[libc++][format] Improves diagnostics.

When attempting to format a type that does not have a valid formatter
specialization the code is ill-formed and diagnostics are generated.

The patch now properly constrains the exposition-only basic_format_arg
constructor (LWG3246). This constructor is called by
- std::make_format_args. This function has the precondition
    The type typename Context::template
    formatter_type<remove_const_t<Ti>> meets the BasicFormatter
    requirements ([formatter.requirements]) for each Ti in Args.
- In libc++ `std::basic_format_string` constructor too. This is not
  specified by the standard, however the requirements for constructor
  allow this.

Since `basic_format_arg` exposition-only constructor's constraint can't
be observed by the user, they can use it to SFINAE their own code.
Currently libc++, libstdc++, and MSVC STL consider the code ill-formed.
This patch keeps that behaviour.

This patch improves the output of these diagnostics:
- Reduces the number of errors generated, which reduces the number of
  lines in the output.
- The error gives a more specific diagnotic in multiple cases. For
  example, the formatter's format function must be const qualified. The
  diagnostic now mentions this specific issue.

Summary of the error messages using the type
no_formatter_specialization, which as the name implies has no
specialization.

std::format (and friends)
Before
- 163 lines of output with 7 errors
- First two diagnostics:
  - error: static assertion failed due to requirement '__arg != __arg_t::__none': the supplied type is not formattable
  - note: in instantiation of function template specialization 'std::__format::__create_format_arg<std::format_context, no_formatter_specialization>' requested here

After
- 25 lines of output with 1 error
- First two diagnostics:
  - error: static assertion failed: The required formatter specialization has not been provided.
  - note: in instantiation of function template specialization 'std::__format::__diagnose_invalid_formatter<std::format_context, no_formatter_specialization>' requested here

std::make_format_args (used by users to call vformat and friends)
Before
- 70 lines of output with 3 errors
- First two diagnostics:
  - error: static assertion failed due to requirement '__arg != __arg_t::__none': the supplied type is not formattable
  - note: in instantiation of function template specialization 'std::__format::__create_format_arg<std::format_context, no_formatter_specialization>' requested here

After
- 25 lines of output with 1 error
- First two diagnostics:
  - error: static assertion failed: The required formatter specialization has not been provided.
  - note: in instantiation of function template specialization 'std::__format::__diagnose_invalid_formatter<std::format_context, no_formatter_specialization>' requested here

Fixes: #100263

Author: mordante

libcxx/include/__format/format_arg.h

@@ -219,7 +219,7 @@ class __basic_format_arg_value {
           __format_([](basic_format_parse_context<_CharT>& __parse_ctx, _Context& __ctx, const void* __ptr) {
             using _Dp = remove_const_t<_Tp>;
             using _Qp = conditional_t<__formattable_with<const _Dp, _Context>, const _Dp, _Dp>;
-            static_assert(__formattable_with<_Qp, _Context>, "Mandated by [format.arg]/10");
+            static_assert(__formattable_with<_Qp, _Context>, "Mandated by [format.arg]/12");
 
             typename _Context::template formatter_type<_Dp> __f;
             __parse_ctx.advance_to(__f.parse(__parse_ctx));
@@ -334,21 +334,16 @@ class _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS basic_format_arg {
 private:
   using char_type = typename _Context::char_type;
 
-  // TODO FMT Implement constrain [format.arg]/4
-  // Constraints: The template specialization
-  //   typename Context::template formatter_type<T>
-  // meets the Formatter requirements ([formatter.requirements]).  The extent
-  // to which an implementation determines that the specialization meets the
-  // Formatter requirements is unspecified, except that as a minimum the
-  // expression
-  //   typename Context::template formatter_type<T>()
-  //    .format(declval<const T&>(), declval<Context&>())
-  // shall be well-formed when treated as an unevaluated operand.
-
 public:
   __basic_format_arg_value<_Context> __value_;
   __format::__arg_t __type_;
 
+  // This constructor is used for the exposition only constructor.
+  // Per [format.arg]/4
+  //   template<class T> explicit basic_format_arg(T& v) noexcept;
+  //     Constraints: T satisfies formattable-with<Context>.
+  //
+  // This constraint is implemented in __create_format_arg
   _LIBCPP_HIDE_FROM_ABI explicit basic_format_arg(__format::__arg_t __type,
                                                   __basic_format_arg_value<_Context> __value) noexcept
       : __value_(__value), __type_(__type) {}

