go/parser: parser to accept ~x as unary expression
This is a port of CL 402255 from the syntax package to go/parser
with adjustments because of the different AST structure.
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 #52559.
Change-Id: I48562cf40ccf5f14c20fcd92c40a0303b2d8b2b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/403696 Reviewed-by: Ian Lance Taylor <iant@google.com> Reviewed-by: David Chase <drchase@google.com>