fix: correct assign from function returning an interface value (#625)

_test/interface37.go

@@ -0,0 +1,23 @@
+package main
+
+type I interface {
+	A() string
+	B() string
+}
+
+type s struct{}
+
+func NewS() (I, error) {
+	return &s{}, nil
+}
+
+func (c *s) A() string { return "a" }
+func (c *s) B() string { return "b" }
+
+func main() {
+	s, _ := NewS()
+	println(s.A())
+}
+
+// Output:
+// a