libcxx/include/__format/format_arg_store.h

@@ -16,11 +16,15 @@
 
 #include <__concepts/arithmetic.h>
 #include <__concepts/same_as.h>
+#include <__concepts/semiregular.h>
 #include <__config>
 #include <__format/concepts.h>
 #include <__format/format_arg.h>
+#include <__format/format_parse_context.h>
 #include <__type_traits/conditional.h>
 #include <__type_traits/extent.h>
+#include <__type_traits/is_assignable.h>
+#include <__type_traits/is_constructible.h>
 #include <__type_traits/remove_const.h>
 #include <cstdint>
 #include <string>
@@ -161,12 +165,11 @@ consteval __arg_t __determine_arg_t() {
 //
 // Modeled after template<class T> explicit basic_format_arg(T& v) noexcept;
 // [format.arg]/4-6
-template <class _Context, class _Tp>
+template <class _Context, __formattable_with<_Context> _Tp>
 _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __value) noexcept {
   using _Dp               = remove_const_t<_Tp>;
   constexpr __arg_t __arg = __format::__determine_arg_t<_Context, _Dp>();
   static_assert(__arg != __arg_t::__none, "the supplied type is not formattable");
-  static_assert(__formattable_with<_Tp, _Context>);
 
   // Not all types can be used to directly initialize the
   // __basic_format_arg_value.  First handle all types needing adjustment, the
@@ -205,6 +208,95 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
     return basic_format_arg<_Context>{__arg, __value};
 }
 
+// Helper function that issues a diagnostic when __formattable_with<_Tp, _Context> is false.
+//
+// Since it's quite easy to make a mistake writing a formatter specialization
+// this function tries to give a better explanation. This should improve the
+// diagnostics when trying to format type that has no properly specialized
+// formatter.
+template <class _Context, class _Tp>
+[[noreturn]] _LIBCPP_HIDE_FROM_ABI constexpr void __diagnose_invalid_formatter() {
+  using _Formatter = typename _Context::template formatter_type<remove_const_t<_Tp>>;
+  constexpr bool __is_disabled =
+      !is_default_constructible_v<_Formatter> && !is_copy_constructible_v<_Formatter> &&
+      !is_move_constructible_v<_Formatter> && !is_copy_assignable_v<_Formatter> && !is_move_assignable_v<_Formatter>;
+  constexpr bool __is_semiregular = semiregular<_Formatter>;
+
+  constexpr bool __has_parse_function =
+      requires(_Formatter& __f, basic_format_parse_context<typename _Context::char_type> __pc) {
+        { __f.parse(__pc) };
+      };
+  constexpr bool __correct_parse_function_return_type =
+      requires(_Formatter& __f, basic_format_parse_context<typename _Context::char_type> __pc) {
+        { __f.parse(__pc) } -> same_as<typename decltype(__pc)::iterator>;
+      };
+
+  // The reason these static_asserts are placed in an if-constexpr-chain is to
+  // only show one error. For example, when the formatter is not specialized it
+  // would show all static_assert messages. With this chain the compiler only
+  // evaluates one static_assert.
+
+  if constexpr (__is_disabled)
+    static_assert(false, "The required formatter specialization has not been provided.");
+  else if constexpr (!__is_semiregular)
+    static_assert(false, "The required formatter specialization is not semiregular.");
+
+  else if constexpr (!__has_parse_function)
+    static_assert(
+        false, "The required formatter specialization does not have a parse function taking the proper arguments.");
+  else if constexpr (!__correct_parse_function_return_type)
+    static_assert(false, "The required formatter specialization's parse function does not return the required type.");
+
+  else {
+    // During constant evaluation this function is called, but the format
+    // member function has not been evaluated. This means these functions
+    // can't be evaluated at that time.
+    //
+    // Note this else branch should never been taken during constant
+    // eveluation, the static_asserts in one of the branches above should
+    // trigger.
+    constexpr bool __has_format_function = requires(_Formatter& __f, _Tp&& __t, _Context __fc) {
+      { __f.format(__t, __fc) };
+    };
+    constexpr bool __is_format_function_const_qualified = requires(const _Formatter& __cf, _Tp&& __t, _Context __fc) {
+      { __cf.format(__t, __fc) };
+    };
+    constexpr bool __correct_format_function_return_type = requires(_Formatter& __f, _Tp&& __t, _Context __fc) {
+      { __f.format(__t, __fc)->template same_as<typename _Context::iterator> };
+    };
+
+    if constexpr (!__has_format_function)
+      static_assert(
+          false, "The required formatter specialization does not have a format function taking the proper arguments.");
+    else if constexpr (!__is_format_function_const_qualified)
+      static_assert(false, "The required formatter specialization's format function is not const qualified.");
+    else if constexpr (!__correct_format_function_return_type)
+      static_assert(
+          false, "The required formatter specialization's format function does not return the required type.");
+
+    else
+      // This should not happen; it makes sure the code is ill-formed.
+      static_assert(false, "The required formatter specialization is not formattable with its context.");
+  }
+}
+
+// The Psuedo constructor is constrained per [format.arg]/4.
+// The only way for a user to call __create_format_arg is from std::make_format_args.
+// Per [format.arg.store]/3
+//   template<class Context = format_context, class... Args>
+//     format-arg-store<Context, Args...> make_format_args(Args&... fmt_args);
+//
+//   Preconditions: The type typename Context::template formatter_type<remove_const_t<Ti>>
+//   meets the BasicFormatter requirements ([formatter.requirements]) for each Ti in Args.
+//
+// This function is only called when the precondition is violated.
+// Without this function the compilation would fail since the overload is
+// SFINAE'ed away. This function is used to provide better diagnostics.
+template <class _Context, class _Tp>
+_LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp&) noexcept {
+  __format::__diagnose_invalid_formatter<_Context, _Tp>();
+}
+
 template <class _Context, class... _Args>
 _LIBCPP_HIDE_FROM_ABI void
 __create_packed_storage(uint64_t& __types, __basic_format_arg_value<_Context>* __values, _Args&... __args) noexcept {

