Simplify format member detection in FormatVariadic

[labath]

Summary

This replaces the format member search, which was quite complicated, with a more
direct approach to detecting whether a class should be formatted using the
format-member method. Instead we use a special type llvm::format_adapter, which
every adapter must inherit from. Then the search can be simply implemented with
the is_base_of type trait.

Aside from the simplification, I like this way more because it makes it more
explicit that you are supposed to use this type only for adapter-like
formattings, and the other approach (format_provider overloads) should be used
as a default (a mistake I made when first trying to use this library).

The only slight change in behaviour here is that now choose the format-adapter
branch even if the format member invocation will fail to compile (e.g. because it is a
non-const member function and we are passing a const adapter), whereas
previously we would have gone on to search for format_providers for the type.
However, I think that is actually a good thing, as it probably means the
programmer did something wrong.

[zturner]

include/llvm/Support/FormatVariadicDetails.h

22–23 ↗

(On Diff #81100)

What if we added virtual void format(llvm::raw_ostream &Stream, StringRef Style) const = 0; to this class?

This would guaranteed that if the is_base_of check ever succeeded, it would never fail to compile because of a missing format member.

25–32 ↗

(On Diff #81100)

Do we actually need the templated version of this class? Why not just have AdapterBase inherit directly from format_adapter? It seems like AdapterBase<T> and format_adapter<T> could be merged into 1 class.

[labath]

Given that you have commented on implementation details rather than the overall idea, I am going to assume you are ok with this approach. I am going to refine this and get back..

include/llvm/Support/FormatVariadicDetails.h

22–23 ↗

(On Diff #81100)

I wasn't sure how much we cared about the performance impact of a virtual call. I can certainly add it. (Note the code will still fail to compile if the member is missing. The abstract function will give the error message a bit earlier though.)

25–32 ↗

(On Diff #81100)

Yeah, that can certainly be cleaned up. I am not sure I like the name AdapterBase though. If this is going to be the main entry point for user-written code, I'd rather call it FormatAdapter or Adapter (be it in CamelCase or snake_form).

[zturner]

include/llvm/Support/FormatVariadicDetails.h

22–23 ↗

(On Diff #81100)

If we mark all adapters final, then there should never be a virtual function call. The performance impact probably won't be a big deal, but I also don't see a reason not to do this.

25–32 ↗

(On Diff #81100)

FormatAdapter sounds fine. BTW, we may not even need any other base class at all. the is_base_of could check for is_base_of<T, FormatAdapter<T>> and we could reduce it down to just 1 base class.

[labath]

• More cleanup

[labath]

If we add the virtual functions, then this class begins to look a lot like format_wrapper, to the point of format_wrapper being obsolete. I tried to simplify this more by removing the wrapper class and using adapters everywhere instead. The basic logic of the formatv function then becomes: "You can either pass in a FormatAdapter directly, or if you don't, we'll create one for you using the format_provider specializations."

The side effect of the virtual function is that it forces us to choose between a const and a non-const version. I think that formatters generally should be deterministic ( == const), but I suppose there could be some occasional situations where they could maintain some aggregate state. So I choose the non-const version, as it is more flexible, but I could be convinced to change it.

include/llvm/Support/FormatVariadicDetails.h

25–32 ↗

(On Diff #81100)

That will not work because at this point, T is the FormatAdapter<RealT>. It may be possible to do some strange recursion here is_base_of<FormatAdapter<typename T::type>, T>, but I think it's not worth it, especially as having a common base type enables other simplifications.

[labath]

Update documentation.

[labath]

I think I've done all the cleanup I wanted to do here. Let me know what you think.

[labath]

Running the unittests for the latest version discovered a new issue. The usage
of && in the FormatAdapter base class forces each class deriving from it to use
std::move. I am not sure how much we care about that, but I am going to look
more into how we can avoid that.

[zturner]

Can you confirm that after your changes, we still trigger the missing_format_adapter error message if you try to format a type that doesn't have a formatter?

For example, just make a dummy class Foo and try to pass an instance of it to formatv. Make sure that the error message says something about missing_format_adapter<T> where T = 'Foo'.

