checker: check non-optional fn return or_block (#9227)

vlib/v/checker/checker.v

@@ -3693,6 +3693,15 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 		}
 		ast.CallExpr {
 			mut ret_type := c.call_expr(mut node)
+			if !ret_type.has_flag(.optional) {
+				if node.or_block.kind == .block {
+					c.error('unexpected `or` block, the function `$node.name` does not return an optional',
+						node.or_block.pos)
+				} else if node.or_block.kind == .propagate {
+					c.error('unexpected `?`, the function `$node.name` does not return an optional',
+						node.or_block.pos)
+				}
+			}
 			if ret_type.has_flag(.optional) && node.or_block.kind != .absent {
 				ret_type = ret_type.clear_flag(.optional)
 			}

vlib/v/checker/tests/fn_return_or_err.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/fn_return_or_err.vv:6:17: error: unexpected `or` block, the function `pop` does not return an optional
+    4 |
+    5 | pub fn next(mut v []Typ) Typ {
+    6 |     return v.pop() or { Typ{} }
+      |                    ~~~~~~~~~~~~
+    7 | }
+    8 |

vlib/v/checker/tests/fn_return_or_err.vv

@@ -0,0 +1,13 @@
+module main
+
+pub struct Typ {}
+
+pub fn next(mut v []Typ) Typ {
+	return v.pop() or { Typ{} }
+}
+
+fn main() {
+	mut v := [Typ{}]
+	last := next(mut v)
+	println('$last')
+}