[libcxx-commits] [PATCH] D116621: [libc++][P2321R2] Add const overloads to tuple swap, construction and assignment

Fri Mar 18 12:28:45 PDT 2022

philnik added inline comments.

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Comment at: libcxx/include/type_traits:506-509
+  template <class...>
+  using _FirstImpl = __nat;
+  template <class...>
+  using _SecondImpl = __nat;
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EricWF wrote:
> philnik wrote:
> > Quuxplusone wrote:
> > > Uh...? This completely changes the meaning of e.g. `_SecondType<int>` from "ill-formed" to "well-formed `__nat`." There's no way that's safe.
> > `_FirstType` and `_SecondType` are exclusively used in `<tuple>` and only in `is_*` contexts. If I'm not mistaken `__nat` should be false in every single case.
> I'm with Arthur, this should not succeed. This needs to be substitution failure.
In which case does it have to be a substitution failure? IIRC there are cases where I need it to //not// be a substitution failure.

Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D116621/new/

https://reviews.llvm.org/D116621