[labath]

Yes, I can. I can also add a static_assert test for that.

As for the std::move issue, the only two options (which do not involve massive
template magic) I can think of are:

• live with it
• have the base class not store the formatted member and leave it up for each derived class to decide how to store it.

[zturner]

I see that you added the missing format provider test. Can you also just verify by inspection that the error message is still nice when it fails? (You may have already done this, but there's no way to really write a test for it). Assuming that's fine and all the tests pass, then lgtm.

[labath]

I wouldn't call any template-related error "nice", but yes, I have tried it, and it's the same error as before.

llvm/trunk/docs/ProgrammersManual.rst

namespace llvm {
       }
     }
     
   This is a useful extensibility mechanism for adding support for formatting your own
   custom types with your own custom Style options.  But it does not help when you want
   to extend the mechanism for formatting a type that the library already knows how to
   format.  For that, we need something else.
     
-2. Provide a **format adapter** with a non-static format method.
+2. Provide a **format adapter** inheriting from ``llvm::FormatAdapter<T>``.
 
   .. code-block:: c++
   
     namespace anything {
-      struct format_int_custom {
-        int N;
-        explicit format_int_custom(int N) : N(N) {}
-        void format(llvm::raw_ostream &Stream, StringRef Style) {
-          // Do whatever is necessary to format ``N`` into ``Stream``
+      struct format_int_custom : public llvm::FormatAdapter<int> {
+        explicit format_int_custom(int N) : llvm::FormatAdapter<int>(N) {}
+        void format(llvm::raw_ostream &Stream, StringRef Style) override {
+          // Do whatever is necessary to format ``this->Item`` into ``Stream``
         }
       };
     }
     namespace llvm {
       void foo() {
         std::string S = formatv("{0}", anything::format_int_custom(42));
       }
     }
     
-  If the search for a specialization of ``format_provider<T>`` for the given type
-  fails, ``formatv`` will subsequently check the argument for an instance method
-  named ``format`` with the signature described above.  If so, it will call the
+  If the type is detected to be derived from ``FormatAdapter<T>``, ``formatv``
+  will call the
   ``format`` method on the argument passing in the specified style.  This allows
   one to provide custom formatting of any type, including one which already has
   a builtin format provider.
 
 ``formatv`` Examples
 ^^^^^^^^^^^^^^^^^^^^
 Below is intended to provide an incomplete set of examples demonstrating
 the usage of ``formatv``.  More information can be found by reading the

llvm/trunk/include/llvm/Support/FormatAdapters.h

 
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/FormatCommon.h"
 #include "llvm/Support/FormatVariadicDetails.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
-template <typename T> class AdapterBase {
+template <typename T> class FormatAdapter : public detail::format_adapter {
 protected:
-  explicit AdapterBase(T &&Item) : Item(Item) {}
+  explicit FormatAdapter(T &&Item) : Item(Item) {}
 
   T Item;
+
   static_assert(!detail::uses_missing_provider<T>::value,
                 "Item does not have a format provider!");
 };
 
 namespace detail {
-template <typename T> class AlignAdapter : public AdapterBase<T> {
+template <typename T> class AlignAdapter final : public FormatAdapter<T> {
   AlignStyle Where;
   size_t Amount;
 
 public:
   AlignAdapter(T &&Item, AlignStyle Where, size_t Amount)
-      : AdapterBase<T>(std::forward<T>(Item)), Where(Where), Amount(Amount) {}
+      : FormatAdapter<T>(std::forward<T>(Item)), Where(Where), Amount(Amount) {}
 
   void format(llvm::raw_ostream &Stream, StringRef Style) {
-    auto Wrapper = detail::build_format_wrapper(std::forward<T>(this->Item));
-    FmtAlign(Wrapper, Where, Amount).format(Stream, Style);
+    auto Adapter = detail::build_format_adapter(std::forward<T>(this->Item));
+    FmtAlign(Adapter, Where, Amount).format(Stream, Style);
   }
 };
 
