On Thu, 11 Jul 2019 at 15:41, Gabriel Dos Reis via Modules
<modules_at_[hidden]> wrote:
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> Maybe my standardese-fu is wrong here, but since I see no provision for the implementation defined set of importable headers to change within a TU, I would expect its membership to remain constant, at least within a TU.
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> There is no ground for such an expectation. Implementation-defined only means that the implementation has to tell you what is importable header. There is no requirement that such a set is fixed. The implementation can very say that an importable header is one that it is told via the very syntax ‘import <x>;’ or one that it can successfully translate.

Furthermore, Nathan wasn't even suggesting that the set necessarily
changes within a TU; it can certainly change between TUs. But sure,
depending on how you parse, it could change within a TU, too. And
there is indeed nothing in the wording that prevents that.