LWG3720 Restrict the valid types of arg-id for width and precision in std-format-spec

Author: burblebee

source/compatibility.tex

@@ -231,6 +231,24 @@
 std::format("{}", t.bit);       // ill-formed, previously returned \tcode{"0"}
 \end{codeblock}
 
+\diffref{format.string.std}
+\change
+Restrict types of formatting arguments
+used as \fmtgrammarterm{width} or \fmtgrammarterm{precision} in
+a \fmtgrammarterm{std-format-spec}.
+\rationale
+Disallow types that do not have useful or portable semantics as
+a formatting width or precision.
+\effect
+Valid \CppXX{} code that passes a boolean or character type as
+\fmtgrammarterm{arg-id} becomes invalid.
+For example:
+\begin{codeblock}
+std::format("{:*^{}}", "", true);   // ill-formed, previously returned \tcode{"*"}
+std::format("{:*^{}}", "", '1');    // ill-formed, previously returned an
+                                    // implementation-defined number of \tcode{'*'} characters
+\end{codeblock}
+
 \rSec2[diff.cpp20.strings]{\ref{strings}: strings library}
 
 \diffref{string.classes}

source/utilities.tex

@@ -14950,7 +14950,8 @@
 If \tcode{\{ \opt{\fmtgrammarterm{arg-id}} \}} is used in
 a \fmtgrammarterm{width} or \fmtgrammarterm{precision},
 the value of the corresponding formatting argument is used in its place.
-If the corresponding formatting argument is not of integral type, or
+If the corresponding formatting argument is
+not of standard signed or unsigned integer type, or
 its value is negative,
 an exception of type \tcode{format_error} is thrown.