checker: detect and error on unreachable code in loops, after `contin…

…ue` and `break` statements (#21294)

vlib/v/checker/checker.v

@@ -2600,7 +2600,10 @@ fn (mut c Checker) stmts_ending_with_expression(mut stmts []ast.Stmt) {
 			}
 		}
 		c.stmt(mut stmt)
-		if stmt is ast.GotoLabel {
+		if !c.inside_anon_fn && c.in_for_count > 0 && stmt is ast.BranchStmt
+			&& stmt.kind in [.key_continue, .key_break] {
+			c.scope_returns = true
+		} else if stmt is ast.GotoLabel {
 			unreachable = token.Pos{
 				line_nr: -1
 			}

vlib/v/checker/tests/unreachable_code.out

@@ -1,7 +1,28 @@
-vlib/v/checker/tests/unreachable_code.vv:3:4: error: unreachable code
-    1 | fn foo() int {
-    2 |     return if 1 == 1 { 1 } else { 2 }
-    3 |     a := 1
+vlib/v/checker/tests/unreachable_code.vv:15:3: error: unreachable code
+   13 |         continue
+   14 |         // ...
+   15 |         eprintln('continue')
+      |         ~~~~~~~~~~~~~~~~~~~~
+   16 |     }
+   17 |     for _ in 0 .. 3 {
+vlib/v/checker/tests/unreachable_code.vv:21:4: error: unreachable code
+   19 |             break
+   20 |             // ...
+   21 |             eprintln('done')
+      |             ~~~~~~~~~~~~~~~~
+   22 |         }
+   23 |         break
+vlib/v/checker/tests/unreachable_code.vv:25:3: error: unreachable code
+   23 |         break
+   24 |         // ...
+   25 |         return 3
+      |         ~~~~~~~~
+   26 |     }
+   27 |     return if 1 == 1 { 1 } else { 2 }
+vlib/v/checker/tests/unreachable_code.vv:28:4: error: unreachable code
+   26 |     }
+   27 |     return if 1 == 1 { 1 } else { 2 }
+   28 |     a := 1
       |       ~~
-    4 |     println(a)
-    5 | }
+   29 |     println(a)
+   30 | }

vlib/v/checker/tests/unreachable_code.vv

@@ -1,8 +1,34 @@
-fn foo() int {
+fn foo(condition bool) int {
+	for _ in 0 .. 1 {
+		if condition {
+			continue // scope should not cause errors.
+		}
+		if !condition {
+			break
+		}
+		eprintln('done')
+		break
+	}
+	for _ in 0 .. 2 {
+		continue
+		// ...
+		eprintln('continue')
+	}
+	for _ in 0 .. 3 {
+		if condition {
+			break
+			// ...
+			eprintln('done')
+		}
+		break
+		// ...
+		return 3
+	}
 	return if 1 == 1 { 1 } else { 2 }
 	a := 1
 	println(a)
 }
+
 fn main() {
-	foo()
+	foo(true)
 }