The new constructors that C++11 added to std::ios_base::failure were
missing for the old ABI. This adds them, but just ignores the
std::error_code argument (because there's nowhere to store it).
This also adds a code() member, which should be provided by the
std::system_error base class, but that base class isn't present in the
old ABI.
This allows the old ios::failure to be used in code that expects the new
API, although with reduced functionality.
libstdc++-v3/ChangeLog:
* include/bits/ios_base.h (ios_base::failure): Add constructors
takeing error_code argument. Add code() member function.
* testsuite/27_io/ios_base/failure/cxx11.cc: Allow test to
run for the old ABI but do not check for derivation from
std::system_error.
* testsuite/27_io/ios_base/failure/error_code.cc: New test.
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