-template <typename T> class PadAdapter : public AdapterBase<T> {
+template <typename T> class PadAdapter final : public FormatAdapter<T> {
   size_t Left;
   size_t Right;
 
 public:
   PadAdapter(T &&Item, size_t Left, size_t Right)
-      : AdapterBase<T>(std::forward<T>(Item)), Left(Left), Right(Right) {}
+      : FormatAdapter<T>(std::forward<T>(Item)), Left(Left), Right(Right) {}
 
   void format(llvm::raw_ostream &Stream, StringRef Style) {
-    auto Wrapper = detail::build_format_wrapper(std::forward<T>(this->Item));
+    auto Adapter = detail::build_format_adapter(std::forward<T>(this->Item));
     Stream.indent(Left);
-    Wrapper.format(Stream, Style);
+    Adapter.format(Stream, Style);
     Stream.indent(Right);
   }
 };
 
-template <typename T> class RepeatAdapter : public AdapterBase<T> {
+template <typename T> class RepeatAdapter final : public FormatAdapter<T> {
   size_t Count;
 
 public:
   RepeatAdapter(T &&Item, size_t Count)
-      : AdapterBase<T>(std::forward<T>(Item)), Count(Count) {}
+      : FormatAdapter<T>(std::forward<T>(Item)), Count(Count) {}
 
   void format(llvm::raw_ostream &Stream, StringRef Style) {
-    auto Wrapper = detail::build_format_wrapper(std::forward<T>(this->Item));
+    auto Adapter = detail::build_format_adapter(std::forward<T>(this->Item));
     for (size_t I = 0; I < Count; ++I) {
-      Wrapper.format(Stream, Style);
+      Adapter.format(Stream, Style);
     }
   }
 };
 }
 
 template <typename T>
 detail::AlignAdapter<T> fmt_align(T &&Item, AlignStyle Where, size_t Amount) {
   return detail::AlignAdapter<T>(std::forward<T>(Item), Where, Amount);
 }
 
 template <typename T>
 detail::PadAdapter<T> fmt_pad(T &&Item, size_t Left, size_t Right) {
   return detail::PadAdapter<T>(std::forward<T>(Item), Left, Right);
 }
 
 template <typename T>
 detail::RepeatAdapter<T> fmt_repeat(T &&Item, size_t Count) {
   return detail::RepeatAdapter<T>(std::forward<T>(Item), Count);
 }
 }
 
-#endif
+#endif
 

llvm/trunk/include/llvm/Support/FormatCommon.h

 #include "llvm/ADT/SmallString.h"
 #include "llvm/Support/FormatVariadicDetails.h"
 #include "llvm/Support/raw_ostream.h"
 
 namespace llvm {
 enum class AlignStyle { Left, Center, Right };
 
 struct FmtAlign {
-  detail::format_wrapper &Wrapper;
+  detail::format_adapter &Adapter;
   AlignStyle Where;
   size_t Amount;
 
-  FmtAlign(detail::format_wrapper &Wrapper, AlignStyle Where, size_t Amount)
-      : Wrapper(Wrapper), Where(Where), Amount(Amount) {}
+  FmtAlign(detail::format_adapter &Adapter, AlignStyle Where, size_t Amount)
+      : Adapter(Adapter), Where(Where), Amount(Amount) {}
 
   void format(raw_ostream &S, StringRef Options) {
     // If we don't need to align, we can format straight into the underlying
     // stream.  Otherwise we have to go through an intermediate stream first
     // in order to calculate how long the output is so we can align it.
     // TODO: Make the format method return the number of bytes written, that
     // way we can also skip the intermediate stream for left-aligned output.
     if (Amount == 0) {
-      Wrapper.format(S, Options);
+      Adapter.format(S, Options);
       return;
     }
     SmallString<64> Item;
     raw_svector_ostream Stream(Item);
 
-    Wrapper.format(Stream, Options);
+    Adapter.format(Stream, Options);
     if (Amount <= Item.size()) {
       S << Item;
       return;
     }
 
     size_t PadAmount = Amount - Item.size();
     switch (Where) {
     case AlignStyle::Left:
default:
       S.indent(PadAmount);
       S << Item;
       break;
     }
   }
 };
 }
 
