cgen: fix callback codegen on generic struct resolution (fix #24947) (#24948)

vlib/v/ast/table.v

@@ -1318,6 +1318,9 @@ pub fn (mut t Table) find_or_register_fn_type(f Fn, is_anon bool, has_decl bool)
 	anon := f.name == '' || is_anon
 	existing_idx := t.type_idxs[name]
 	if existing_idx > 0 && t.type_symbols[existing_idx].kind != .placeholder {
+		if t.type_symbols[existing_idx].info is FnType && !has_decl {
+			t.type_symbols[existing_idx].info.has_decl = has_decl
+		}
 		return existing_idx
 	}
 	return t.register_sym(

vlib/v/tests/structs/struct_anon_generic_fn_field_test.v

@@ -0,0 +1,37 @@
+pub struct ChunksArrayDecoder[T] {
+	callback fn (t T) = unsafe { nil }
+}
+
+pub fn new_chunks_array_decoder[T](callback fn (t T)) &ChunksArrayDecoder[T] {
+	return &ChunksArrayDecoder[T]{
+		callback: callback
+	}
+}
+
+struct Person {
+	name string
+	age  int
+	kgs  f32
+}
+
+@[heap]
+struct People {
+mut:
+	persons int
+	kgs     f32
+}
+
+fn (mut people People) callback(person Person) {
+	people.persons++
+	people.kgs += person.kgs
+}
+
+fn module_callback(person Person) {
+	println('person: ${person}')
+}
+
+fn test_main() {
+	mut people := &People{}
+	mut decoder := new_chunks_array_decoder[Person](people.callback)
+	_ = decoder
+}