cmd/compile/internal/syntax: parser to accept ~x as unary expression

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>

1 files changed, 8 insertions, 6 deletions

diff --git a/src/cmd/compile/internal/syntax/testdata/typeset.go b/src/cmd/compile/internal/syntax/testdata/typeset.go
index 19b74f28ea..fe5c3f45a8 100644
--- a/src/cmd/compile/internal/syntax/testdata/typeset.go
+++ b/src/cmd/compile/internal/syntax/testdata/typeset.go
@@ -65,15 +65,17 @@ func _[_ t[t] | t[t]]() {}
 
 // Single-expression type parameter lists and those that don't start
 // with a (type parameter) name are considered array sizes.
-// The term must be a valid expression (it could be a type - and then
-// a type-checker will complain - but we don't allow ~ in the expr).
+// The term must be a valid expression (it could be a type incl. a
+// tilde term) but the type-checker will complain.
 type (
         _[t] t
-        _[/* ERROR unexpected ~ */ ~t] t
         _[t|t] t
-        _[/* ERROR unexpected ~ */ ~t|t] t
-        _[t| /* ERROR unexpected ~ */ ~t] t
-        _[/* ERROR unexpected ~ */ ~t|~t] t
+
+        // These are invalid and the type-checker will complain.
+        _[~t] t
+        _[~t|t] t
+        _[t|~t] t
+        _[~t|~t] t
 )
 
 type (