Restore the 'B' type by https://github.com/gcflymoto and add tests.

format.h

@@ -729,7 +729,7 @@ BasicWriter<Char> &BasicWriter<Char>::operator<<(
     }
     break;
   }
-  case 'b': {
+  case 'b': case 'B': {
     UnsignedType n = abs_value;
     bool print_prefix = f.hash_flag();
     if (print_prefix) size += 2;
@@ -742,7 +742,7 @@ BasicWriter<Char> &BasicWriter<Char>::operator<<(
       *p-- = '0' + (n & 1);
     } while ((n >>= 1) != 0);
     if (print_prefix) {
-      *p-- = 'b';
+      *p-- = f.type();
       *p = '0';
     }
     break;

format_test.cc

@@ -316,6 +316,14 @@ TEST(WriterTest, WriteWideString) {
   //fmt::WWriter() << "abc";
 }
 
+TEST(WriterTest, bin) {
+  using fmt::bin;
+  EXPECT_EQ("1100101011111110", str(Writer() << bin(0xcafe)));
+  EXPECT_EQ("1011101010111110", str(Writer() << bin(0xbabeu)));
+  EXPECT_EQ("1101111010101101", str(Writer() << bin(0xdeadl)));
+  EXPECT_EQ("1011111011101111", str(Writer() << bin(0xbeeful)));
+}
+
 TEST(WriterTest, oct) {
   using fmt::oct;
   EXPECT_EQ("12", str(Writer() << oct(static_cast<short>(012))));
@@ -325,14 +333,6 @@ TEST(WriterTest, oct) {
   EXPECT_EQ("70", str(Writer() << oct(070ul)));
 }
 
-TEST(WriterTest, bin) {
-  using fmt::bin;
-  EXPECT_EQ("1100101011111110", str(Writer() << bin(0xcafe)));
-  EXPECT_EQ("1011101010111110", str(Writer() << bin(0xbabeu)));
-  EXPECT_EQ("1101111010101101", str(Writer() << bin(0xdeadl)));
-  EXPECT_EQ("1011111011101111", str(Writer() << bin(0xbeeful)));
-}
-
 TEST(WriterTest, hex) {
   using fmt::hex;
   fmt::IntFormatter<int, fmt::TypeSpec<'x'> > (*phex)(int value) = hex;
@@ -685,6 +685,9 @@ TEST(FormatterTest, SpaceSign) {
 TEST(FormatterTest, HashFlag) {
   EXPECT_EQ("42", str(Format("{0:#}") << 42));
   EXPECT_EQ("-42", str(Format("{0:#}") << -42));
+  EXPECT_EQ("0b101010", str(Format("{0:#b}") << 0x42));
+  EXPECT_EQ("0B101010", str(Format("{0:#B}") << 0x42));
+  EXPECT_EQ("-0b101010", str(Format("{0:#b}") << -0x42));
   EXPECT_EQ("0x42", str(Format("{0:#x}") << 0x42));
   EXPECT_EQ("0X42", str(Format("{0:#X}") << 0x42));
   EXPECT_EQ("-0x42", str(Format("{0:#x}") << -0x42));