[all-commits] [llvm/llvm-project] fa6580: [clang] Implement CWG2398 provisional TTP matching...

[Matheus Izvekov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fa6580470547411667b866362941db0b02e25578
      https://github.com/llvm/llvm-project/commit/fa6580470547411667b866362941db0b02e25578
  Author: Matheus Izvekov <mizvekov at gmail.com>
  Date:   2024-09-07 (Sat, 07 Sep 2024)

  Changed paths:
    M clang-tools-extra/clangd/DumpAST.cpp
    M clang-tools-extra/clangd/SemanticHighlighting.cpp
    M clang/docs/ReleaseNotes.rst
    M clang/include/clang/AST/ASTContext.h
    M clang/include/clang/AST/ASTImporter.h
    M clang/include/clang/AST/DependenceFlags.h
    M clang/include/clang/AST/PropertiesBase.td
    M clang/include/clang/AST/TemplateName.h
    M clang/include/clang/Sema/Sema.h
    M clang/lib/AST/ASTContext.cpp
    M clang/lib/AST/ASTDiagnostic.cpp
    M clang/lib/AST/ASTImporter.cpp
    M clang/lib/AST/ASTStructuralEquivalence.cpp
    M clang/lib/AST/Decl.cpp
    M clang/lib/AST/ItaniumMangle.cpp
    M clang/lib/AST/ODRHash.cpp
    M clang/lib/AST/TemplateName.cpp
    M clang/lib/AST/TextNodeDumper.cpp
    M clang/lib/AST/Type.cpp
    M clang/lib/AST/TypePrinter.cpp
    M clang/lib/Sema/SemaDeclCXX.cpp
    M clang/lib/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    M clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    M clang/test/SemaTemplate/cwg2398.cpp
    M clang/tools/libclang/CIndex.cpp

  Log Message:
  -----------
  [clang] Implement CWG2398 provisional TTP matching to class templates (#94981)

This extends default argument deduction to cover class templates as
well, applying only to partial ordering, adding to the provisional
wording introduced in https://github.com/llvm/llvm-project/pull/89807.

This solves some ambuguity introduced in P0522 regarding how template
template parameters are partially ordered, and should reduce the
negative impact of enabling `-frelaxed-template-template-args` by
default.

Given the following example:
```C++
template <class T1, class T2 = float> struct A;
template <class T3> struct B;

template <template <class T4> class TT1, class T5> struct B<TT1<T5>>;   // #1
template <class T6, class T7>                      struct B<A<T6, T7>>; // #2

template struct B<A<int>>;
```
Prior to P0522, `#2` was picked. Afterwards, this became ambiguous. This
patch restores the pre-P0522 behavior, `#2` is picked again.



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