-#endif
+#endif
 

llvm/trunk/include/llvm/Support/FormatProviders.h

public:
   static void format(const llvm::iterator_range<IterT> &V,
                      llvm::raw_ostream &Stream, StringRef Style) {
     StringRef Sep;
     StringRef ArgStyle;
     std::tie(Sep, ArgStyle) = parseOptions(Style);
     auto Begin = V.begin();
     auto End = V.end();
     if (Begin != End) {
-      auto Wrapper =
-          detail::build_format_wrapper(std::forward<reference>(*Begin));
-      Wrapper.format(Stream, ArgStyle);
+      auto Adapter =
+          detail::build_format_adapter(std::forward<reference>(*Begin));
+      Adapter.format(Stream, ArgStyle);
       ++Begin;
     }
     while (Begin != End) {
       Stream << Sep;
-      auto Wrapper =
-          detail::build_format_wrapper(std::forward<reference>(*Begin));
-      Wrapper.format(Stream, ArgStyle);
+      auto Adapter =
+          detail::build_format_adapter(std::forward<reference>(*Begin));
+      Adapter.format(Stream, ArgStyle);
       ++Begin;
     }
   }
 };
 }
 
 #endif

llvm/trunk/include/llvm/Support/FormatVariadic.h

 protected:
   // The parameters are stored in a std::tuple, which does not provide runtime
   // indexing capabilities.  In order to enable runtime indexing, we use this
   // structure to put the parameters into a std::vector.  Since the parameters
   // are not all the same type, we use some type-erasure by wrapping the
   // parameters in a template class that derives from a non-template superclass.
   // Essentially, we are converting a std::tuple<Derived<Ts...>> to a
   // std::vector<Base*>.
-  struct create_wrappers {
+  struct create_adapters {
     template <typename... Ts>
-    std::vector<detail::format_wrapper *> operator()(Ts &... Items) {
-      return std::vector<detail::format_wrapper *>{&Items...};
+    std::vector<detail::format_adapter *> operator()(Ts &... Items) {
+      return std::vector<detail::format_adapter *>{&Items...};
     }
   };
 
   StringRef Fmt;
-  std::vector<detail::format_wrapper *> Wrappers;
+  std::vector<detail::format_adapter *> Adapters;
   std::vector<ReplacementItem> Replacements;
 
   static bool consumeFieldLayout(StringRef &Spec, AlignStyle &Where,
                                  size_t &Align, char &Pad);
 
   static std::pair<ReplacementItem, StringRef>
   splitLiteralAndReplacement(StringRef Fmt);
 
 public:
   formatv_object_base(StringRef Fmt, std::size_t ParamCount)
       : Fmt(Fmt), Replacements(parseFormatString(Fmt)) {
-    Wrappers.reserve(ParamCount);
+    Adapters.reserve(ParamCount);
   }
 
   void format(raw_ostream &S) const {
     for (auto &R : Replacements) {
       if (R.Type == ReplacementType::Empty)
         continue;
       if (R.Type == ReplacementType::Literal) {
         S << R.Spec;
         continue;
       }
-      if (R.Index >= Wrappers.size()) {
+      if (R.Index >= Adapters.size()) {
         S << R.Spec;
         continue;
       }
 
-      auto W = Wrappers[R.Index];
+      auto W = Adapters[R.Index];
 
       FmtAlign Align(*W, R.Where, R.Align);
       Align.format(S, R.Options);
     }
   }
   static std::vector<ReplacementItem> parseFormatString(StringRef Fmt);
 
   static Optional<ReplacementItem> parseReplacementItem(StringRef Spec);
public:
   }
 
   template <unsigned N> operator SmallString<N>() const { return sstr<N>(); }
 
   operator std::string() const { return str(); }
 };
 
 template <typename Tuple> class formatv_object : public formatv_object_base {
-  // Storage for the parameter wrappers.  Since the base class erases the type
+  // Storage for the parameter adapters.  Since the base class erases the type
   // of the parameters, we have to own the storage for the parameters here, and
   // have the base class store type-erased pointers into this tuple.
   Tuple Parameters;
 
 public:
   formatv_object(StringRef Fmt, Tuple &&Params)
       : formatv_object_base(Fmt, std::tuple_size<Tuple>::value),
         Parameters(std::move(Params)) {
-    Wrappers = apply_tuple(create_wrappers(), Parameters);
+    Adapters = apply_tuple(create_adapters(), Parameters);
   }
 };
 
