[LWG motion 20] P2757R3 Type checking format args

source/support.tex

@@ -628,7 +628,7 @@
 #define @\defnlibxname{cpp_lib_filesystem}@                        201703L // also in \libheader{filesystem}
 #define @\defnlibxname{cpp_lib_flat_map}@                          202207L // also in \libheader{flat_map}
 #define @\defnlibxname{cpp_lib_flat_set}@                          202207L // also in \libheader{flat_set}
-#define @\defnlibxname{cpp_lib_format}@                            202304L // also in \libheader{format}
+#define @\defnlibxname{cpp_lib_format}@                            202305L // also in \libheader{format}
 #define @\defnlibxname{cpp_lib_format_ranges}@                     202207L // also in \libheader{format}
 #define @\defnlibxname{cpp_lib_formatters}@                        202302L // also in \libheader{stacktrace}, \libheader{thread}
 #define @\defnlibxname{cpp_lib_forward_like}@                      202207L // freestanding, also in \libheader{utility}

source/utilities.tex

@@ -15442,9 +15442,9 @@
 If \tcode{\{ \opt{\fmtgrammarterm{arg-id}} \}} is used in
 a \fmtgrammarterm{width} or \fmtgrammarterm{precision} option,
 the value of the corresponding formatting argument is used as the value of the option.
-If the corresponding formatting argument is
-not of standard signed or unsigned integer type, or
-its value is negative,
+The option is valid only if the corresponding formatting argument is
+of standard signed or unsigned integer type.
+If its value is negative,
 an exception of type \tcode{format_error} is thrown.
 
 \pnum
@@ -16555,8 +16555,7 @@
     size_t num_args_;                                   // \expos
 
   public:
-    constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt,
-                                                  size_t num_args = 0) noexcept;
+    constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt) noexcept;
     basic_format_parse_context(const basic_format_parse_context&) = delete;
     basic_format_parse_context& operator=(const basic_format_parse_context&) = delete;
 
@@ -16566,6 +16565,11 @@
 
     constexpr size_t next_arg_id();
     constexpr void check_arg_id(size_t id);
+
+    template<class... Ts>
+      constexpr void check_dynamic_spec(size_t id) noexcept;
+    constexpr void check_dynamic_spec_integral(size_t id) noexcept;
+    constexpr void check_dynamic_spec_string(size_t id) noexcept;
   };
 }
 \end{codeblock}
@@ -16578,8 +16582,7 @@
 
 \indexlibraryctor{basic_format_parse_context}%
 \begin{itemdecl}
-constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt,
-                                              size_t num_args = 0) noexcept;
+constexpr explicit basic_format_parse_context(basic_string_view<charT> fmt) noexcept;
 \end{itemdecl}
 
 \begin{itemdescr}
@@ -16590,7 +16593,13 @@
 \tcode{end_} with \tcode{fmt.end()},
 \tcode{indexing_} with \tcode{unknown},
 \tcode{next_arg_id_} with \tcode{0}, and
-\tcode{num_args_} with \tcode{num_args}.
+\tcode{num_args_} with \tcode{0}.
+\begin{note}
+Any call to
+\tcode{next_arg_id}, \tcode{check_arg_id}, or \tcode{check_dynamic_spec}
+on an instance of \tcode{basic_format_parse_context}
+initialized using this constructor is not a core constant expression.
+\end{note}
 \end{itemdescr}
 
 \indexlibrarymember{begin}{basic_format_parse_context}%
@@ -16683,6 +16692,68 @@
 core constant expressions\iref{expr.const}.
 \end{itemdescr}
 
+\indexlibrarymember{check_dynamic_spec}{basic_format_parse_context}%
+\begin{itemdecl}
+template<class... Ts>
+  constexpr void check_dynamic_spec(size_t id) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\mandates
+The types in \tcode{Ts...} are unique.
+Each type in \tcode{Ts...} is one of
+\keyword{bool},
+\tcode{char_type},
+\keyword{int},
+\tcode{\keyword{unsigned} \keyword{int}},
+\tcode{\keyword{long} \keyword{long} \keyword{int}},
+\tcode{\keyword{unsigned} \keyword{long} \keyword{long} \keyword{int}},
+\keyword{float},
+\keyword{double},
+\tcode{\keyword{long} \keyword{double}},
+\tcode{\keyword{const} char_type*},
+\tcode{basic_string_view<char_type>}, or
+\tcode{\keyword{const} \keyword{void}*}.
+
+\pnum
+\remarks
+Call expressions where
+\tcode{id >= num_args_} or
+the type of the corresponding format argument
+(after conversion to \tcode{basic_format_arg<Context>})
+is not one of the types in \tcode{Ts...}
+are not core constant expressions\iref{expr.const}.
+\end{itemdescr}
+
+\indexlibrarymember{check_dynamic_spec_integral}{basic_format_parse_context}%
+\begin{itemdecl}
+constexpr void check_dynamic_spec_integral(size_t id) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+check_dynamic_spec<int, unsigned int, long long int, unsigned long long int>(id);
+\end{codeblock}
+\end{itemdescr}
+
+\indexlibrarymember{check_dynamic_spec_string}{basic_format_parse_context}%
+\begin{itemdecl}
+constexpr void check_dynamic_spec_string(size_t id) noexcept;
+\end{itemdecl}
+
+\begin{itemdescr}
+\pnum
+\effects
+Equivalent to:
+\begin{codeblock}
+check_dynamic_spec<const char_type*, basic_string_view<char_type>>(id);
+\end{codeblock}
+\end{itemdescr}
+
 \rSec3[format.context]{Class template \tcode{basic_format_context}}
 
 \indexlibraryglobal{basic_format_context}%