expression in the parser but recognize
such expressions as invalid in the type checker.

This change opens the door to recognizing complex type constraint
literals such as `*E|~int` in `[P *E|~int]` and parse them correctly
instead of reporting a parse error because `P*E|~int` syntactically
looks like an incorrect array length expression (binary expression
where the RHS of | is an invalid unary expression ~int).

As a result, the parser is more forgiving with expressions but the
type checker will reject invalid uses as before.

We could pass extra information into the binary/unary expression
parse functions to prevent the use of ~ in invalid situations but
it doesn't seem worth the trouble. In fact it may be advantageous
to allow a more liberal expression syntax especially in the presence
of errors (better parser synchronization after an error).

Preparation for fixing #49482.

Change-Id: I119e8bd9445dfa6460fcd7e0658e3554a34b2769
Reviewed-on: https://go-review.googlesource.com/c/go/+/402255
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>

…erals

Without this change, the type parameter list "[P T | T]" is printed
as "[P T | T,]" in an attempt to avoid an ambiguity. But the type
parameter P cannot syntactically combine with the constraint T | T
and make a new valid expression.

This change introduces a specific combinesWithName predicate that
reports whether a constraint expression can combine with a type
parameter name to form a new valid (value) expression.

Use combinesWithName to accurately determine when a comma is needed.

For #49482.

Change-Id: Id1d17a18f0c9af04495da7b0453e83798f32b04a
Reviewed-on: https://go-review.googlesource.com/c/go/+/404397
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>