[dev.typeparams] cmd/compile/internal/types2: implement TypeParam.Constraint

Change-Id: I95a96f9dbd199cee3a4be8f42cd64e7f44ba5e5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/336989
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/types2/typeparam.go

@@ -53,20 +53,28 @@ func (t *TypeParam) SetId(id uint64) {
 	t.id = id
 }
 
+// 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 := 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)
+	if iface, _ := under(t.Constraint()).(*Interface); iface != nil {
-	iface, _ := under(t.bound).(*Interface)
+		return iface
-	if iface == nil {
-		return &emptyInterface
 	}
-	// use the type bound position if we have one
+	return &emptyInterface
-	pos := nopos
-	if n, _ := t.bound.(*Named); n != nil {
-		pos = n.obj.pos
-	}
-	// TODO(gri) switch this to an unexported method on Checker.
-	computeTypeSet(t.check, pos, iface)
-	return iface
 }
 
 func (t *TypeParam) SetBound(bound Type) {