From fe0d910a7de68710171bfcc994802fe8a840c547 Mon Sep 17 00:00:00 2001
From: Victor Zverovich <viz@meta.com>
Date: Mon, 15 Jan 2024 06:48:10 -0800
Subject: [PATCH] Replace multiple error reporting mechanisms with report_error

---
 include/fmt/base.h       | 59 ++++++++++++++++++++--------------------
 include/fmt/format-inl.h |  2 +-
 include/fmt/format.h     | 32 ++++++++++------------
 include/fmt/printf.h     | 20 ++++++--------
 include/fmt/ranges.h     |  7 ++---
 test/format-test.cc      |  2 +-
 test/scan-test.cc        |  2 +-
 test/scan.h              | 20 +++++++-------
 8 files changed, 70 insertions(+), 74 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 4d0fb703..432a6f0a 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -683,8 +683,17 @@ enum {
 };
 }  // namespace detail
 
-/** Throws ``format_error`` with a given message. */
-FMT_NORETURN FMT_API void throw_format_error(const char* message);
+/**
+  Reports a format error at compile time or, via a ``format_error`` exception,
+  at runtime.
+ */
+// This function is intentionally not constexpr to give a compile-time error.
+FMT_NORETURN FMT_API void report_error(const char* message);
+
+FMT_DEPRECATED FMT_NORETURN inline void throw_format_error(
+    const char* message) {
+  report_error(message);
+}
 
 /** String's character (code unit) type. */
 template <typename S,
@@ -738,8 +747,7 @@ template <typename Char> class basic_format_parse_context {
    */
   FMT_CONSTEXPR auto next_arg_id() -> int {
     if (next_arg_id_ < 0) {
-      throw_format_error(
-          "cannot switch from manual to automatic argument indexing");
+      report_error("cannot switch from manual to automatic argument indexing");
       return 0;
     }
     int id = next_arg_id_++;
@@ -753,8 +761,7 @@ template <typename Char> class basic_format_parse_context {
    */
   FMT_CONSTEXPR void check_arg_id(int id) {
     if (next_arg_id_ > 0) {
-      throw_format_error(
-          "cannot switch from automatic to manual argument indexing");
+      report_error("cannot switch from automatic to manual argument indexing");
       return;
     }
     next_arg_id_ = -1;
@@ -787,20 +794,20 @@ class compile_parse_context : public basic_format_parse_context<Char> {
 
   FMT_CONSTEXPR auto next_arg_id() -> int {
     int id = base::next_arg_id();
-    if (id >= num_args_) throw_format_error("argument not found");
+    if (id >= num_args_) report_error("argument not found");
     return id;
   }
 
   FMT_CONSTEXPR void check_arg_id(int id) {
     base::check_arg_id(id);
-    if (id >= num_args_) throw_format_error("argument not found");
+    if (id >= num_args_) report_error("argument not found");
   }
   using base::check_arg_id;
 
   FMT_CONSTEXPR void check_dynamic_spec(int arg_id) {
     detail::ignore_unused(arg_id);
     if (arg_id < num_args_ && types_ && !is_integral_type(types_[arg_id]))
-      throw_format_error("width/precision is not integer");
+      report_error("width/precision is not integer");
   }
 };
 
@@ -1081,7 +1088,7 @@ FMT_CONSTEXPR void basic_format_parse_context<Char>::do_check_arg_id(int id) {
       (!FMT_GCC_VERSION || FMT_GCC_VERSION >= 1200)) {
     using context = detail::compile_parse_context<Char>;
     if (id >= static_cast<context*>(this)->num_args())
-      throw_format_error("argument not found");
+      report_error("argument not found");
   }
 }
 
@@ -1916,9 +1923,6 @@ class context {
   }
   auto args() const -> const basic_format_args<context>& { return args_; }
 
-  // This function is intentionally not constexpr to give a compile-time error.
-  void on_error(const char* message) { throw_format_error(message); }
-
   // Returns an iterator to the beginning of the output range.
   FMT_CONSTEXPR auto out() -> iterator { return out_; }
 
