interp: fix case behavior for values converted to empty interface

Fixes #1465.

_test/issue-1465.go

@@ -0,0 +1,24 @@
+package main
+
+import (
+	"fmt"
+)
+
+func SomeFunc[T int | string](defaultValue T) T {
+	switch v := any(&defaultValue).(type) {
+	case *string:
+		*v = *v + " abc"
+	case *int:
+		*v -= 234
+	}
+	return defaultValue
+}
+
+func main() {
+	fmt.Println(SomeFunc("test"))
+	fmt.Println(SomeFunc(1234))
+}
+
+// Output:
+// test abc
+// 1000

interp/run.go

@@ -3010,15 +3010,17 @@ func _case(n *node) {
 					}
 					return fnext
 				}
-				vi := v.Interface().(valueInterface)
-				if vi.node == nil {
-					if typ.cat == nilT {
-						return tnext
+				if vi, ok := v.Interface().(valueInterface); ok {
+					if vi.node != nil {
+						if vi.node.typ.id() == typ.id() {
+							destValue(f).Set(vi.value)
+							return tnext
+						}
 					}
 					return fnext
 				}
-				if vi.node.typ.id() == typ.id() {
-					destValue(f).Set(vi.value)
+				if v.Type() == typ.TypeOf() {
+					destValue(f).Set(v)
 					return tnext
 				}
 				return fnext
@@ -3053,13 +3055,23 @@ func _case(n *node) {
 					}
 					return fnext
 				}
-				if v := val.Interface().(valueInterface).node; v != nil {
-					for _, typ := range types {
-						if v.typ.id() == typ.id() {
-							destValue(f).Set(val)
-							return tnext
+				if vi, ok := val.Interface().(valueInterface); ok {
+					if v := vi.node; v != nil {
+						for _, typ := range types {
+							if v.typ.id() == typ.id() {
+								destValue(f).Set(val)
+								return tnext
+							}
 						}
 					}
+					return fnext
+				}
+				vt := val.Type()
+				for _, typ := range types {
+					if vt == typ.TypeOf() {
+						destValue(f).Set(val)
+						return tnext
+					}
 				}
 				return fnext
 			}