Trying to improve errors in the unformattable case. by brevzin · Pull Request #3478 · fmtlib/fmt

brevzin · 2023-06-04T17:35:16Z

This is my attempt to improve some some of the compile errors that we get when types aren't formattable at all.

void test(X x, Y y, Z z) {
    fmt::vprint("{} {} {}\n", fmt::make_format_args(x, y, z));
}

Before - you know something is unformattable, but you don't know which one: is it the X, the Y, or the Z?

<source>: In instantiation of 'constexpr fmt::v10::detail::value<Context> fmt::v10::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; T = Y; typename std::enable_if<PACKED, int>::type <anonymous> = 0]':
<source>:1805:51:   required from 'constexpr fmt::v10::format_arg_store<Context, Args>::format_arg_store(T& ...) [with T = {X, Y, Z}; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; Args = {X, Y, Z}]'
<source>:1823:18:   required from 'constexpr fmt::v10::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<T>::type>::type ...> fmt::v10::make_format_args(T& ...) [with Context = basic_format_context<appender, char>; T = {X, Y, Z}]'
<source>:2936:52:   required from here
<source>:1577:7: error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1577 |       formattable,
      |       ^~~~~~~~~~~
<source>:1577:7: note: 'formattable' evaluates to false
ASM generation compiler returned: 1
<source>: In instantiation of 'constexpr fmt::v10::detail::value<Context> fmt::v10::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; T = Y; typename std::enable_if<PACKED, int>::type <anonymous> = 0]':
<source>:1805:51:   required from 'constexpr fmt::v10::format_arg_store<Context, Args>::format_arg_store(T& ...) [with T = {X, Y, Z}; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; Args = {X, Y, Z}]'
<source>:1823:18:   required from 'constexpr fmt::v10::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<T>::type>::type ...> fmt::v10::make_format_args(T& ...) [with Context = basic_format_context<appender, char>; T = {X, Y, Z}]'
<source>:2936:52:   required from here
<source>:1577:7: error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1577 |       formattable,
      |       ^~~~~~~~~~~
<source>:1577:7: note: 'formattable' evaluates to false

After: obviously the Y:

<source>: In instantiation of 'constexpr fmt::v10::detail::value<Context> fmt::v10::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; T = Y; typename std::enable_if<PACKED, int>::type <anonymous> = 0]':
<source>:1805:51:   required from 'constexpr fmt::v10::format_arg_store<Context, Args>::format_arg_store(T& ...) [with T = {X, Y, Z}; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; Args = {X, Y, Z}]'
<source>:1823:18:   required from 'constexpr fmt::v10::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<T>::type>::type ...> fmt::v10::make_format_args(T& ...) [with Context = basic_format_context<appender, char>; T = {X, Y, Z}]'
<source>:2936:52:   required from here
<source>:1573:63: error: 'fmt::v10::detail::type_is_unformattable_for<Y, char> _' has incomplete type
 1573 |     type_is_unformattable_for<T, typename Context::char_type> _;
      |                                                               ^
<source>:1577:7: error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1577 |       formattable,
      |       ^~~~~~~~~~~
<source>:1577:7: note: 'formattable' evaluates to false
ASM generation compiler returned: 1
<source>: In instantiation of 'constexpr fmt::v10::detail::value<Context> fmt::v10::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; T = Y; typename std::enable_if<PACKED, int>::type <anonymous> = 0]':
<source>:1805:51:   required from 'constexpr fmt::v10::format_arg_store<Context, Args>::format_arg_store(T& ...) [with T = {X, Y, Z}; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; Args = {X, Y, Z}]'
<source>:1823:18:   required from 'constexpr fmt::v10::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<T>::type>::type ...> fmt::v10::make_format_args(T& ...) [with Context = basic_format_context<appender, char>; T = {X, Y, Z}]'
<source>:2936:52:   required from here
<source>:1573:63: error: 'fmt::v10::detail::type_is_unformattable_for<Y, char> _' has incomplete type
 1573 |     type_is_unformattable_for<T, typename Context::char_type> _;
      |                                                               ^
<source>:1577:7: error: static assertion failed: Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1577 |       formattable,
      |       ^~~~~~~~~~~
<source>:1577:7: note: 'formattable' evaluates to false

Example 2, which is the more typical way people run into this:

void test(X x, Y y, Z z) {
    fmt::print("{} {} {}\n", x, y, z);
}

Before (just copying the first error). Here, you can tell that Y isn't formattable, but the message is about something not being default constructible, which is a little cryptic?

<source>: In instantiation of 'constexpr decltype (ctx.begin()) fmt::v10::detail::parse_format_specs(ParseContext&) [with T = Y; ParseContext = compile_parse_context<char>; decltype (ctx.begin()) = const char*]':
<source>:2607:22:   required from 'constexpr fmt::v10::detail::format_string_checker<Char, Args>::format_string_checker(fmt::v10::basic_string_view<Char>) [with Char = char; Args = {X, Y, Z}]'
<source>:2937:15:   required from here
<source>:2549:10: error: use of deleted function 'fmt::v10::formatter<T, Char, Enable>::formatter() [with T = Y; Char = char; Enable = void]'
 2549 |   return formatter<mapped_type, char_type>().parse(ctx);
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:1066:3: note: declared here
 1066 |   formatter() = delete;
      |   ^~~~~~~~~

After: clearly Y is unformattable:

