[all-commits] [llvm/llvm-project] 28bc15: [clang] Implement CWG2398 provisional TTP matching...

[Matheus Izvekov via All-commits]

  Branch: refs/heads/users/mizvekov/clang-cwg2398-ttp-matches-class-template
  Home:   https://github.com/llvm/llvm-project
  Commit: 28bc152e72d458e91aee90d46763eea04091ebe2
      https://github.com/llvm/llvm-project/commit/28bc152e72d458e91aee90d46763eea04091ebe2
  Author: Matheus Izvekov <mizvekov at gmail.com>
  Date:   2024-06-10 (Mon, 10 Jun 2024)

  Changed paths:
    M clang-tools-extra/clangd/DumpAST.cpp
    M clang-tools-extra/clangd/SemanticHighlighting.cpp
    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/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/Sema/SemaTemplate.cpp
    M clang/lib/Sema/SemaTemplateDeduction.cpp
    M clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
    M clang/test/CXX/temp/temp.decls/temp.alias/p2.cpp
    A clang/test/CodeGenCXX/mangle-cwg2398.cpp
    M clang/test/SemaTemplate/cwg2398.cpp
    M clang/tools/libclang/CIndex.cpp
    M clang/unittests/AST/ASTImporterTest.cpp

  Log Message:
  -----------
  [clang] Implement CWG2398 provisional TTP matching to class templates

This extends default argument deduction to cover class templates as
well, and also applies outside of 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.

As the consequences are not restricted to partial ordering,
the following code becomes valid:
```C++
template<class T, class U> struct A {};
A<int, float> v;
template<template<class> class TT> void f(TT<int>);

// OK: TT picks 'float' as the default argument for the second parameter.
void g() { f(v); }
```

Also, since 'f' deduced from `A<int, float>` is different from 'f'
deduced from `A<int, double>`, this implements an additional mangling
rule.

---

Since this changes provisional implementation of CWG2398 which has
not been released yet, and already contains a changelog entry,
we don't provide a changelog entry here.



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