Support %P (lowercase am/pm) glibc extension

Fun fact: in glibc you can also spell %P %#p, # meaning “swap case”.
I haven’t implemented glibc’s ^ or # flags.

doc/syntax.rst

@@ -315,8 +315,8 @@ Format specifications for chrono duration and time point types as well as
    modifier: "E" | "O"
    chrono_type: "a" | "A" | "b" | "B" | "c" | "C" | "d" | "D" | "e" | "F" |
               : "g" | "G" | "h" | "H" | "I" | "j" | "m" | "M" | "n" | "p" |
-              : "q" | "Q" | "r" | "R" | "S" | "t" | "T" | "u" | "U" | "V" |
+              : "P" | "q" | "Q" | "r" | "R" | "S" | "t" | "T" | "u" | "U" |
-              : "w" | "W" | "x" | "X" | "y" | "Y" | "z" | "Z" | "%"
+              : "V" | "w" | "W" | "x" | "X" | "y" | "Y" | "z" | "Z" | "%"
 
 Literal chars are copied unchanged to the output. Precision is valid only for
 ``std::chrono::duration`` types with a floating-point representation type.
@@ -396,6 +396,8 @@ The available presentation types (*chrono_type*) are:
 +---------+--------------------------------------------------------------------+
 | ``'p'`` | The AM/PM designations associated with a 12-hour clock.            |
 +---------+--------------------------------------------------------------------+
+| ``'P'`` | ``%p``, but lowercase (am/pm). (GNU/glibc extension.)              |
++---------+--------------------------------------------------------------------+
 | ``'q'`` | The duration's unit suffix.                                        |
 +---------+--------------------------------------------------------------------+
 | ``'Q'`` | The duration's numeric value (as if extracted via ``.count()``).   |

include/fmt/chrono.h

@@ -838,6 +838,9 @@ FMT_CONSTEXPR const Char* parse_chrono_format(const Char* begin,
     case 'p':
       handler.on_am_pm();
       break;
+    case 'P':
+      handler.on_am_pm_lower();
+      break;
     case 'Q':
       handler.on_duration_value();
       break;
@@ -976,6 +979,7 @@ template <typename Derived> struct null_chrono_spec_handler {
   FMT_CONSTEXPR void on_24_hour_time() { unsupported(); }
   FMT_CONSTEXPR void on_iso_time() { unsupported(); }
   FMT_CONSTEXPR void on_am_pm() { unsupported(); }
+  FMT_CONSTEXPR void on_am_pm_lower() { unsupported(); }
   FMT_CONSTEXPR void on_duration_value() { unsupported(); }
   FMT_CONSTEXPR void on_duration_unit() { unsupported(); }
   FMT_CONSTEXPR void on_utc_offset(numeric_system) { unsupported(); }
@@ -1019,6 +1023,7 @@ struct tm_format_checker : null_chrono_spec_handler<tm_format_checker> {
   FMT_CONSTEXPR void on_24_hour_time() {}
   FMT_CONSTEXPR void on_iso_time() {}
   FMT_CONSTEXPR void on_am_pm() {}
+  FMT_CONSTEXPR void on_am_pm_lower() {}
   FMT_CONSTEXPR void on_utc_offset(numeric_system) {}
   FMT_CONSTEXPR void on_tz_name() {}
 };
@@ -1636,6 +1641,15 @@ class tm_writer {
     }
   }
 
+  void on_am_pm_lower() {
+    if (is_classic_) {
+      *out_++ = tm_hour() < 12 ? 'a' : 'p';
+      *out_++ = 'm';
+    } else {
+      format_localized('P');
+    }
+  }
+
   // These apply to chrono durations but not tm.
   void on_duration_value() {}
   void on_duration_unit() {}
@@ -1656,6 +1670,7 @@ struct chrono_format_checker : null_chrono_spec_handler<chrono_format_checker> {
   FMT_CONSTEXPR void on_24_hour_time() {}
   FMT_CONSTEXPR void on_iso_time() {}
   FMT_CONSTEXPR void on_am_pm() {}
+  FMT_CONSTEXPR void on_am_pm_lower() {}
   FMT_CONSTEXPR void on_duration_value() const {
     if (has_precision_integral) {
       FMT_THROW(format_error("precision not allowed for this argument type"));
@@ -2011,6 +2026,11 @@ struct chrono_formatter {
     format_tm(time(), &tm_writer_type::on_am_pm);
   }
 
+  void on_am_pm_lower() {
+    if (handle_nan_inf()) return;
+    format_tm(time(), &tm_writer_type::on_am_pm_lower);
+  }
+
   void on_duration_value() {
     if (handle_nan_inf()) return;
     write_sign();

test/chrono-test.cc

@@ -973,4 +973,12 @@ TEST(chrono_test, glibc_extensions) {
     EXPECT_EQ(fmt::format("{:%_S}", d), " 3.140000");
     EXPECT_EQ(fmt::format("{:%-S}", d), "3.140000");
   }
+
+  {
+    // %p is standard, %P is glibc
+    const auto tm_am = make_tm(1970, 1, 1, 0, 0, 0);
+    const auto tm_pm = make_tm(1970, 1, 1, 12, 0, 0);
+    EXPECT_EQ(fmt::format("{:%p,%P}", tm_am), "AM,am");
+    EXPECT_EQ(fmt::format("{:%p,%P}", tm_pm), "PM,pm");
+  }
 }