go/types: add Alias.Rhs

This method returns the type on the right-hand side of an
alias declaration such as type L = R.

Fixes #66559

Change-Id: I396f2d999680ad251f47cdde20856ae20fc1c40a
Reviewed-on: https://go-review.googlesource.com/c/go/+/581615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/go/types/api_test.go b/src/go/types/api_test.go
index 564bbc2423..6f8dddb936 100644
--- a/src/go/types/api_test.go
+++ b/src/go/types/api_test.go
@@ -3014,3 +3014,21 @@ type B = T[A]
 		t.Errorf("Unalias(type B = T[A]) = %q, want %q", got, want)
 	}
 }
+
+func TestAlias_Rhs(t *testing.T) {
+	t.Setenv("GODEBUG", "gotypesalias=1")
+	const src = `package p
+
+type A = B
+type B = C
+type C = int
+`
+
+	pkg := mustTypecheck(src, nil, nil)
+	A := pkg.Scope().Lookup("A")
+
+	got, want := A.Type().(*Alias).Rhs().String(), "p.B"
+	if got != want {
+		t.Errorf("A.Rhs = %s, want %s", got, want)
+	}
+}