interp: improve type assertions

In type assertion at compile time, compare signatures between function types only.

Make `itype.numOut()` return the correct value for Go builtins (this was not strictly necessary due to above fix, but it is correct and improves maintainability).

Fixes #1454.

_test/issue-1454.go

@@ -0,0 +1,22 @@
+package main
+
+type I2 interface {
+	I2() string
+}
+
+type I interface {
+	I2
+}
+
+type S struct{}
+
+func (*S) I2() string { return "foo" }
+
+func main() {
+	var i I
+	_, ok := i.(*S)
+	println(ok)
+}
+
+// Output:
+// false

interp/cfg.go

@@ -1060,7 +1060,7 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				}
 
 				n.gen = c0.sym.builtin
-				c0.typ = &itype{cat: builtinT}
+				c0.typ = &itype{cat: builtinT, name: bname}
 				if n.typ, err = nodeType(interp, sc, n); err != nil {
 					return
 				}

interp/type.go

@@ -1249,8 +1249,13 @@ func (t *itype) numOut() int {
 		if t.rtype.Kind() == reflect.Func {
 			return t.rtype.NumOut()
 		}
+	case builtinT:
+		switch t.name {
+		case "append", "cap", "complex", "copy", "imag", "len", "make", "new", "real", "recover":
+			return 1
+		}
 	}
-	return 1
+	return 0
 }
 
 func (t *itype) out(i int) *itype {

interp/typecheck.go

@@ -625,6 +625,11 @@ func (check typecheck) typeAssertionExpr(n *node, typ *itype) error {
 			return n.cfgErrorf("impossible type assertion: %s does not implement %s as %q method has a pointer receiver", typ.id(), n.typ.id(), name)
 		}
 
+		if im.cat != funcT || tm.cat != funcT {
+			// It only makes sense to compare in/out parameter types if both types are functions.
+			continue
+		}
+
 		err := n.cfgErrorf("impossible type assertion: %s does not implement %s", typ.id(), n.typ.id())
 		if im.numIn() != tm.numIn() || im.numOut() != tm.numOut() {
 			return err