[fs.filesystem_error.members] says that constructors of filesystem_error have the postcondition that runtime_error::what() has the value what_arg.c_str(). That's obviously incorrect: these are pointers to distinct copies of the string in any sane implementation and cannot possibly compare equal.

The requirement seems suspect for a further reason: it mandates the content of the string returned by runtime_error::what(), but filesystem_error has no direct control over the construction of its indirect non-virtual base class runtime_error. Instead, what is passed to runtime_error's constructor is determined by system_error's constructor, which in many implementations is an eagerly crafted error string. This is permitted by the specification of system_error (see [syserr.syserr]) but would make the requirement unimplementable.

The proposed wording below adjusts the postcondition using the formula of system_error's constructor. As an editorial change, it also replaces the postcondition tables with normal postcondition clauses, in the spirit of editorial issue 1875.