checker: disallow <- (channel push) on right-hand side of assignment (fix #12309) (#12321)

Author: x3ro

vlib/v/checker/checker.v

@@ -3711,6 +3711,12 @@ pub fn (mut c Checker) assign_stmt(mut node ast.AssignStmt) {
 				right_len = 0
 			}
 		}
+		if right is ast.InfixExpr {
+			if right.op == .arrow {
+				c.error('cannot use `<-` on the right-hand side of an assignment, as it does not return any values',
+					right.pos)
+			}
+		}
 	}
 	if node.left.len != right_len {
 		if right_first is ast.CallExpr {

vlib/v/checker/tests/assign_expr_channel_push.out

@@ -0,0 +1,4 @@
+vlib/v/checker/tests/assign_expr_channel_push.vv:2:9: error: cannot use `<-` on the right-hand side of an assignment, as it does not return any values
+    1 | ch := chan f64{}
+    2 | _ := ch <- 4.56
+      |

vlib/v/checker/tests/assign_expr_channel_push.vv

@@ -0,0 +1,2 @@
+ch := chan f64{}
+_ := ch <- 4.56