@@ -2215,13 +2219,13 @@ FMT_CONSTEXPR auto do_parse_arg_id(const Char* begin, const Char* end,
     else
       ++begin;
     if (begin == end || (*begin != '}' && *begin != ':'))
-      throw_format_error("invalid format string");
+      report_error("invalid format string");
     else
       handler.on_index(index);
     return begin;
   }
   if (!is_name_start(c)) {
-    throw_format_error("invalid format string");
+    report_error("invalid format string");
     return begin;
   }
   auto it = begin;
@@ -2274,13 +2278,13 @@ FMT_CONSTEXPR auto parse_dynamic_spec(const Char* begin, const Char* end,
     if (val != -1)
       value = val;
     else
-      throw_format_error("number is too big");
+      report_error("number is too big");
   } else if (*begin == '{') {
     ++begin;
     auto handler = dynamic_spec_id_handler<Char>{ctx, ref};
     if (begin != end) begin = parse_arg_id(begin, end, handler);
     if (begin != end && *begin == '}') return ++begin;
-    throw_format_error("invalid format string");
+    report_error("invalid format string");
   }
   return begin;
 }
@@ -2292,7 +2296,7 @@ FMT_CONSTEXPR auto parse_precision(const Char* begin, const Char* end,
     -> const Char* {
   ++begin;
   if (begin == end || *begin == '}') {
-    throw_format_error("invalid precision");
+    report_error("invalid precision");
     return begin;
   }
   return parse_dynamic_spec(begin, end, value, ref, ctx);
@@ -2319,7 +2323,7 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
     state current_state = state::start;
     FMT_CONSTEXPR void operator()(state s, bool valid = true) {
       if (current_state >= s || !valid)
-        throw_format_error("invalid format specifier");
+        report_error("invalid format specifier");
       current_state = s;
     }
   } enter_state;
@@ -2334,7 +2338,7 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
     FMT_CONSTEXPR auto operator()(pres pres_type, int set) -> const Char* {
       if (!in(arg_type, set)) {
         if (arg_type == type::none_type) return begin;
-        throw_format_error("invalid format specifier");
+        report_error("invalid format specifier");
       }
       specs.type = pres_type;
       return begin + 1;
@@ -2378,7 +2382,7 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
       enter_state(state::zero);
       if (!is_arithmetic_type(arg_type)) {
         if (arg_type == type::none_type) return begin;
-        throw_format_error("format specifier requires numeric argument");
+        report_error("format specifier requires numeric argument");
       }
       if (specs.align == align::none) {
         // Ignore 0 if align is specified for compatibility with std::format.
@@ -2448,8 +2452,7 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
     case 'a':
       return parse_presentation_type(pres::hexfloat, float_set);
     case 'c':
-      if (arg_type == type::bool_type)
-        throw_format_error("invalid format specifier");
+      if (arg_type == type::bool_type) report_error("invalid format specifier");
       return parse_presentation_type(pres::chr, integral_set);
     case 's':
       return parse_presentation_type(pres::string,
@@ -2466,11 +2469,11 @@ FMT_CONSTEXPR auto parse_format_specs(const Char* begin, const Char* end,
       // Parse fill and alignment.
       auto fill_end = begin + code_point_length(begin);
       if (end - fill_end <= 0) {
-        throw_format_error("invalid format specifier");
+        report_error("invalid format specifier");
         return begin;
       }
       if (*begin == '{') {
-        throw_format_error("invalid fill character '{'");
+        report_error("invalid fill character '{'");
         return begin;
       }
       auto align = parse_align(to_ascii(*fill_end));
@@ -2610,7 +2613,7 @@ FMT_CONSTEXPR auto check_char_specs(const format_specs<Char>& specs) -> bool {
     return false;
   }
   if (specs.align == align::numeric || specs.sign != sign::none || specs.alt)
-    throw_format_error("invalid format specifier for char");
+    report_error("invalid format specifier for char");
   return true;
 }
 
@@ -2687,9 +2690,7 @@ template <typename Char, typename... Args> class format_string_checker {
     return id >= 0 && id < num_args ? parse_funcs_[id](context_) : begin;
   }
 
