checker: fix a bug checking generic closures (#18489)

vlib/v/checker/fn.v

@@ -459,12 +459,12 @@ fn (mut c Checker) anon_fn(mut node ast.AnonFn) ast.Type {
 		}
 		node.decl.scope.update_var_type(var.name, var.typ)
 	}
-	c.stmts(node.decl.stmts)
-	c.fn_decl(mut node.decl)
 	if has_generic && node.decl.generic_names.len == 0 {
 		c.error('generic closure fn must specify type parameter, e.g. fn [foo] [T]()',
 			node.decl.pos)
 	}
+	c.stmts(node.decl.stmts)
+	c.fn_decl(mut node.decl)
 	return node.typ
 }
 

vlib/v/checker/tests/generic_closure_fn_decl_err_a.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/generic_closure_fn_decl_err.vv:5:2: error: generic closure fn must specify type parameter, e.g. fn [foo] [T]()
+vlib/v/checker/tests/generic_closure_fn_decl_err_a.vv:5:2: error: generic closure fn must specify type parameter, e.g. fn [foo] [T]()
     3 |
     4 | pub fn (mut app App) register[T](service T) {
     5 |     fn [service] () {

vlib/v/checker/tests/generic_closure_fn_decl_err_b.out

@@ -0,0 +1,7 @@
+vlib/v/checker/tests/generic_closure_fn_decl_err_b.vv:13:2: error: generic closure fn must specify type parameter, e.g. fn [foo] [T]()
+   11 |     println(typeof(my_plugin).name) // MyPlugin
+   12 |
+   13 |     fn[my_plugin](){ my_plugin.on_update() }()
+      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+   14 | }
+   15 |

vlib/v/checker/tests/generic_closure_fn_decl_err_b.vv

@@ -0,0 +1,18 @@
+struct MyPlugin {
+}
+
+fn (p MyPlugin) on_update() {
+	println('[MyPlugin.on_update]')
+}
+
+fn foo[T](my_plugin T) {
+	println('[foo]')
+	println(T.name) // MyPlugin
+	println(typeof(my_plugin).name) // MyPlugin
+
+	fn[my_plugin](){ my_plugin.on_update() }()
+}
+
+fn main() {
+	foo[MyPlugin](MyPlugin{})
+}