checker: fix inferring method receiver types (#10166)

vlib/v/checker/check_types.v

@@ -503,7 +503,8 @@ pub fn (mut c Checker) infer_fn_generic_types(f ast.Fn, mut call_expr ast.CallEx
 				if sym.kind == .struct_ {
 					info := sym.info as ast.Struct
 					receiver_generic_names := info.generic_types.map(c.table.get_type_symbol(it).name)
-					if gt_name in receiver_generic_names {
+					if gt_name in receiver_generic_names
+						&& info.generic_types.len == info.concrete_types.len {
 						idx := receiver_generic_names.index(gt_name)
 						typ = info.concrete_types[idx]
 					}

vlib/v/tests/generics_struct_anon_fn_type_test.v

@@ -40,14 +40,30 @@ fn holder_call_21<T>(func T, a int) int {
 	return h.call(a)
 }
 
+fn holder_call_12<T>(func T, a int) int {
+	return FnHolder1{func}.call(a)
+}
+
+fn holder_call_22<T>(func T, a int) int {
+	return FnHolder2{func}.call(a)
+}
+
 fn test_generic_struct_with_anon_fn_parameter() {
 	mut ret := holder_call_1(neg, 1)
 	assert ret == -1
 	ret = holder_call_11(neg, 2)
 	assert ret == -2
+	ret = holder_call_12(neg, 3)
+	assert ret == -3
+	ret = FnHolder1<fn (int) int>{neg}.call(4)
+	assert ret == -4
 
 	ret = holder_call_2(neg, 3)
 	assert ret == -3
 	ret = holder_call_21(neg, 4)
 	assert ret == -4
+	ret = holder_call_22(neg, 5)
+	assert ret == -5
+	ret = FnHolder2<fn (int) int>{neg}.call(6)
+	assert ret == -6
 }