Support width and alignment in std::exception formatter#4860
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The formatter for std::exception (and std::exception_ptr) previously
accepted only the optional 't' type-name specifier, so standard fill,
align and width specifiers were rejected with "unknown format
specifier". This made it impossible to pad or align exception messages,
e.g. when laying them out in a log column.
Parse the standard fill/align/width specifiers (as the std::filesystem::
path formatter already does) and apply them when writing the message.
Dynamic width ({:{}}) and the 'none'/'unknown exception' cases for
exception_ptr are handled too. Existing behavior ({}, {:t}, nested
exception unwinding) is unchanged.
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The formatter for std::exception (and std::exception_ptr) previously accepted only the optional 't' type-name specifier, so standard fill, align and width specifiers were rejected with "unknown format specifier". This made it impossible to pad or align exception messages, e.g. when laying them out in a log column.
Parse the standard fill/align/width specifiers (as the std::filesystem:: path formatter already does) and apply them when writing the message. Dynamic width ({:{}}) and the 'none'/'unknown exception' cases for exception_ptr are handled too. Existing behavior ({}, {:t}, nested exception unwinding) is unchanged.