-  FMT_CONSTEXPR void on_error(const char* message) {
-    throw_format_error(message);
-  }
+  FMT_CONSTEXPR void on_error(const char* message) { report_error(message); }
 };
 
 // A base class for compile-time strings.
diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index 15649194..05ea41ac 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -123,7 +123,7 @@ FMT_FUNC auto write_loc(appender out, loc_value value,
 }
 }  // namespace detail
 
-FMT_FUNC void throw_format_error(const char* message) {
+FMT_FUNC void report_error(const char* message) {
   FMT_THROW(format_error(message));
 }
 
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 558b6dd1..12f5ba09 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -1033,8 +1033,6 @@ template <typename OutputIt, typename Char> class generic_context {
     return args_;
   }
 
-  void on_error(const char* message) { throw_format_error(message); }
-
   FMT_CONSTEXPR auto out() -> iterator { return out_; }
 
   void advance_to(iterator it) {
@@ -2333,7 +2331,7 @@ FMT_CONSTEXPR auto write(OutputIt out, const Char* s,
     -> OutputIt {
   if (specs.type == presentation_type::pointer)
     return write_ptr<Char>(out, bit_cast<uintptr_t>(s), &specs);
-  if (!s) throw_format_error("string pointer is null");
+  if (!s) report_error("string pointer is null");
   return write(out, basic_string_view<Char>(s), specs, {});
 }
 
@@ -2384,7 +2382,7 @@ FMT_CONSTEXPR auto parse_align(const Char* begin, const Char* end,
         auto c = *begin;
         if (c == '}') return begin;
         if (c == '{') {
-          throw_format_error("invalid fill character '{'");
+          report_error("invalid fill character '{'");
           return begin;
         }
         specs.fill = {begin, to_unsigned(p - begin)};
@@ -3600,7 +3598,7 @@ FMT_CONSTEXPR20 auto write_float(OutputIt out, T value,
                       : 6;
   if (fspecs.format == float_format::exp) {
     if (precision == max_value<int>())
-      throw_format_error("number is too big");
+      report_error("number is too big");
     else
       ++precision;
   } else if (fspecs.format != float_format::fixed && precision == 0) {
@@ -3707,7 +3705,7 @@ FMT_CONSTEXPR auto write(OutputIt out, Char value) -> OutputIt {
 template <typename Char, typename OutputIt>
 FMT_CONSTEXPR20 auto write(OutputIt out, const Char* value) -> OutputIt {
   if (value) return write(out, basic_string_view<Char>(value));
-  throw_format_error("string pointer is null");
+  report_error("string pointer is null");
   return out;
 }
 
@@ -3785,13 +3783,13 @@ template <typename Char> struct arg_formatter {
 struct width_checker {
   template <typename T, FMT_ENABLE_IF(is_integer<T>::value)>
   FMT_CONSTEXPR auto operator()(T value) -> unsigned long long {
-    if (is_negative(value)) throw_format_error("negative width");
+    if (is_negative(value)) report_error("negative width");
     return static_cast<unsigned long long>(value);
   }
 
   template <typename T, FMT_ENABLE_IF(!is_integer<T>::value)>
   FMT_CONSTEXPR auto operator()(T) -> unsigned long long {
-    throw_format_error("width is not integer");
+    report_error("width is not integer");
     return 0;
   }
 };
@@ -3799,13 +3797,13 @@ struct width_checker {
 struct precision_checker {
   template <typename T, FMT_ENABLE_IF(is_integer<T>::value)>
   FMT_CONSTEXPR auto operator()(T value) -> unsigned long long {
-    if (is_negative(value)) throw_format_error("negative precision");
+    if (is_negative(value)) report_error("negative precision");
     return static_cast<unsigned long long>(value);
   }
 
   template <typename T, FMT_ENABLE_IF(!is_integer<T>::value)>
   FMT_CONSTEXPR auto operator()(T) -> unsigned long long {
-    throw_format_error("precision is not integer");
+    report_error("precision is not integer");
     return 0;
   }
 };
@@ -3813,15 +3811,14 @@ struct precision_checker {
 template <typename Handler, typename FormatArg>
 FMT_CONSTEXPR auto get_dynamic_spec(FormatArg arg) -> int {
   unsigned long long value = arg.visit(Handler());
-  if (value > to_unsigned(max_value<int>()))
-    throw_format_error("number is too big");
+  if (value > to_unsigned(max_value<int>())) report_error("number is too big");
   return static_cast<int>(value);
 }
 
