From 5e184888990d68a563533461edffc94b3be8b934 Mon Sep 17 00:00:00 2001
From: Howard Hinnant <howard.hinnant@gmail.com>
Date: Sat, 5 Sep 2020 16:33:44 -0400
Subject: [PATCH] Fix subtle bug in fractional_width

---
 include/date/date.h                  | 12 ++++++-----
 test/date_test/detail/width.pass.cpp | 30 ++++++++++++++--------------
 2 files changed, 22 insertions(+), 20 deletions(-)

diff --git a/include/date/date.h b/include/date/date.h
index 00b2993..6c7787c 100644
--- a/include/date/date.h
+++ b/include/date/date.h
@@ -3685,11 +3685,12 @@ struct undocumented {explicit undocumented() = default;};
 // Example:  width<4>::value    ==  2
 // Example:  width<10>::value   ==  1
 // Example:  width<1000>::value ==  3
-template <std::uint64_t n, std::uint64_t d = 10, unsigned w = 0,
-          bool should_continue = !(n < 2) && d != 0 && (w < 19)>
+template <std::uint64_t n, std::uint64_t d, unsigned w = 0,
+          bool should_continue = n%d != 0 && w < 19>
 struct width
 {
-    static CONSTDATA unsigned value = 1 + width<n, d%n*10, w+1>::value;
+    static_assert(d > 0, "width called with zero denominator");
+    static CONSTDATA unsigned value = 1 + width<n%d*10, d, w+1>::value;
 };
 
 template <std::uint64_t n, std::uint64_t d, unsigned w>
@@ -3718,9 +3719,10 @@ class decimal_format_seconds
 {
     using CT = typename std::common_type<Duration, std::chrono::seconds>::type;
     using rep = typename CT::rep;
+    static unsigned CONSTDATA trial_width =
+        detail::width<CT::period::num, CT::period::den>::value;
 public:
-    static unsigned CONSTDATA width = detail::width<CT::period::den>::value < 19 ?
-                                      detail::width<CT::period::den>::value : 6u;
+    static unsigned CONSTDATA width = trial_width < 19 ? trial_width : 6u;
     using precision = std::chrono::duration<rep,
                                             std::ratio<1, static_pow10<width>::value>>;
 
diff --git a/test/date_test/detail/width.pass.cpp b/test/date_test/detail/width.pass.cpp
index 6fbb7b9..94f57b7 100644
--- a/test/date_test/detail/width.pass.cpp
+++ b/test/date_test/detail/width.pass.cpp
@@ -46,19 +46,19 @@ int
 main()
 {
     using namespace date::detail;
-    static_assert(width<0>::value == 0, "");
-    static_assert(width<1>::value == 0, "");
-    static_assert(width<2>::value == 1, "");
-    static_assert(width<3>::value == 19, "");
-    static_assert(width<4>::value == 2, "");
-    static_assert(width<5>::value == 1, "");
-    static_assert(width<6>::value == 19, "");
-    static_assert(width<7>::value == 19, "");
-    static_assert(width<8>::value == 3, "");
-    static_assert(width<9>::value == 19, "");
-    static_assert(width<10>::value == 1, "");
-    static_assert(width<100>::value == 2, "");
-    static_assert(width<1000>::value == 3, "");
-    static_assert(width<10000>::value == 4, "");
-    static_assert(width<625>::value == 4, "");
+    static_assert(width<0, 1>::value == 0, "");
+    static_assert(width<1, 1>::value == 0, "");
+    static_assert(width<1, 2>::value == 1, "");
+    static_assert(width<1, 3>::value == 19, "");
+    static_assert(width<1, 4>::value == 2, "");
+    static_assert(width<1, 5>::value == 1, "");
+    static_assert(width<1, 6>::value == 19, "");
+    static_assert(width<1, 7>::value == 19, "");
+    static_assert(width<1, 8>::value == 3, "");
+    static_assert(width<1, 9>::value == 19, "");
+    static_assert(width<1, 10>::value == 1, "");
+    static_assert(width<1, 100>::value == 2, "");
+    static_assert(width<1, 1000>::value == 3, "");
+    static_assert(width<1, 10000>::value == 4, "");
+    static_assert(width<756, 625>::value == 4, "");
 }