interp: fix computation of array size from constant expression

The result of the expression giving the size of an array may be an `int` instead of `constant.Value` in case of an out of order declaration. Handle both cases.

Fixes #1536.

_test/issue-1536.go

@@ -0,0 +1,15 @@
+package main
+
+var a [len(prefix+path) + 2]int
+
+const (
+	prefix = "/usr/"
+	path   = prefix + "local/bin"
+)
+
+func main() {
+	println(len(a))
+}
+
+// Output:
+// 21

interp/type.go

@@ -484,19 +484,22 @@ 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.
+			ok := false
 			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)
-			if !ok {
-				incomplete = true
-				break
+			if length, ok = c0.rval.Interface().(int); !ok {
+				v, ok := c0.rval.Interface().(constant.Value)
+				if !ok {
+					incomplete = true
+					break
+				}
+				length = constToInt(v)
 			}
-			length = constToInt(v)
 		}
 		val, err := nodeType2(interp, sc, n.child[1], seen)
 		if err != nil {