From 5bcc04e66a3f2697adc4da3e7d792e4ab6ea5f2c Mon Sep 17 00:00:00 2001
From: Swastik Baranwal <swstkbaranwal@gmail.com>
Date: Thu, 4 May 2023 23:53:34 +0530
Subject: [PATCH] checker: disallow `_ = <- quit` (#18104)

---
 vlib/v/checker/checker.v                      |  6 +++
 .../blank_ident_select_branch_send_err.out    |  7 ++++
 .../blank_ident_select_branch_send_err.vv     | 39 +++++++++++++++++++
 3 files changed, 52 insertions(+)
 create mode 100644 vlib/v/checker/tests/blank_ident_select_branch_send_err.out
 create mode 100644 vlib/v/checker/tests/blank_ident_select_branch_send_err.vv

diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index a65b8a2257..2bbbbf3603 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3609,6 +3609,12 @@ fn (mut c Checker) select_expr(mut node ast.SelectExpr) ast.Type {
 						c.error('`<-` receive expression expected', branch.stmt.right[0].pos())
 					}
 				}
+				if mut branch.stmt.left[0] is ast.Ident {
+					ident := branch.stmt.left[0] as ast.Ident
+					if ident.kind == .blank_ident && branch.stmt.op != .decl_assign {
+						c.error('cannot send on `_`, use `_ := <- quit` instead', branch.stmt.left[0].pos())
+					}
+				}
 			}
 			else {
 				if !branch.is_else {
diff --git a/vlib/v/checker/tests/blank_ident_select_branch_send_err.out b/vlib/v/checker/tests/blank_ident_select_branch_send_err.out
new file mode 100644
index 0000000000..b2cd5ffb70
--- /dev/null
+++ b/vlib/v/checker/tests/blank_ident_select_branch_send_err.out
@@ -0,0 +1,7 @@
+vlib/v/checker/tests/blank_ident_select_branch_send_err.vv:13:4: error: cannot send on `_`, use `_ := <- quit` instead
+   11 |                 x, y = y, x + y
+   12 |             }
+   13 |             _ = <-quit {
+      |             ^
+   14 |                 println('quit')
+   15 |                 return
diff --git a/vlib/v/checker/tests/blank_ident_select_branch_send_err.vv b/vlib/v/checker/tests/blank_ident_select_branch_send_err.vv
new file mode 100644
index 0000000000..1977d09fda
--- /dev/null
+++ b/vlib/v/checker/tests/blank_ident_select_branch_send_err.vv
@@ -0,0 +1,39 @@
+import time
+
+fn fibonacci(c chan int, quit chan int) {
+	println('fibonacci')
+	mut x, mut y := 0, 1
+
+	for {
+		select {
+			c <- x {
+				println('send')
+				x, y = y, x + y
+			}
+			_ = <-quit {
+				println('quit')
+				return
+			}
+			2 * time.second {
+				println('timeout')
+				return
+			}
+		}
+	}
+}
+
+fn main() {
+	ch := chan int{}
+	quit := chan int{}
+
+	spawn fn [ch, quit] () {
+		for _ in 0 .. 5 {
+			println('pop')
+			println(<-ch)
+		}
+		println('quit')
+		quit <- 0
+	}()
+
+	fibonacci(ch, quit)
+}