From 242d6fa1ff55357c3abc639a372dad24a9b6e8c4 Mon Sep 17 00:00:00 2001
From: yuyi <yuyi98@163.com>
Date: Mon, 6 Feb 2023 17:58:03 +0800
Subject: [PATCH] checker: disallow `for mut letter in 'abc' {` (#17234)

---
 vlib/v/checker/for.v                       |  5 +++--
 vlib/v/checker/tests/for_in_mut_string.out | 12 ++++++------
 vlib/v/checker/tests/for_in_mut_string.vv  |  5 +++++
 3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/vlib/v/checker/for.v b/vlib/v/checker/for.v
index b5c9bf47ce..20a3e6749d 100644
--- a/vlib/v/checker/for.v
+++ b/vlib/v/checker/for.v
@@ -113,6 +113,9 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 				}
 				else {}
 			}
+		} else if node.val_is_mut {
+			c.error('string type is immutable, it cannot be changed', node.pos)
+			return
 		}
 		if sym.kind == .struct_ {
 			// iterators
@@ -139,8 +142,6 @@ fn (mut c Checker) for_in_stmt(mut node ast.ForInStmt) {
 			node.kind = sym.kind
 			node.val_type = val_type
 			node.scope.update_var_type(node.val_var, val_type)
-		} else if sym.kind == .string && node.val_is_mut {
-			c.error('string type is immutable, it cannot be changed', node.pos)
 		} else if sym.kind == .any {
 			node.cond_type = typ
 			node.kind = sym.kind
diff --git a/vlib/v/checker/tests/for_in_mut_string.out b/vlib/v/checker/tests/for_in_mut_string.out
index 989006ae91..e46da21c6b 100644
--- a/vlib/v/checker/tests/for_in_mut_string.out
+++ b/vlib/v/checker/tests/for_in_mut_string.out
@@ -5,10 +5,10 @@ vlib/v/checker/tests/for_in_mut_string.vv:19:6: error: string type is immutable,
       |         ~~~
    20 |         println(ch)
    21 |     }
-vlib/v/checker/tests/for_in_mut_string.vv:20:3: error: `println` can not print void expressions
-   18 | fn wrap_text(mut gv Grid) {
-   19 |     for mut ch in gv.header {
-   20 |         println(ch)
-      |         ~~~~~~~~~~~
+vlib/v/checker/tests/for_in_mut_string.vv:23:6: error: string type is immutable, it cannot be changed
    21 |     }
-   22 | }
+   22 |
+   23 |     for mut letter in 'abc' {
+      |         ~~~
+   24 |         bit := letter - 1
+   25 |         println(bit)
diff --git a/vlib/v/checker/tests/for_in_mut_string.vv b/vlib/v/checker/tests/for_in_mut_string.vv
index d9779394cd..2a36f7f608 100644
--- a/vlib/v/checker/tests/for_in_mut_string.vv
+++ b/vlib/v/checker/tests/for_in_mut_string.vv
@@ -19,4 +19,9 @@ fn wrap_text(mut gv Grid) {
 	for mut ch in gv.header {
 		println(ch)
 	}
+
+	for mut letter in 'abc' {
+		bit := letter - 1
+		println(bit)
+	}
 }