checker: disallow _ = <- quit (#18104)

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 {

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

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)
+}