From 419db8baa1a8102eb333c5a7aa269efd198e1977 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@fb.com>
Date: Wed, 22 Jan 2020 18:03:34 -0800
Subject: [PATCH] Fix length computation of constexpr C strings

---
 include/fmt/format.h | 9 ++++++++-
 test/format-test.cc  | 3 +++
 2 files changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/fmt/format.h b/include/fmt/format.h
index cc9e5b4e..cd3bb03f 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -3449,6 +3449,13 @@ template <typename Char> struct udl_arg {
   }
 };
 
+// A constexpr version of strlen.
+template <typename Char> FMT_CONSTEXPR size_t compute_length(const Char* s) {
+  size_t len = 0;
+  while (*s++) ++len;
+  return len;
+}
+
 }  // namespace internal
 
 inline namespace literals {
@@ -3512,7 +3519,7 @@ FMT_END_NAMESPACE
       using char_type = fmt::remove_cvref_t<decltype(*s)>;   \
       __VA_ARGS__ FMT_CONSTEXPR                              \
       operator fmt::basic_string_view<char_type>() const {   \
-        return {s, sizeof(s) / sizeof(char_type) - 1};       \
+        return {s, fmt::internal::compute_length(s)};        \
       }                                                      \
     };                                                       \
     return FMT_STRING();                                     \
diff --git a/test/format-test.cc b/test/format-test.cc
index 8bce69be..f479ceb6 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -1849,7 +1849,10 @@ TEST(FormatTest, UnpackedArgs) {
 struct string_like {};
 fmt::string_view to_string_view(string_like) { return "foo"; }
 
+FMT_CONSTEXPR_DECL const char* format_str_ptr = "0123456789";
+
 TEST(FormatTest, CompileTimeString) {
+  EXPECT_EQ(format_str_ptr, fmt::format(FMT_STRING(format_str_ptr)));
   EXPECT_EQ("42", fmt::format(FMT_STRING("{}"), 42));
   EXPECT_EQ(L"42", fmt::format(FMT_STRING(L"{}"), 42));
   EXPECT_EQ("foo", fmt::format(FMT_STRING("{}"), string_like()));