checker: fix struct field init with generic anon fn (add the test from …

…#18294) (#20888)

vlib/v/tests/struct_field_init_with_generic_anon_fn_test.v

@@ -0,0 +1,30 @@
+pub struct Module {
+}
+
+pub struct Service {
+	callback fn () = unsafe { nil }
+}
+
+pub fn (mut self Module) do_anything[T]() {
+}
+
+pub fn (mut self Module) register[T]() {
+	_ := Service{
+		callback: fn [mut self] [T]() {
+			self.do_anything[T]()
+		}
+	}
+}
+
+struct Something {
+}
+
+struct SomethingDifferent {
+}
+
+fn test_struct_field_init_with_generic_anon_fn() {
+	mut mod := Module{}
+	mod.register[Something]()
+	mod.register[SomethingDifferent]()
+	assert true
+}