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)) {}
 
@@ -381,11 +384,12 @@ template <typename Char> class basic_string_view {
       : data_(s.data()),
         size_(s.size()) {}
 
-  template <
-      typename S,
-      FMT_ENABLE_IF(std::is_same<S, internal::std_string_view<Char>>::value)>
-  FMT_CONSTEXPR basic_string_view(S s) FMT_NOEXCEPT : data_(s.data()),
-                                                      size_(s.size()) {}
+#if __cplusplus >= 201703L  // C++17's std::string_view::size() is constexpr.
+  FMT_CONSTEXPR
+#endif
+  basic_string_view(internal::std_string_view<Char> s)
+      FMT_NOEXCEPT : data_(s.data()),
+                     size_(s.size()) {}
 
   /** Returns a pointer to the string data. */
   FMT_CONSTEXPR const Char* data() const { return data_; }

include/fmt/format.h

@@ -2236,6 +2236,7 @@ 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)
@@ -2266,8 +2267,8 @@ struct dynamic_format_specs : basic_format_specs<Char> {
   arg_ref<Char> precision_ref;
 };
 
-// Format spec handler that saves references to arguments representing dynamic
-// width and precision to be resolved at formatting time.
+// Format spec handler that saves references to arguments representing
+// dynamic width and precision to be resolved at formatting time.
 template <typename ParseContext>
 class dynamic_specs_handler
     : public specs_setter<typename ParseContext::char_type> {
@@ -2579,8 +2580,9 @@ FMT_CONSTEXPR void parse_format_string(basic_string_view<Char> format_str,
   auto begin = format_str.data();
   auto end = begin + format_str.size();
   while (begin != end) {
-    // Doing two passes with memchr (one for '{' and another for '}') is up to
-    // 2.5x faster than the naive one-pass implementation on big format strings.
+    // Doing two passes with memchr (one for '{' and another for '}') is up
+    // to 2.5x faster than the naive one-pass implementation on big format
+    // strings.
     const Char* p = begin;
     if (*begin != '{' && !find<IS_CONSTEXPR>(begin + 1, end, '{', p))
       return write(begin, end);
@@ -3523,9 +3525,22 @@ 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 - ((N > 0 && 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
 
@@ -3583,18 +3598,17 @@ FMT_CONSTEXPR internal::udl_arg<wchar_t> operator"" _a(const wchar_t* s,
 #endif  // FMT_USE_USER_DEFINED_LITERALS
 FMT_END_NAMESPACE
 
-#define FMT_STRING_IMPL(s, ...)                              \
-  [] {                                                       \
-    /* 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)>;   \
-      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_COMPILE_STRING();                             \
+#define FMT_STRING_IMPL(s, ...)                                     \
+  [] {                                                              \
+    /* 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[0])>;        \
+      FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR                    \
+      operator fmt::basic_string_view<char_type>() const {          \
+        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;
 
@@ -1853,10 +1854,25 @@ TEST(FormatTest, UnpackedArgs) {
 struct string_like {};
 fmt::string_view to_string_view(string_like) { return "foo"; }
 
+#if __cplusplus >= 201703L
+namespace {
+FMT_CONSTEXPR char withNull[3] = {'{', '}', '\0'};
+FMT_CONSTEXPR char noNull[2] = {'{', '}'};
+}  // namespace
+#endif
+
 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()));
+#if __cplusplus >= 201703L
+  EXPECT_EQ("42", fmt::format(FMT_STRING(withNull), 42));
+  EXPECT_EQ("42", fmt::format(FMT_STRING(noNull), 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) {