 // \brief Format text given a format string and replacement parameters.
 //
 // ===General Description===
 //
 // Formats textual output.  `Fmt` is a string consisting of one or more

 //
 // In the case of a format string which does not match the grammar described
 // above, the output is undefined.  With asserts enabled, LLVM will trigger an
 // assertion.  Otherwise, it will try to do something reasonable, but in general
 // the details of what that is are undefined.
 //
 template <typename... Ts>
 inline auto formatv(const char *Fmt, Ts &&... Vals) -> formatv_object<decltype(
-    std::make_tuple(detail::build_format_wrapper(std::forward<Ts>(Vals))...))> {
+    std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...))> {
   using ParamTuple = decltype(
-      std::make_tuple(detail::build_format_wrapper(std::forward<Ts>(Vals))...));
+      std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...));
   return formatv_object<ParamTuple>(
       Fmt,
-      std::make_tuple(detail::build_format_wrapper(std::forward<Ts>(Vals))...));
+      std::make_tuple(detail::build_format_adapter(std::forward<Ts>(Vals))...));
 }
 
 } // end namespace llvm
 
 #endif // LLVM_SUPPORT_FORMATVARIADIC_H

llvm/trunk/include/llvm/Support/FormatVariadicDetails.h

 #include "llvm/Support/raw_ostream.h"
 
 #include <type_traits>
 
 namespace llvm {
 template <typename T, typename Enable = void> struct format_provider {};
 
 namespace detail {
-
-class format_wrapper {
+class format_adapter {
 protected:
-  virtual ~format_wrapper() {}
+  virtual ~format_adapter() {}
 
 public:
-  virtual void format(llvm::raw_ostream &S, StringRef Options) = 0;
+  virtual void format(raw_ostream &S, StringRef Options) = 0;
 };
 
-template <typename T> class member_format_wrapper : public format_wrapper {
+template <typename T> class provider_format_adapter : public format_adapter {
   T Item;
 
 public:
-  explicit member_format_wrapper(T &&Item) : Item(Item) {}
-
-  void format(llvm::raw_ostream &S, StringRef Options) override {
-    Item.format(S, Options);
-  }
-};
-
-template <typename T> class provider_format_wrapper : public format_wrapper {
-  T Item;
-
-public:
-  explicit provider_format_wrapper(T &&Item) : Item(Item) {}
+  explicit provider_format_adapter(T &&Item) : Item(Item) {}
 
   void format(llvm::raw_ostream &S, StringRef Options) override {
     format_provider<typename std::decay<T>::type>::format(Item, S, Options);
   }
 };
 
-template <typename T> class missing_format_wrapper;
-
-// Test if T is a class that contains a member function with the signature:
-//
-// void format(raw_ostream &, StringRef);
-//
-// It is assumed T is a non-reference type.
-template <class T, class Enable = void> class has_FormatMember {
-public:
-  static bool const value = false;
-};
-
-template <class T>
-class has_FormatMember<T,
-                       typename std::enable_if<std::is_class<T>::value &&
-                                               std::is_const<T>::value>::type> {
-  using CleanT = typename std::remove_volatile<T>::type;
-  using Signature_format = void (CleanT::*)(llvm::raw_ostream &S,
-                                            StringRef Options) const;
-
-  template <typename U>
-  static char test2(SameType<Signature_format, &U::format> *);
-
-  template <typename U> static double test2(...);
-
-public:
-  static bool const value = (sizeof(test2<CleanT>(nullptr)) == 1);
-};
-
-template <class T>
-class has_FormatMember<
-    T, typename std::enable_if<std::is_class<T>::value &&
-                               !std::is_const<T>::value>::type> {
-  using CleanT = typename std::remove_cv<T>::type;
-  using Signature_format = void (CleanT::*)(llvm::raw_ostream &S,
-                                            StringRef Options);
-
-  template <typename U>
-  static char test2(SameType<Signature_format, &U::format> *);
-
-  template <typename U> static double test2(...);
-
-public:
-  static bool const value =
-      (sizeof(test2<CleanT>(nullptr)) == 1) || has_FormatMember<const T>::value;
-};
+template <typename T> class missing_format_adapter;
 
