interp: retry type definition if an array size is undefined

In case of forward definition of a constant, a symbol may be undefined when attempting to compute the array size in type analysis. Just mark the type as incomplete instead of aborting directly, to allow resolution at a second pass.

Fixes #1470.

_test/issue-1470.go

@@ -0,0 +1,15 @@
+package main
+
+type T struct {
+	num [tnum + 2]int
+}
+
+const tnum = 23
+
+func main() {
+	t := T{}
+	println(len(t.num))
+}
+
+// Output:
+// 25

interp/type.go

@@ -483,7 +483,11 @@ func nodeType2(interp *Interpreter, sc *scope, n *node, seen []*node) (t *itype,
 			length = int(vInt(sym.rval))
 		default:
 			// Size is defined by a numeric constant expression.
-			if _, err = interp.cfg(c0, sc, sc.pkgID, sc.pkgName); err != nil {
+			if _, err := interp.cfg(c0, sc, sc.pkgID, sc.pkgName); err != nil {
+				if strings.Contains(err.Error(), " undefined: ") {
+					incomplete = true
+					break
+				}
 				return nil, err
 			}
 			v, ok := c0.rval.Interface().(constant.Value)