checker: disallow calls to a shared receiver method, on a non-shared var (fix #25577) (#25656)

Author: kbkpbot

vlib/v/checker/fn.v

@@ -2716,6 +2716,10 @@ fn (mut c Checker) method_call(mut node ast.CallExpr, mut continue_check &bool)
 	} else {
 		node.receiver_type = method.params[0].typ
 	}
+	if node.receiver_type.has_flag(.shared_f) && !node.left_type.has_flag(.shared_f) {
+		c.error('cannot use shared method `${node.name}` as `${node.left}` is not a shared var',
+			node.left.pos())
+	}
 	// if receiver_type is T, then `receiver_concrete_type` is concrete type, otherwise it is the same as `receiver_type`.
 	node.receiver_concrete_type = if is_method_from_embed {
 		node.from_embed_types.last().derive(method.params[0].typ)

vlib/v/checker/tests/shared_bad_receiver.out

@@ -0,0 +1,6 @@
+vlib/v/checker/tests/shared_bad_receiver.vv:16:2: error: cannot use shared method `inc` as `c` is not a shared var
+   14 | fn main() {
+   15 |     mut c := Counter{1}
+   16 |     c.inc()
+      |     ^
+   17 | }

vlib/v/checker/tests/shared_bad_receiver.vv

@@ -0,0 +1,17 @@
+module main
+
+struct Counter {
+mut:
+	value int
+}
+
+fn (shared c Counter) inc() {
+	lock c {
+		c.value += 1
+	}
+}
+
+fn main() {
+	mut c := Counter{1}
+	c.inc()
+}