backport commit 0eb51bf from master:

fix #36: do not crash on a := [...]int{1,2,3}; len(a)

all_test.go

@@ -667,7 +667,8 @@ var testcases = []TestCase{
 	TestCase{A, "builtin_append_1", "append(vs,0,1,2)", []byte{0, 1, 2}, nil},
 	TestCase{A, "builtin_append_2", "append(vn,3,4)", []byte{3, 4}, nil},
 	TestCase{A, "builtin_cap", "cap(va)", 2, nil},
-	TestCase{A, "builtin_len", "len(v5)", len("8y57riuh@#$"), nil},
+	TestCase{A, "builtin_len_1", "len(v5)", len("8y57riuh@#$"), nil},
+	TestCase{A, "builtin_len_2", "{ a := [...]int{1,2,3}; len(a) }", nil, none},
 	TestCase{A, "builtin_new", "new(int)", new(int), nil},
 	TestCase{A, "builtin_make_1", "make(map[int]int)", make(map[int]int), nil},
 	TestCase{A, "builtin_make_2", "make(map[int]int, 10)", make(map[int]int), nil}, // capacity is ignored

fast/builtin.go

@@ -526,6 +526,9 @@ func compileLen(c *Comp, sym Symbol, node *ast.CallExpr) *Call {
 		fun.Value = func(_ r.Value) int {
 			return n
 		}
+		// since we currently optimize len() by evaluating it at compile time,
+		// actual arg may not exist yet. optimize it away.
+		arg = exprLit(Lit{Type: tin, Value: xr.Zero(tin).Interface()}, nil)
 	}
 	return newCall1(fun, arg, isarray || arg.Const(), tout)
 }