[PATCH] Add ability for can_write to indicate why the file is not writable

David Blaikie dblaikie at gmail.com
Thu Sep 4 17:01:44 PDT 2014

On Wed, Sep 3, 2014 at 6:36 PM, Richard Smith <richard at metafoo.co.uk> wrote:

> Hi dblaikie,
> Add an optional output parameter to `can_write` to allow it to indicate
> why a file is not considered writable (along with a sample use from Clang
> where we currently produce stupid diagnostics saying `"can't open output
> file: Success"`).
> I'm not entirely sure this is the right design, and in particular it's a
> somewhat different interface from the other functions in this file. I also
> considered:
>   std::error_code can_write(const Twine &Path, bool &result);
> ... which seems redundant because `result` is set to `true` iff the error
> code is `success`, and ...
>   std::error_code can_write(const Twine &Path);
> ... which is terrible because `bool(can_write(Path))` is `true` iff the
> file is *not* writable, and ...
>   std::error_code cannot_write(const Twine &Path);
> ... which is inconsistent with other functions near here and ugly.

I'll just put this out there. No idea whether it's sane:

ErrorOr<void>? (not sure if the ErrorOr template supports void... I suppose
it'd need a whole separate specialization)

(alternatively, struct Success {}; ErrorOr<Success>)

> http://reviews.llvm.org/D5182
> Files:
>   include/llvm/Support/FileSystem.h
>   lib/Frontend/CompilerInstance.cpp
>   lib/Support/Unix/Path.inc
>   lib/Support/Windows/Path.inc
>   test/Frontend/output-failures.c
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