From 6365d9e070be37c1b26b348fd9ada605359dfd7b Mon Sep 17 00:00:00 2001
From: Makhnev Petr <51853996+i582@users.noreply.github.com>
Date: Thu, 26 Jan 2023 19:40:10 +0400
Subject: [PATCH] checker: forbid var declaration in post statement of for loop
 (#17120)

---
 vlib/v/checker/for.v                                      | 8 +++++++-
 .../for_c_stmt_with_var_declaration_in_post_stmt.out      | 5 +++++
 .../tests/for_c_stmt_with_var_declaration_in_post_stmt.vv | 3 +++
 3 files changed, 15 insertions(+), 1 deletion(-)
 create mode 100644 vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.out
 create mode 100644 vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.vv

diff --git a/vlib/v/checker/for.v b/vlib/v/checker/for.v
index c8cf84d930..b5c9bf47ce 100644
--- a/vlib/v/checker/for.v
+++ b/vlib/v/checker/for.v
@@ -14,7 +14,13 @@ fn (mut c Checker) for_c_stmt(node ast.ForCStmt) {
 	c.expr(node.cond)
 	if node.has_inc {
 		if node.inc is ast.AssignStmt {
-			for right in node.inc.right {
+			assign := node.inc
+
+			if assign.op == .decl_assign {
+				c.error('for loop post statement cannot be a variable declaration', assign.pos)
+			}
+
+			for right in assign.right {
 				if right is ast.CallExpr {
 					if right.or_block.stmts.len > 0 {
 						c.error('options are not allowed in `for statement increment` (yet)',
diff --git a/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.out b/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.out
new file mode 100644
index 0000000000..32cccdf389
--- /dev/null
+++ b/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.out
@@ -0,0 +1,5 @@
+vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.vv:1:26: error: for loop post statement cannot be a variable declaration
+    1 | for i := 100; i < 200; j := 100 {
+      |                          ~~
+    2 |     println(j)
+    3 | }
\ No newline at end of file
diff --git a/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.vv b/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.vv
new file mode 100644
index 0000000000..07706ee122
--- /dev/null
+++ b/vlib/v/checker/tests/for_c_stmt_with_var_declaration_in_post_stmt.vv
@@ -0,0 +1,3 @@
+for i := 100; i < 200; j := 100 {
+	println(j)
+}