libstdc++: Fix output stream spanbuf openmode

Ensures `basic_ospanstream` correctly opens its internal span buffer for output.

By Jonathan Wakely August 24, 2026 committed

A copy-paste error in the basic_ospanstream constructor for libstdc++ caused it to incorrectly initialize its basic_spanbuf. This commit corrects the openmode parameter, ensuring the span buffer is properly opened for output operations, and includes a new test case to verify the fix.

In Details

The constructor for std::basic_ospanstream in libstdc++ was erroneously passing an incorrect openmode to its constituent basic_spanbuf. This commit corrects the constructor to pass the appropriate std::ios_base::openmode flags required for output operations on the basic_spanbuf, fixing PR127006.

For Context

basic_ospanstream: A C++ standard library stream class that writes to a contiguous sequence of elements in memory (a span).
spanbuf: The stream buffer associated with basic_spanstream classes, handling the actual input/output operations on a memory span.
openmode: A set of flags (e.g., ios_base::in, ios_base::out, ios_base::trunc) used to specify how a file or stream buffer should be opened.

Filed Under: libstdc++bugfixstreams

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