[all-commits] [llvm/llvm-project] 6afe56: [clang] Finish implementation of P0522 (#96023)

[Matheus Izvekov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 6afe56732a172d3f2cbd0330b1fcb34bbfd002a9
      https://github.com/llvm/llvm-project/commit/6afe56732a172d3f2cbd0330b1fcb34bbfd002a9
  Author: Matheus Izvekov <mizvekov at gmail.com>
  Date:   2024-10-01 (Tue, 01 Oct 2024)

  Changed paths:
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/Basic/DiagnosticSemaKinds.td
    M clang/include/clang/Sema/Sema.h
    M clang/lib/Frontend/FrontendActions.cpp
    M clang/lib/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    M clang/lib/Sema/SemaTemplateInstantiate.cpp
    M clang/test/CXX/temp/temp.arg/temp.arg.template/p3-0x.cpp
    M clang/test/CXX/temp/temp.param/p12.cpp
    M clang/test/Modules/cxx-templates.cpp
    M clang/test/SemaCXX/make_integer_seq.cpp
    M clang/test/SemaTemplate/cwg2398.cpp
    M clang/test/SemaTemplate/temp_arg_nontype.cpp
    M clang/test/SemaTemplate/temp_arg_template.cpp
    M clang/test/SemaTemplate/temp_arg_template_p0522.cpp
    M clang/test/Templight/templight-empty-entries-fix.cpp
    M clang/test/Templight/templight-prior-template-arg.cpp

  Log Message:
  -----------
  [clang] Finish implementation of P0522 (#96023)

This finishes the clang implementation of P0522, getting rid of the
fallback to the old, pre-P0522 rules.

Before this patch, when partial ordering template template parameters,
we would perform, in order:
* If the old rules would match, we would accept it. Otherwise, don't
generate diagnostics yet.
* If the new rules would match, just accept it. Otherwise, don't
generate any diagnostics yet again.
* Apply the old rules again, this time with diagnostics.

This situation was far from ideal, as we would sometimes:
* Accept some things we shouldn't.
* Reject some things we shouldn't.
* Only diagnose rejection in terms of the old rules.

With this patch, we apply the P0522 rules throughout.

This needed to extend template argument deduction in order to accept the
historial rule for TTP matching pack parameter to non-pack arguments.
This change also makes us accept some combinations of historical and
P0522 allowances we wouldn't before.

It also fixes a bunch of bugs that were documented in the test suite,
which I am not sure there are issues already created for them.

This causes a lot of changes to the way these failures are diagnosed,
with related test suite churn.

The problem here is that the old rules were very simple and
non-recursive, making it easy to provide customized diagnostics, and to
keep them consistent with each other.

The new rules are a lot more complex and rely on template argument
deduction, substitutions, and they are recursive.

The approach taken here is to mostly rely on existing diagnostics, and
create a new instantiation context that keeps track of things.

So for example when a substitution failure occurs, we use the error
produced there unmodified, and just attach notes to it explaining that
it occurred in the context of partial ordering this template argument
against that template parameter.

This diverges from the old diagnostics, which would lead with an error
pointing to the template argument, explain the problem in subsequent
notes, and produce a final note pointing to the parameter.



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