interp: support two more type assertion cases

Fixes #967

Author: mpl

_test/assert1.go

@@ -0,0 +1,36 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+type TestStruct struct{}
+
+func (t TestStruct) String() string {
+	return "hello world"
+}
+
+func main() {
+	var t interface{}
+	t = time.Nanosecond
+	s, ok := t.(fmt.Stringer)
+	if !ok {
+		fmt.Println("time.Nanosecond does not implement fmt.Stringer")
+		return
+	}
+	fmt.Println(s.String())
+
+	var tt interface{}
+	tt = TestStruct{}
+	ss, ok := tt.(fmt.Stringer)
+	if !ok {
+		fmt.Println("TestStuct does not implement fmt.Stringer")
+		return
+	}
+	fmt.Println(ss.String())
+}
+
+// Output:
+// 1ns
+// hello world

interp/run.go

@@ -380,14 +380,54 @@ func typeAssert2(n *node) {
 		}
 	case isInterface(typ):
 		n.exec = func(f *frame) bltn {
-			v := value(f).Elem()
-			ok := v.IsValid() && canAssertTypes(v.Type(), rtype)
-			if ok {
+			var leftType reflect.Type
+			v := value(f)
+			val, ok := v.Interface().(valueInterface)
+			defer func() {
+				assertOk := ok
+				if setStatus {
+					value1(f).SetBool(assertOk)
+				}
+			}()
+			if ok && val.node.typ.cat != valueT {
+				m0 := val.node.typ.methods()
+				m1 := typ.methods()
+				if len(m0) < len(m1) {
+					ok = false
+					return next
+				}
+
+				for k, meth1 := range m1 {
+					var meth0 string
+					meth0, ok = m0[k]
+					if !ok {
+						return next
+					}
+					if meth0 != meth1 {
+						ok = false
+						return next
+					}
+				}
+
+				v = genInterfaceWrapper(val.node, rtype)(f)
 				value0(f).Set(v)
+				ok = true
+				return next
 			}
-			if setStatus {
-				value1(f).SetBool(ok)
+
+			if ok {
+				v = val.value
+				leftType = val.node.typ.rtype
+			} else {
+				v = v.Elem()
+				leftType = v.Type()
+				ok = true
 			}
+			ok = v.IsValid() && canAssertTypes(leftType, rtype)
+			if !ok {
+				return next
+			}
+			value0(f).Set(v)
 			return next
 		}
 	case n.child[0].typ.cat == valueT || n.child[0].typ.cat == errorT:
@@ -844,6 +884,9 @@ func genFunctionWrapper(n *node) func(*frame) reflect.Value {
 				if src.Type().Kind() != dest.Type().Kind() {
 					dest.Set(src.Addr())
 				} else {
+					if wrappedSrc, ok := src.Interface().(valueInterface); ok {
+						src = wrappedSrc.value
+					}
 					dest.Set(src)
 				}
 				d = d[numRet+1:]