[PATCH] D159351: [Sema] Change order of displayed overloads in diagnostics

[Ilya Biryukov via Phabricator via cfe-commits]

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Make it a strict weak order.

Fixes #64121.

Current implementation uses the definition the definition of ordering
in the C++ standard. The definition provides only a partial order and
cannot be used in sorting algorithms.

The debug builds of libc++ are capable of detecting that problem
and this failure was found when building Clang with libc++ and
those extra checks enabled, see #64121.

The new ordering takes attempts to be a strict weak order and still
pushes most interesting functions to the start of the list.
In some cases, it leads to better results, e.g.

  struct Foo {
    operator int();
    operator const char*();
  };
  
  void test() { Foo() - Foo(); }

Now produces a list with two most relevant builtin operators at the top,
i.e. `operator-(int, int)` and `operator-(const char*, const char*)`.
Previously `operator-(const char*, const char*)` was the first element,
but `operator-(int, int)` was only the 13th element in the output.
This is a consequence of `stable_sort` now being able to compare those
two candidates, which are indistinguishable in the semantic partial order
despite being two local minimums in their respective comparable
subsets.

However, new implementation does not take into account some aspects of
C++ semantics, e.g. which function template is more specialized. This
can also lead to worse ordering sometimes.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D159351

Files:
  clang/lib/Sema/SemaOverload.cpp
  clang/test/SemaCXX/overloaded-operators-display-order-crash.cpp

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