cmd/compile/internal/syntax: handle parentheses around constraints consistently

Generally, the parser strips (i.e., does not record in the syntax tree)
unnecessary parentheses. Specifically, given a type parameter list of
the form

        [P (C),]

it records it as

        [P C]

and then no comma is required when printing. However it did only strip
one level of parentheses, and

        [P ((C)),]

made it through, causing a panic when printing. Somewhat related,
the printer stripped parentheses around constraints as well.

This CL implements a more consistent behavior:

1) The parser strips all parentheses around constraints. For testing
   purposes, a local flag (keep_parens) can be set to retain the
   parentheses.

2) The printer code now correctly intruces a comma if parentheses
   are present (e.g., when testing with keep_parens). This case does
   not occur in normal operation.

3) The printer does not strip parentheses around constraints since
   the parser does it already.

For #69206.

Change-Id: I974a800265625e8daf9477faa9ee4dd74dbd17ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/610758
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/src/cmd/compile/internal/syntax/parser.go b/src/cmd/compile/internal/syntax/parser.go
index 20106f4e61..cd6b6696a2 100644
--- a/src/cmd/compile/internal/syntax/parser.go
+++ b/src/cmd/compile/internal/syntax/parser.go
@@ -721,8 +721,20 @@ func extractName(x Expr, force bool) (*Name, Expr) {
 	case *CallExpr:
 		if name, _ := x.Fun.(*Name); name != nil {
 			if len(x.ArgList) == 1 && !x.HasDots && (force || isTypeElem(x.ArgList[0])) {
-				// x = name "(" x.ArgList[0] ")"
-				return name, x.ArgList[0]
+				// The parser doesn't keep unnecessary parentheses.
+				// Set the flag below to keep them, for testing
+				// (see go.dev/issues/69206).
+				const keep_parens = false
+				if keep_parens {
+					// x = name (x.ArgList[0])
+					px := new(ParenExpr)
+					px.pos = x.pos // position of "(" in call
+					px.X = x.ArgList[0]
+					return name, px
+				} else {
+					// x = name x.ArgList[0]
+					return name, Unparen(x.ArgList[0])
+				}
 			}
 		}
 	}