ychen added inline comments.

================
Comment at: clang/lib/Sema/SemaTemplateDeduction.cpp:5184
 
-  // FIXME: This mimics what GCC implements, but doesn't match up with the
-  // proposed resolution for core issue 692. This area needs to be sorted out,
----------------
ychen wrote:
> aaron.ballman wrote:
> > ychen wrote:
> > > aaron.ballman wrote:
> > > > aaron.ballman wrote:
> > > > > We tend not to use top-level const on locals in the project as a 
> > > > > matter of style.
> > > > Does GCC still implement it this way?
> > > > 
> > > > One concern I have is that this will be an ABI breaking change, and I'm 
> > > > not certain how disruptive it will be. If GCC already made that change, 
> > > > it may be reasonable for us to also break it without having to add ABI 
> > > > tags or something special. But if these changes diverge us from GCC, 
> > > > that may require some extra effort to mitigate.
> > > Unfortunately, GCC is still using the old/non-conforming behavior. 
> > > https://clang.godbolt.org/z/5K4916W71. What is the usual way to mitigate 
> > > this?
> > You would use the `ClangABI` enumeration to alter behavior between ABI 
> > versions: 
> > https://github.com/llvm/llvm-project/blob/main/clang/include/clang/Basic/LangOptions.h#L174
> >  like done in: 
> > https://github.com/llvm/llvm-project/blob/main/clang/lib/AST/DeclCXX.cpp#L929
> Looking at how `ClangABI` is currently used, it looks like it is for 
> low-level object layout ABI compatibility. For library/language ABI 
> compatibility, maybe we should not use `ClangABI`? Looking at 
> https://reviews.llvm.org/rG86a1b135f0c67a022ef628f092c6691892752876, I guess 
> the practice is just committed and see? If it breaks important or many 
> existing libraries, just revert or add compiler options?
> I guess the practice is just committed and see? If it breaks important or 
> many existing libraries, just revert or add compiler options?

I'm fairly optimistic considering `check-runtimes` passes. 


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D128745/new/

https://reviews.llvm.org/D128745

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