<source>: In instantiation of 'constexpr decltype (ctx.begin()) fmt::v10::detail::parse_format_specs(ParseContext&) [with T = Y; ParseContext = compile_parse_context<char>; decltype (ctx.begin()) = const char*]':
<source>:2607:22:   required from 'constexpr fmt::v10::detail::format_string_checker<Char, Args>::format_string_checker(fmt::v10::basic_string_view<Char>) [with Char = char; Args = {X, Y, Z}]'
<source>:2937:15:   required from here
<source>:2545:45: error: 'fmt::v10::detail::type_is_unformattable_for<Y, char> _' has incomplete type
 2545 |     type_is_unformattable_for<T, char_type> _;
      |                                             ^
<source>: In instantiation of 'constexpr fmt::v10::detail::value<Context> fmt::v10::detail::make_arg(T&) [with bool PACKED = true; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; T = Y; typename std::enable_if<PACKED, int>::type <anonymous> = 0]':
<source>:1805:51:   required from 'constexpr fmt::v10::format_arg_store<Context, Args>::format_arg_store(T& ...) [with T = {X, Y, Z}; Context = fmt::v10::basic_format_context<fmt::v10::appender, char>; Args = {X, Y, Z}]'
<source>:1823:18:   required from 'constexpr fmt::v10::format_arg_store<Context, typename std::remove_cv<typename std::remove_reference<T>::type>::type ...> fmt::v10::make_format_args(T& ...) [with Context = basic_format_context<appender, char>; T = {X, Y, Z}]'
<source>:2870:44:   required from 'void fmt::v10::print(format_string<T ...>, T&& ...) [with T = {X&, Y&, Z&}; format_string<T ...> = basic_format_string<char, X&, Y&, Z&>]'
<source>:2937:15:   required from here
<source>:1573:63: error: 'fmt::v10::detail::type_is_unformattable_for<Y, char> _' has incomplete type
 1573 |     type_is_unformattable_for<T, typename Context::char_type> _;
      |                                                               ^

Maybe it's possible to do better than this, I'm not sure, but this is the best I've been able to come up with so far at least.

vitaut · 2023-06-04T21:14:42Z

Having the type name displayed in diagnostic would be nice but unfortunately you can't use if constexpr unconditionally.

vitaut · 2023-06-16T11:54:45Z

@brevzin, do you plan to update this PR?

brevzin · 2023-06-18T14:09:31Z

@brevzin, do you plan to update this PR?

Yeah I will. Are you okay with a solution that just gives the better error in C++17, and maintains status quo before then?

vitaut · 2023-06-18T14:51:24Z

Are you okay with a solution that just gives the better error in C++17, and maintains status quo before then?

I am fine with that.

brevzin · 2023-06-19T18:37:55Z

Are you okay with a solution that just gives the better error in C++17, and maintains status quo before then?

I am fine with that.

Done.

vitaut · 2023-06-20T12:21:19Z

+#if defined(__cpp_if_constexpr)
+  if constexpr (!formattable_char) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
+#endif


I think the message below better explains the error, so let's remove this.

You still get both errors - the static_assert message is still printed. This way you also get which character type (and pointer type, for the next one) you were trying to print, which I think is still helpful info.

Hmm, is this information actually useful? You cannot make the types in question formattable and I think having two errors may be confusing.

vitaut · 2023-06-20T12:21:30Z

+#if defined(__cpp_if_constexpr)
+  if constexpr (!formattable_pointer) {
+    type_is_unformattable_for<T, typename Context::char_type> _;
+  }
+#endif


vitaut · 2023-06-20T12:23:26Z

+#if defined(__cpp_if_constexpr)
+  if constexpr (std::is_default_constructible_v<
+                    formatter<mapped_type, char_type>>) {
+    return formatter<mapped_type, char_type>().parse(ctx);
+  } else {
+    type_is_unformattable_for<T, char_type> _;
+    return ctx.begin();
+  }
+#else


Why do we need to duplicate the logic here?

So I could do something like this instead:

if constexpr (/* not default constructible */) { type_is_unformattable_for<T, char_type> _; } return formatter<mapped_type, char_type>().parse(Ctx);

But doing it this way gives you both the ~~good~~ slightly better error message with type_is_unformattable and also the "not default constructible" error message when you actually try to construct the thing. But all the latter one tells you is that... something failed, whereas the former tells you what failed. I think it's better to just provide the one?

I am not sure why we care about checking default constructibility at all. It should be uncommon and not worth any extra diagnostics.

I am not sure why we care about checking default constructibility at all. It should be uncommon and not worth any extra diagnostics.

Because that's what actually fails here. Today, if MyType is not formattable and you try to format a MyType, the error that you get is:

<source>:2549:10: error: use of deleted function 'fmt::v10::formatter<T, Char, Enable>::formatter() [with T = MyType; Char = char; Enable = void]' 2549 | return formatter<mapped_type, char_type>().parse(ctx); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That's the only way of checking if something is formattable, as far as I'm aware. has_formatter does the same thing.

vitaut · 2023-07-01T13:46:13Z

Merged, thanks!

Trying to improve errors in the unformattable case.

b5bd632

Adding guards around if constexpr

805e84f

phprus reviewed Jun 19, 2023

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Comment thread include/fmt/core.h Outdated

brevzin added 2 commits June 19, 2023 13:55

Something something msvc.

9b2ae5e

Something something msvc.

422181f

vitaut requested changes Jun 20, 2023

View reviewed changes

Removing these two

1323cd4

vitaut merged commit de4705f into fmtlib:master Jul 1, 2023

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brevzin commented Jun 4, 2023

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vitaut commented Jun 4, 2023

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vitaut commented Jun 16, 2023

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brevzin commented Jun 18, 2023

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vitaut commented Jun 18, 2023

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brevzin commented Jun 19, 2023

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vitaut commented Jul 1, 2023

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