Update cppfmt to current git.

vendor/Fmt/test/core-test.cc

@@ -151,7 +151,9 @@ TEST(buffer_test, indestructible) {
 template <typename T> struct mock_buffer final : buffer<T> {
   MOCK_METHOD1(do_grow, size_t(size_t capacity));
 
-  void grow(size_t capacity) { this->set(this->data(), do_grow(capacity)); }
+  void grow(size_t capacity) override {
+    this->set(this->data(), do_grow(capacity));
+  }
 
   mock_buffer(T* data = nullptr, size_t buf_capacity = 0) {
     this->set(data, buf_capacity);
@@ -452,7 +454,7 @@ struct check_custom {
     struct test_buffer final : fmt::detail::buffer<char> {
       char data[10];
       test_buffer() : fmt::detail::buffer<char>(data, 0, 10) {}
-      void grow(size_t) {}
+      void grow(size_t) override {}
     } buffer;
     auto parse_ctx = fmt::format_parse_context("");
     auto ctx = fmt::format_context(fmt::detail::buffer_appender<char>(buffer),
@@ -845,3 +847,43 @@ TEST(core_test, adl) {
   fmt::print("{}", s);
   fmt::print(stdout, "{}", s);
 }
+
+struct const_formattable {};
+struct nonconst_formattable {};
+
+FMT_BEGIN_NAMESPACE
+template <> struct formatter<const_formattable> {
+  auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  auto format(const const_formattable&, format_context& ctx)
+      -> decltype(ctx.out()) {
+    auto test = string_view("test");
+    return std::copy_n(test.data(), test.size(), ctx.out());
+  }
+};
+
+template <> struct formatter<nonconst_formattable> {
+  auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
+    return ctx.begin();
+  }
+
+  auto format(nonconst_formattable&, format_context& ctx)
+      -> decltype(ctx.out()) {
+    auto test = string_view("test");
+    return std::copy_n(test.data(), test.size(), ctx.out());
+  }
+};
+FMT_END_NAMESPACE
+
+TEST(core_test, is_const_formattable) {
+  EXPECT_TRUE((fmt::detail::is_const_formattable<const_formattable,
+                                                 fmt::format_context>()));
+  EXPECT_FALSE((fmt::detail::is_const_formattable<nonconst_formattable,
+                                                  fmt::format_context>()));
+}
+
+TEST(core_test, format_nonconst) {
+  EXPECT_EQ(fmt::format("{}", nonconst_formattable()), "test");
+}