 template <typename Context, typename ID>
 FMT_CONSTEXPR auto get_arg(Context& ctx, ID id) -> decltype(ctx.arg(id)) {
   auto arg = ctx.arg(id);
-  if (!arg) ctx.on_error("argument not found");
+  if (!arg) report_error("argument not found");
   return arg;
 }
 
@@ -3893,6 +3890,7 @@ using format_func = void (*)(detail::buffer<char>&, int, const char*);
 FMT_API void format_error_code(buffer<char>& out, int error_code,
                                string_view message) noexcept;
 
+using fmt::report_error;
 FMT_API void report_error(format_func func, int error_code,
                           const char* message) noexcept;
 }  // namespace detail
@@ -4253,7 +4251,7 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
   auto out = basic_appender<Char>(buf);
   if (fmt.size() == 2 && equal2(fmt.data(), "{}")) {
     auto arg = args.get(0);
-    if (!arg) throw_format_error("argument not found");
+    if (!arg) report_error("argument not found");
     arg.visit(default_arg_formatter<Char>{out, args, loc});
     return;
   }
@@ -4280,7 +4278,7 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
     }
     FMT_CONSTEXPR auto on_arg_id(basic_string_view<Char> id) -> int {
       int arg_id = context.arg_id(id);
-      if (arg_id < 0) throw_format_error("argument not found");
+      if (arg_id < 0) report_error("argument not found");
       return arg_id;
     }
 
@@ -4303,13 +4301,13 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
       detail::handle_dynamic_spec<detail::precision_checker>(
           specs.precision, specs.precision_ref, context);
       if (begin == end || *begin != '}')
-        throw_format_error("missing '}' in format string");
+        report_error("missing '}' in format string");
       context.advance_to(arg.visit(
           arg_formatter<Char>{context.out(), specs, context.locale()}));
       return begin;
     }
 
-    void on_error(const char* message) { throw_format_error(message); }
+    void on_error(const char* message) { report_error(message); }
   };
   detail::parse_format_string<false>(fmt, format_handler(out, fmt, args, loc));
 }
diff --git a/include/fmt/printf.h b/include/fmt/printf.h
index 13b4df6c..30f41404 100644
--- a/include/fmt/printf.h
+++ b/include/fmt/printf.h
@@ -52,8 +52,6 @@ template <typename Char> class basic_printf_context {
   auto arg(int id) const -> basic_format_arg<basic_printf_context> {
     return args_.get(id);
   }
-
-  void on_error(const char* message) { throw_format_error(message); }
 };
 
 namespace detail {
@@ -80,13 +78,13 @@ struct printf_precision_handler {
   template <typename T, FMT_ENABLE_IF(std::is_integral<T>::value)>
   auto operator()(T value) -> int {
     if (!int_checker<std::numeric_limits<T>::is_signed>::fits_in_int(value))
-      throw_format_error("number is too big");
+      report_error("number is too big");
     return (std::max)(static_cast<int>(value), 0);
   }
 
   template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
   auto operator()(T) -> int {
-    throw_format_error("precision is not integer");
+    report_error("precision is not integer");
     return 0;
   }
 };
@@ -208,13 +206,13 @@ template <typename Char> class printf_width_handler {
       width = 0 - width;
     }
     unsigned int_max = max_value<int>();
-    if (width > int_max) throw_format_error("number is too big");
+    if (width > int_max) report_error("number is too big");
     return static_cast<unsigned>(width);
   }
 
