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C++17 P0522R0 (Matching template template parameters to compatible arguments) only partially implemented #55894

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ackelcn opened this issue Jun 6, 2022 · 9 comments
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C++17 P0522R0 (Matching template template parameters to compatible arguments) only partially implemented #55894

ackelcn opened this issue Jun 6, 2022 · 9 comments
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c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" duplicate Resolved as duplicate

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@ackelcn
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ackelcn commented Jun 6, 2022

I encounter a commit that repaired a clang bug. This commit is from WEB++:
the-moisrex/webpp@97a5947

 - typename istl::filter_parameters<istl::templated_negation<stl::is_void>::type,
 +typename istl::filter_parameters<istl::templated_negation<stl::is_void>::template type,
                                   typename details::resource_extractor_impl<AllocDescTypes>::type>::type

Looks like a clang bug?
@ackelcn ackelcn mentioned this issue Jun 6, 2022
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@the-moisrex
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This bug is known and described here.

@EugeneZelenko EugeneZelenko added c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" and removed new issue labels Jun 6, 2022
@llvmbot
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llvmbot commented Jun 6, 2022

@llvm/issue-subscribers-clang-frontend

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llvmbot commented Jun 6, 2022

@llvm/issue-subscribers-c-20

@fhahn
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fhahn commented Jun 6, 2022

@the-moisrex thanks, I updated the title with the info you provided.

@fhahn fhahn changed the title A clang bug? C++17 P0522R0 (Matching template template parameters to compatible arguments) only partially implemented Jun 6, 2022
@mizvekov
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mizvekov commented Jun 6, 2022

I worked on an implementation of this some time ago.
See: https://reviews.llvm.org/D109496

We tried first just implementing the wording as is, without any resolution to the new issues. There was negative push back, so we reverted. I implemented a workaround, but then @zygoloid suggested a better approach, but I did not have the time to follow that up yet.

@ackelcn
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ackelcn commented Jun 9, 2022

@mizvekov Many thanks for your effort.
@the-moisrex provided a code sample, and explained how to reproduce the bug. Hope that this problem can be fully resolved.

the-moisrex/webpp#132

@pinskia pinskia mentioned this issue Apr 24, 2024
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pinskia commented Apr 24, 2024

Cross reference to GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114841 .

mizvekov added a commit to mizvekov/llvm-project that referenced this issue Apr 26, 2024
@mizvekov
[clang] Enable C++17 relaxed template template argument matching by d…
3f6e50e 
…efault

In order to implement this as a DR and avoid breaking reasonable code
that worked before P0522, this patch implements a provisional resolution
for CWG2398: When deducing template template parameters against each other,
and the argument side names a template specialization, instead of just
deducing A, we instead deduce a synthesized template template parameter based
on A, but with it's parameters using the template specialization's arguments
as defaults.

The driver flag is deprecated with a warning.

With this patch, we finally mark C++17 support in clang as complete.

Closes llvm#55894
mizvekov added a commit to mizvekov/llvm-project that referenced this issue Apr 26, 2024
@mizvekov
[clang] Enable C++17 relaxed template template argument matching by d…
43f813d 
…efault

In order to implement this as a DR and avoid breaking reasonable code
that worked before P0522, this patch implements a provisional resolution
for CWG2398: When deducing template template parameters against each other,
and the argument side names a template specialization, instead of just
deducing A, we instead deduce a synthesized template template parameter based
on A, but with it's parameters using the template specialization's arguments
as defaults.

The driver flag is deprecated with a warning.

With this patch, we finally mark C++17 support in clang as complete.

Closes llvm#55894
@mizvekov mizvekov mentioned this issue Apr 27, 2024
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@mizvekov
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Closing as duplicate in favor of older issue.

Duplicate of #42224

@mizvekov mizvekov closed this as not planned Won't fix, can't repro, duplicate, stale Apr 27, 2024
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Duplicate of #42224

@mizvekov mizvekov marked this as a duplicate of #42224 Apr 27, 2024
@EugeneZelenko EugeneZelenko added the duplicate Resolved as duplicate label Apr 27, 2024
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c++20 clang:frontend Language frontend issues, e.g. anything involving "Sema" duplicate Resolved as duplicate
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@fhahn @mizvekov @pinskia @EugeneZelenko @the-moisrex @ackelcn @llvmbot