[dev.typeparams] go/types: implement TypeParam.Constraint

This is a clean port of CL 336989 to go/types.

Change-Id: Ib8dbe03f420d28ada6d5fc7003ab0c82c7e06c41
Reviewed-on: https://go-review.googlesource.com/c/go/+/339650
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 21 insertions, 13 deletions

diff --git a/src/go/types/typeparam.go b/src/go/types/typeparam.go
index a3d60c1648..8cb44ea25e 100644
--- a/src/go/types/typeparam.go
+++ b/src/go/types/typeparam.go
@@ -60,24 +60,32 @@ func (t *TypeParam) _SetId(id uint64) {
 	t.id = id
 }
 
-// TODO(rfindley): document the Bound and SetBound methods.
+// Constraint returns the type constraint specified for t.
+func (t *TypeParam) Constraint() Type {
+	// compute the type set if possible (we may not have an interface)
+	if iface, _ := under(t.bound).(*Interface); iface != nil {
+		// use the type bound position if we have one
+		pos := token.NoPos
+		if n, _ := t.bound.(*Named); n != nil {
+			pos = n.obj.pos
+		}
+		computeTypeSet(t.check, pos, iface)
+	}
+	return t.bound
+}
 
+// Bound returns the underlying type of the type parameter's
+// constraint.
+// Deprecated for external use. Use Constraint instead.
 func (t *TypeParam) Bound() *Interface {
-	// we may not have an interface (error reported elsewhere)
-	iface, _ := under(t.bound).(*Interface)
-	if iface == nil {
-		return &emptyInterface
+	if iface, _ := under(t.Constraint()).(*Interface); iface != nil {
+		return iface
 	}
-	// use the type bound position if we have one
-	pos := token.NoPos
-	if n, _ := t.bound.(*Named); n != nil {
-		pos = n.obj.pos
-	}
-	// TODO(rFindley) switch this to an unexported method on Checker.
-	computeTypeSet(t.check, pos, iface)
-	return iface
+	return &emptyInterface
 }
 
+// TODO(rfindley): document the SetBound methods.
+
 func (t *TypeParam) SetBound(bound Type) {
 	if bound == nil {
 		panic("internal error: bound must not be nil")