 // Test if format_provider<T> is defined on T and contains a member function
 // with the signature:
 //   static void format(const T&, raw_stream &, StringRef);
 //
 template <class T> class has_FormatProvider {
 public:
   using Decayed = typename std::decay<T>::type;

 };
 
 // Simple template that decides whether a type T should use the member-function
 // based format() invocation.
 template <typename T>
 struct uses_format_member
     : public std::integral_constant<
           bool,
-          has_FormatMember<typename std::remove_reference<T>::type>::value> {};
+          std::is_base_of<format_adapter,
+                          typename std::remove_reference<T>::type>::value> {};
 
 // Simple template that decides whether a type T should use the format_provider
 // based format() invocation.  The member function takes priority, so this test
 // will only be true if there is not ALSO a format member.
 template <typename T>
 struct uses_format_provider
     : public std::integral_constant<
           bool, !uses_format_member<T>::value && has_FormatProvider<T>::value> {
 };
 
 // Simple template that decides whether a type T has neither a member-function
 // nor format_provider based implementation that it can use.  Mostly used so
 // that the compiler spits out a nice diagnostic when a type with no format
 // implementation can be located.
 template <typename T>
 struct uses_missing_provider
     : public std::integral_constant<bool,
                                     !uses_format_member<T>::value &&
                                         !uses_format_provider<T>::value> {};
 
 template <typename T>
-typename std::enable_if<uses_format_member<T>::value,
-                        member_format_wrapper<T>>::type
-build_format_wrapper(T &&Item) {
-  return member_format_wrapper<T>(std::forward<T>(Item));
+typename std::enable_if<uses_format_member<T>::value, T>::type
+build_format_adapter(T &&Item) {
+  return std::forward<T>(Item);
 }
 
 template <typename T>
 typename std::enable_if<uses_format_provider<T>::value,
-                        provider_format_wrapper<T>>::type
-build_format_wrapper(T &&Item) {
-  return provider_format_wrapper<T>(std::forward<T>(Item));
+                        provider_format_adapter<T>>::type
+build_format_adapter(T &&Item) {
+  return provider_format_adapter<T>(std::forward<T>(Item));
 }
 
 template <typename T>
 typename std::enable_if<uses_missing_provider<T>::value,
-                        missing_format_wrapper<T>>::type
-build_format_wrapper(T &&Item) {
-  return missing_format_wrapper<T>();
+                        missing_format_adapter<T>>::type
+build_format_adapter(T &&Item) {
+  return missing_format_adapter<T>();
 }
 }
 }
 
 #endif

llvm/trunk/unittests/Support/FormatVariadicTest.cpp

 //===- FormatVariadicTest.cpp - Unit tests for string formatting ----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/FormatAdapters.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "gtest/gtest.h"
 
 using namespace llvm;
 
