Make format_arg() accept class hierarchies

If a base class can be formatted, it is assumed
that the hierarchy can be formatted from the base
class. The idiom is not uncommon with ostreams.

doc/api.rst

@@ -84,6 +84,36 @@ Note in the example above the ``format_arg`` function ignores the contents of
 ``format_arg`` in :file:`fmt/time.h` for an advanced example of how to use
 the ``format_str`` argument to customize the formatted output.
 
+This technique can also be used for formatting class hierarchies::
+
+  namespace local {
+  struct Parent {
+    Parent(int p) : p(p) {}
+    virtual void write(fmt::Writer &w) const {
+      w.write("Parent : p={}", p);
+    }
+    int p;
+  };
+
+  struct Child : Parent {
+    Child(int c, int p) : Parent(p), c(c) {}
+    virtual void write(fmt::Writer &w) const {
+      w.write("Child c={} : ", c);
+      Parent::write(w);
+    }
+    int c;
+  };
+
+  void format_arg(fmt::BasicFormatter<char> &f,
+                  const char *&format_str, const Parent &p) {
+    p.write(f.writer());
+  }
+  }
+  Local::Child c(1,2);
+  Local::Parent &p = c;
+  fmt::print("via ref to base: {}\n", p);
+  fmt::print("direct to child: {}\n", c);
+
 This section shows how to define a custom format function for a user-defined
 type. The next section describes how to get ``fmt`` to use a conventional stream
 output ``operator<<`` when one is defined for a user-defined type.

fmt/format.h

@@ -1255,9 +1255,9 @@ inline fmt::StringRef thousands_sep(...) { return ""; }
   typedef int FMT_CONCAT_(Assert, __LINE__)[(cond) ? 1 : -1] FMT_UNUSED
 #endif
 
-template <typename Formatter, typename Char, typename T>
-void format_arg(Formatter &, const Char *, const T &) {
-  FMT_STATIC_ASSERT(FalseType<T>::value,
+template <typename Formatter>
+void format_arg(Formatter&, ...) {
+  FMT_STATIC_ASSERT(FalseType<Formatter>::value,
                     "Cannot format argument. To enable the use of ostream "
                     "operator<< include fmt/ostream.h. Otherwise provide "
                     "an overload of format_arg.");