`uniform_int_distribution<T>` is UB unless `T` is one of the non-character, non-boolean integer types (`short` or larger). However, libc++ has never enforced this. D114129 accidentally made `uniform_int_distribution<bool>` into an error. Make it now *intentionally* an error; and likewise for the character types and all user-defined class and enum types; but permit `__[u]int128_t` to continue working. Apply the same static_assert to all the integer distributions.
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fwolff ldionne var-const Mordante mclow.lists leonardchan - Group Reviewers
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- rGa3255f219a86: [libc++] Explicitly reject `uniform_int_distribution<bool>` and `<char>`.
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libcxx/test/std/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/eval.pass.cpp | ||
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84–91 | Before you ask: yeah, these error bounds are crazy ad-hoc. But:
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libcxx/test/std/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/eval.pass.cpp | ||
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84–91 | One of those "oh boy" moments when looking at the existing trunk code (e.g. mean of zero comment). |
Let's just *not* support uniform_int_distribution<bool>, and make it into a static_assert.
Also, the test improvements should be made as a different patch. probably before this one cause it is needed to test with a wide variety of types like you do (which is a good improvement BTW).
Let's just *not* support uniform_int_distribution<bool>, and make it into a static_assert.
Works for me, but (1) I'll feel better once we hear from @leonardchan, and (2) what do you want to do about char, signed char, char16_t, etc.? Also disallow them via static_assert, I'd guess?
Yeah, I'd do the same. If something is library UB and it's easy to diagnose, I would diagnose it.
libcxx/include/__random/is_valid_inttype.h | ||
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21–40 ↗ | (On Diff #392064) | I'm not thrilled about this helper or its placement/naming, but it seemed like the best option. |
Couple of comments.
Also, I'm going to try this patch out on a significant code base just to confirm it doesn't result in crazy breakage -- requesting changes until that's done.
Ref for myself: 86111407&86111417
libcxx/include/__random/is_valid_inttype.h | ||
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21–40 ↗ | (On Diff #392064) | Yeah, let's use __libcpp_*. I don't like that name, but I think it's worth doing it if it reduces the likelihood of users messing around with our internal details. |
37 ↗ | (On Diff #392064) | This is where we could document that we support this as an extension, in a comment. |
libcxx/test/std/numerics/rand/rand.dis/rand.dist.uni/rand.dist.uni.int/eval.pass.cpp | ||
13 | Any appetite for improving tests for other random distributions like this? It's *much* better than what we had before. |
Okay, so I have some initial results of my investigation. I found some uses of uniform_int_distribution with things like uint8_t, char, and unsigned char. There was definitely some breakage, however nothing that seemed entirely unmanageable.
I say we go for it, but we definitely need to add a release note. Also, we'll want to sync up with some folks that build their code bases from libc++ trunk (like I think @thakis @rnk @phosek) or they might encounter issues. They should proof their code bases before we land this (or if that can't be done in a reasonable time frame, then they can revert this commit internally).
One thing we could do to make things easier would be to have the static_asserts in a different commit, that way folks could just revert that one locally until they have solved their problems.
@ldionne: Makes perfect sense to me. Would you care to suggest wording for a release note? Perhaps
(API Changes) The integer distributions ``binomial_distribution``, ``discrete_distribution``, ``geometric_distribution``, ``negative_binomial_distribution``, ``poisson_distribution``, and ``uniform_int_distribution`` now conform to the Standard by rejecting template parameter types other than ``short``, ``int``, ``long``, ``long long``, (as an extension) ``__int128_t``, and the unsigned versions thereof. In particular, ``uniform_int_distribution<int8_t>`` is no longer supported.
Also, since "everything but the static_asserts" is really just "the test file," shall I go ahead and land the test-file change by itself, soonish?
libcxx/include/__random/is_valid_inttype.h | ||
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21–40 ↗ | (On Diff #392064) | Will rename to __libcpp_random_is_valid_inttype. Also yuck, libcxx/benchmarks/vector_operations.bench.cpp uses uniform_int_distribution<char> in order to get random example data for its vectors. Maybe we should just support that. It would match libstdc++, too. uniform_int_distribution<char/int8_t/uint8_t>: GCC accepts, MSVC rejects with static_assert. |
Yes, that sounds perfect to me.
Also, since "everything but the static_asserts" is really just "the test file," shall I go ahead and land the test-file change by itself, soonish?
Yes! You can do this as a NFC commit and repurpose this patch to be only the static asserts and the release note, and fixing the benchmarks.
libcxx/include/__random/is_valid_inttype.h | ||
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21–40 ↗ | (On Diff #392064) | So basically we'd only exclude bool? IMO we should strive for strict conformance, however we should also write a half-page paper to support it for char and other small types cause it kind of makes sense. |
Add a release note.
Fix the benchmarks.
Scope-creep to [libc++] Explicitly reject URNG types with signed result_types (fixes https://github.com/llvm/llvm-project/issues/48965 ) while I'm here.
Can we add libc++ specific tests to check that we properly diagnose those? One big .verify.cpp test for all the distributions would be great. LGTM apart from that.
Rebase. Split out "Explicitly reject URNG types with signed result_types" into its own PR.
If CI on this part is green, I'll probably just land it on Monday, since it's been approved since forever.
Any appetite for improving tests for other random distributions like this? It's *much* better than what we had before.