-// Compile-time tests for the uses_format_member template
+// Compile-time tests templates in the detail namespace.
 namespace {
-struct ConstFormat {
-  void format(raw_ostream &OS, StringRef Opt) const { OS << "ConstFormat"; }
-};
-
-struct Format {
-  void format(raw_ostream &OS, StringRef Opt) { OS << "Format"; }
+struct Format : public FormatAdapter<int> {
+  Format(int N) : FormatAdapter<int>(std::move(N)) {}
+  void format(raw_ostream &OS, StringRef Opt) override { OS << "Format"; }
 };
 
 using detail::uses_format_member;
+using detail::uses_missing_provider;
 
 static_assert(uses_format_member<Format>::value, "");
 static_assert(uses_format_member<Format &>::value, "");
 static_assert(uses_format_member<Format &&>::value, "");
-static_assert(!uses_format_member<const Format>::value, "");
-static_assert(!uses_format_member<const Format &>::value, "");
-static_assert(!uses_format_member<const volatile Format>::value, "");
-static_assert(!uses_format_member<const volatile Format &>::value, "");
-
-static_assert(uses_format_member<ConstFormat>::value, "");
-static_assert(uses_format_member<ConstFormat &>::value, "");
-static_assert(uses_format_member<ConstFormat &&>::value, "");
-static_assert(uses_format_member<const ConstFormat>::value, "");
-static_assert(uses_format_member<const ConstFormat &>::value, "");
-static_assert(uses_format_member<const volatile ConstFormat>::value, "");
-static_assert(uses_format_member<const volatile ConstFormat &>::value, "");
+static_assert(uses_format_member<const Format>::value, "");
+static_assert(uses_format_member<const Format &>::value, "");
+static_assert(uses_format_member<const volatile Format>::value, "");
+static_assert(uses_format_member<const volatile Format &>::value, "");
+
+struct NoFormat {};
+static_assert(uses_missing_provider<NoFormat>::value, "");
 }
 
 TEST(FormatVariadicTest, EmptyFormatString) {
   auto Replacements = formatv_object_base::parseFormatString("");
   EXPECT_EQ(0U, Replacements.size());
 }
 
 TEST(FormatVariadicTest, NoReplacements) {
TEST(FormatVariadicTest, Range) {
 
   // 6. Empty separator and style.
   EXPECT_EQ("11235813",
             formatv("{0:$[]@<>}", make_range(IntRange.begin(), IntRange.end()))
                 .str());
 }
 
 TEST(FormatVariadicTest, Adapter) {
-  class Negative {
-    int N;
-
+  class Negative : public FormatAdapter<int> {
   public:
-    explicit Negative(int N) : N(N) {}
-    void format(raw_ostream &S, StringRef Options) const { S << -N; }
+    explicit Negative(int N) : FormatAdapter<int>(std::move(N)) {}
+    void format(raw_ostream &S, StringRef Options) override { S << -Item; }
   };
 
   EXPECT_EQ("-7", formatv("{0}", Negative(7)).str());
 
   int N = 171;
 
   EXPECT_EQ("  171  ",
             formatv("{0}", fmt_align(N, AlignStyle::Center, 7)).str());

 TEST(FormatVariadicTest, ImplicitConversions) {
   std::string S = formatv("{0} {1}", 1, 2);
   EXPECT_EQ("1 2", S);
 
   SmallString<4> S2 = formatv("{0} {1}", 1, 2);
   EXPECT_EQ("1 2", S2);
 }
 
-TEST(FormatVariadicTest, FormatMember) {
-  EXPECT_EQ("Format", formatv("{0}", Format()).str());
+TEST(FormatVariadicTest, FormatAdapter) {
+  EXPECT_EQ("Format", formatv("{0}", Format(1)).str());
 
-  Format var;
+  Format var(1);
   EXPECT_EQ("Format", formatv("{0}", var).str());
   EXPECT_EQ("Format", formatv("{0}", std::move(var)).str());
 
   // Not supposed to compile
-  // const Format cvar{};
+  // const Format cvar(1);
   // EXPECT_EQ("Format", formatv("{0}", cvar).str());
 }
-
-TEST(FormatVariadicTest, FormatMemberConst) {
-  EXPECT_EQ("ConstFormat", formatv("{0}", ConstFormat()).str());
-
-  ConstFormat var;
-  EXPECT_EQ("ConstFormat", formatv("{0}", var).str());
-  EXPECT_EQ("ConstFormat", formatv("{0}", std::move(var)).str());
-
-  const ConstFormat cvar{};
-  EXPECT_EQ("ConstFormat", formatv("{0}", cvar).str());
-}