   template <typename T, FMT_ENABLE_IF(!std::is_integral<T>::value)>
   auto operator()(T) -> unsigned {
-    throw_format_error("width is not integer");
+    report_error("width is not integer");
     return 0;
   }
 };
@@ -352,7 +350,7 @@ auto parse_header(const Char*& it, const Char* end, format_specs<Char>& specs,
       if (value != 0) {
         // Nonzero value means that we parsed width and don't need to
         // parse it or flags again, so return now.
-        if (value == -1) throw_format_error("number is too big");
+        if (value == -1) report_error("number is too big");
         specs.width = value;
         return arg_index;
       }
@@ -363,7 +361,7 @@ auto parse_header(const Char*& it, const Char* end, format_specs<Char>& specs,
   if (it != end) {
     if (*it >= '0' && *it <= '9') {
       specs.width = parse_nonnegative_int(it, end, -1);
-      if (specs.width == -1) throw_format_error("number is too big");
+      if (specs.width == -1) report_error("number is too big");
     } else if (*it == '*') {
       ++it;
       specs.width = static_cast<int>(
@@ -457,7 +455,7 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
 
     // Parse argument index, flags and width.
     int arg_index = parse_header(it, end, specs, get_arg);
-    if (arg_index == 0) throw_format_error("argument not found");
+    if (arg_index == 0) report_error("argument not found");
 
     // Parse precision.
     if (it != end && *it == '.') {
@@ -539,7 +537,7 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
     }
 
     // Parse type.
-    if (it == end) throw_format_error("invalid format string");
+    if (it == end) report_error("invalid format string");
     char type = static_cast<char>(*it++);
     if (arg.is_integral()) {
       // Normalize type.
@@ -556,7 +554,7 @@ void vprintf(buffer<Char>& buf, basic_string_view<Char> format,
     bool upper = false;
     specs.type = parse_printf_presentation_type(type, arg.type(), upper);
     if (specs.type == presentation_type::none)
-      throw_format_error("invalid format specifier");
+      report_error("invalid format specifier");
     specs.upper = upper;
 
     start = it;
diff --git a/include/fmt/ranges.h b/include/fmt/ranges.h
index 07d8d38b..9499fa14 100644
--- a/include/fmt/ranges.h
+++ b/include/fmt/ranges.h
@@ -310,8 +310,7 @@ struct formatter<Tuple, Char,
   template <typename ParseContext>
   FMT_CONSTEXPR auto parse(ParseContext& ctx) -> decltype(ctx.begin()) {
     auto it = ctx.begin();
-    if (it != ctx.end() && *it != '}')
-      throw_format_error("invalid format specifier");
+    if (it != ctx.end() && *it != '}') report_error("invalid format specifier");
     detail::for_each(formatters_, detail::parse_empty_specs<ParseContext>{ctx});
     return it;
   }
@@ -417,7 +416,7 @@ struct range_formatter<
     }
 
     if (it != end && *it != '}') {
-      if (*it != ':') throw_format_error("invalid format specifier");
+      if (*it != ':') report_error("invalid format specifier");
       ++it;
     } else {
       detail::maybe_set_debug_format(underlying_, true);
@@ -650,7 +649,7 @@ struct formatter<tuple_join_view<Char, T...>, Char> {
     if (N > 1) {
       auto end1 = do_parse(ctx, std::integral_constant<size_t, N - 1>());
       if (end != end1)
-        throw_format_error("incompatible format specs for tuple elements");
+        report_error("incompatible format specs for tuple elements");
     }
 #endif
     return end;
diff --git a/test/format-test.cc b/test/format-test.cc
index e78fc6b1..05575643 100644
--- a/test/format-test.cc
+++ b/test/format-test.cc
@@ -447,7 +447,7 @@ TEST(memory_buffer_test, max_size_allocator_overflow) {
 }
 
 TEST(format_test, exception_from_lib) {
-  EXPECT_THROW_MSG(fmt::throw_format_error("test"), format_error, "test");
+  EXPECT_THROW_MSG(fmt::report_error("test"), format_error, "test");
 }
 