libcxx/include/__format/format_context.h

@@ -65,9 +65,13 @@ __format_context_create(_OutIt __out_it, basic_format_args<basic_format_context<
 }
 #  endif
 
-using format_context = basic_format_context<back_insert_iterator<__format::__output_buffer<char>>, char>;
+template <class _CharT>
+using __format_context _LIBCPP_NODEBUG =
+    basic_format_context<back_insert_iterator<__format::__output_buffer<_CharT>>, _CharT>;
+
+using format_context = __format_context<char>;
 #  if _LIBCPP_HAS_WIDE_CHARACTERS
-using wformat_context = basic_format_context< back_insert_iterator<__format::__output_buffer<wchar_t>>, wchar_t>;
+using wformat_context = __format_context<wchar_t>;
 #  endif
 
 template <class _OutIt, class _CharT>

libcxx/include/__format/format_functions.h

@@ -13,6 +13,7 @@
 #include <__algorithm/clamp.h>
 #include <__concepts/convertible_to.h>
 #include <__concepts/same_as.h>
+#include <__concepts/semiregular.h>
 #include <__config>
 #include <__format/buffer.h>
 #include <__format/format_arg.h>
@@ -78,6 +79,27 @@ template <class... _Args>
 
 namespace __format {
 
+/// A "watered-down" __formattable_with for usage during constant evaluation.
+///
+/// The member function "format" can have a decuded return type. When
+/// constraining the __compile_time_handle this leads to a nested concept
+/// eveluation that is always false. Instead ignore the "format" member function.
+///
+/// In theory the same could happen to the "parse" member function. However
+/// thisfunction does not is less likely to have odd cased, and this function
+/// is "required" to be called during constant evaluation. So if this
+/// function's return type needs to be deduced things will probably not work.
+///
+/// See https://github.com/llvm/llvm-project/issues/81590 and
+/// test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp for
+/// an example of a problematic "format" member function.
+template <class _Tp, class _Context, class _Formatter = typename _Context::template formatter_type<remove_const_t<_Tp>>>
+concept __formattable_with_constexpr =
+    semiregular<_Formatter> &&
+    requires(_Formatter& __f, _Tp&& __t, basic_format_parse_context<typename _Context::char_type> __pc) {
+      { __f.parse(__pc) } -> same_as<typename decltype(__pc)::iterator>;
+    };
+
 /// Helper class parse and handle argument.
 ///
 /// When parsing a handle which is not enabled the code is ill-formed.
@@ -91,13 +113,22 @@ class _LIBCPP_TEMPLATE_VIS __compile_time_handle {
   }
 
   template <class _Tp>
+    requires __formattable_with_constexpr<_Tp, __format_context<_CharT>>
   _LIBCPP_HIDE_FROM_ABI constexpr void __enable() {
     __parse_ = [](basic_format_parse_context<_CharT>& __ctx) {
       formatter<_Tp, _CharT> __f;
       __ctx.advance_to(__f.parse(__ctx));
     };
   }
 
+  template <class _Tp>
+  _LIBCPP_HIDE_FROM_ABI constexpr void __enable() {
+    // Avoids the throw set in the constructor.
+    // Otherwise two diagnostics will be issued.
+    __parse_ = [](basic_format_parse_context<_CharT>&) {};
+    __format::__diagnose_invalid_formatter<__format_context<_CharT>, _Tp>();
+  }
+
   // Before calling __parse the proper handler needs to be set with __enable.
   // The default handler isn't a core constant expression.
   _LIBCPP_HIDE_FROM_ABI constexpr __compile_time_handle()