Enable FMT_STRING() use with types other than string literals

include/fmt/core.h

@@ -371,6 +371,9 @@ template <typename Char> class basic_string_view {
     the size with ``std::char_traits<Char>::length``.
     \endrst
    */
+#if __cplusplus >= 201703L  // C++17's char_traits::length() is constexpr.
+  FMT_CONSTEXPR
+#endif
   basic_string_view(const Char* s)
       : data_(s), size_(std::char_traits<Char>::length(s)) {}
 

include/fmt/format.h

@@ -2238,6 +2238,7 @@ enum class arg_id_kind { none, index, name };
 // An argument reference.
 template <typename Char> struct arg_ref {
   FMT_CONSTEXPR arg_ref() : kind(arg_id_kind::none), val() {}
+
   FMT_CONSTEXPR explicit arg_ref(int index)
       : kind(arg_id_kind::index), val(index) {}
   FMT_CONSTEXPR explicit arg_ref(basic_string_view<Char> name)
@@ -3525,9 +3526,21 @@ template <typename Char> struct udl_arg {
   }
 };
 
+// Converts string literals to basic_string_view.
 template <typename Char, size_t N>
-FMT_CONSTEXPR basic_string_view<Char> literal_to_view(const Char (&s)[N]) {
-  return {s, N - 1};
+FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
+    const Char (&s)[N]) {
+  // Remove trailing null character if needed. Won't be present if this is used
+  // with raw character array (i.e. not defined as a string).
+  return {s,
+          N - ((std::char_traits<Char>::to_int_type(s[N - 1]) == 0) ? 1 : 0)};
+}
+
+// Converts string_view to basic_string_view.
+template <typename Char>
+FMT_CONSTEXPR basic_string_view<Char> compile_string_to_view(
+    const std_string_view<Char>& s) {
+  return {s.data(), s.size()};
 }
 }  // namespace internal
 
@@ -3589,11 +3602,10 @@ FMT_END_NAMESPACE
   [] {                                                              \
     /* Use a macro-like name to avoid shadowing warnings. */        \
     struct FMT_COMPILE_STRING : fmt::compile_string {               \
-      using char_type = fmt::remove_cvref_t<decltype(*s)>;   \
+      using char_type = fmt::remove_cvref_t<decltype(s[0])>;        \
       FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR                    \
       operator fmt::basic_string_view<char_type>() const {          \
-        /* FMT_STRING only accepts string literals. */       \
-        return fmt::internal::literal_to_view(s);            \
+        return fmt::internal::compile_string_to_view<char_type>(s); \
       }                                                             \
     };                                                              \
     return FMT_COMPILE_STRING();                                    \

test/format-test.cc

@@ -46,6 +46,7 @@ using fmt::format_error;
 using fmt::memory_buffer;
 using fmt::string_view;
 using fmt::wmemory_buffer;
+using fmt::wstring_view;
 using fmt::internal::basic_writer;
 using fmt::internal::max_value;
 
@@ -1854,10 +1855,23 @@ TEST(FormatTest, UnpackedArgs) {
 struct string_like {};
 fmt::string_view to_string_view(string_like) { return "foo"; }
 
+constexpr char with_null[3] = {'{', '}', '\0'};
+constexpr char no_null[2] = {'{', '}'};
+
 TEST(FormatTest, CompileTimeString) {
   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()));
+  (void)with_null;
+  (void)no_null;
+#if __cplusplus >= 201703L
+  EXPECT_EQ("42", fmt::format(FMT_STRING(with_null), 42));
+  EXPECT_EQ("42", fmt::format(FMT_STRING(no_null), 42));
+#endif
+#if defined(FMT_USE_STRING_VIEW) && __cplusplus >= 201703L
+  EXPECT_EQ("42", fmt::format(FMT_STRING(std::string_view("{}")), 42));
+  EXPECT_EQ(L"42", fmt::format(FMT_STRING(std::wstring_view(L"{}")), 42));
+#endif
 }
 
 TEST(FormatTest, CustomFormatCompileTimeString) {