 TEST(format_test, escape) {
diff --git a/test/scan-test.cc b/test/scan-test.cc
index 76d75f16..e9ffbe79 100644
--- a/test/scan-test.cc
+++ b/test/scan-test.cc
@@ -87,7 +87,7 @@ template <> struct scanner<num> {
       hex = true;
       ++it;
     }
-    if (it != end && *it != '}') throw_format_error("invalid format");
+    if (it != end && *it != '}') report_error("invalid format");
     return it;
   }
 
diff --git a/test/scan.h b/test/scan.h
index 5fd15baf..67b5d5ba 100644
--- a/test/scan.h
+++ b/test/scan.h
@@ -453,7 +453,7 @@ template <typename T, FMT_ENABLE_IF(std::is_unsigned<T>::value)>
 auto read(scan_iterator it, T& value) -> scan_iterator {
   if (it == scan_sentinel()) return it;
   char c = *it;
-  if (c < '0' || c > '9') throw_format_error("invalid input");
+  if (c < '0' || c > '9') report_error("invalid input");
 
   int num_digits = 0;
   T n = 0, prev = 0;
@@ -477,7 +477,7 @@ auto read(scan_iterator it, T& value) -> scan_iterator {
       prev * 10ull + unsigned(prev_digit - '0') <= max) {
     value = n;
   } else {
-    throw_format_error("number is too big");
+    report_error("number is too big");
   }
   return it;
 }
@@ -486,7 +486,7 @@ template <typename T, FMT_ENABLE_IF(std::is_unsigned<T>::value)>
 auto read_hex(scan_iterator it, T& value) -> scan_iterator {
   if (it == scan_sentinel()) return it;
   int digit = to_hex_digit(*it);
-  if (digit < 0) throw_format_error("invalid input");
+  if (digit < 0) report_error("invalid input");
 
   int num_digits = 0;
   T n = 0;
@@ -501,7 +501,7 @@ auto read_hex(scan_iterator it, T& value) -> scan_iterator {
   if (num_digits <= (std::numeric_limits<T>::digits >> 2))
     value = n;
   else
-    throw_format_error("number is too big");
+    report_error("number is too big");
   return it;
 }
 
@@ -518,7 +518,7 @@ auto read(scan_iterator it, T& value, const format_specs<>& specs = {})
   bool negative = it != scan_sentinel() && *it == '-';
   if (negative) {
     ++it;
-    if (it == scan_sentinel()) throw_format_error("invalid input");
+    if (it == scan_sentinel()) report_error("invalid input");
   }
   using unsigned_type = typename std::make_unsigned<T>::type;
   unsigned_type abs_value = 0;
@@ -538,7 +538,7 @@ auto read(scan_iterator it, string_view& value, const format_specs<>& = {})
     -> scan_iterator {
   auto range = to_contiguous(it);
   // This could also be checked at compile time in scan.
-  if (!range) throw_format_error("string_view requires contiguous input");
+  if (!range) report_error("string_view requires contiguous input");
   auto p = range.begin;
   while (p != range.end && *p != ' ') ++p;
   size_t size = to_unsigned(p - range.begin);
@@ -624,7 +624,7 @@ struct scan_handler {
     return begin;
   }
 
-  void on_error(const char* message) { throw_format_error(message); }
+  void on_error(const char* message) { report_error(message); }
 };
 }  // namespace detail
 
@@ -640,7 +640,7 @@ template <typename T> class scan_value {
  public:
   scan_value(T value) : value_(std::move(value)) {}
 
-  const T& value() const { return value_; }
+  auto value() const -> const T& { return value_; }
 };
 
 // A rudimentary version of std::expected for testing the API shape.
@@ -651,7 +651,7 @@ template <typename T> class expected {
  public:
   expected(T value) : value_(std::move(value)) {}
 
-  const T* operator->() const { return &value_; }
+  auto operator->() const -> const T* { return &value_; }
 };
 
 template <typename T> using scan_result = expected<scan_value<T>>;
@@ -688,7 +688,7 @@ auto scan_to(InputRange&& input, string_view fmt, T&... args)
 }
 
 template <typename... T>
-bool scan_to(FILE* f, string_view fmt, T&... args) {
+auto scan_to(FILE* f, string_view fmt, T&... args) -> bool {
   auto&& buf = detail::file_scan_buffer(f);
   vscan(buf, fmt, make_scan_args(args...));
   return buf.begin() != buf.end();