go/internal/gcimporter: prevent importReader reading type parameter twice

This is port of CL 349009 to go/internal/gcimporter.

Updates #48280

Change-Id: I7d40d8b67333538ca58fe012535d54e891d0ed16
Reviewed-on: https://go-review.googlesource.com/c/go/+/349010
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

src/go/internal/gcimporter/iimport.go

@@ -284,6 +284,8 @@ type importReader struct {
 	prevColumn int64
 }
 
+// obj reads import declaration for an object. It may not read
+// the entire declaration, e.g, for recursive type.
 func (r *importReader) obj(name string) {
 	tag := r.byte()
 	pos := r.pos()
@@ -309,16 +311,17 @@ func (r *importReader) obj(name string) {
 		r.declare(types.NewFunc(pos, r.currPkg, name, sig))
 
 	case 'T', 'U':
-		var tparams []*types.TypeParam
-		if tag == 'U' {
-			tparams = r.tparamList()
-		}
 		// Types can be recursive. We need to setup a stub
 		// declaration before recursing.
 		obj := types.NewTypeName(pos, r.currPkg, name, nil)
 		named := types.NewNamed(obj, nil, nil)
-		named.SetTypeParams(tparams)
+		// Declare obj before calling r.tparamList, so the new type name is recognized
+		// if used in the constraint of one of its own typeparams (see #48280).
 		r.declare(obj)
+		if tag == 'U' {
+			tparams := r.tparamList()
+			named.SetTypeParams(tparams)
+		}
 
 		underlying := r.p.typAt(r.uint64(), named).Underlying()
 		named.SetUnderlying(underlying)