[PATCH] D49439: [Sema] Fix a crash while converting constructors to deduction guides

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added inline comments.


================
Comment at: clang/lib/Sema/SemaTemplate.cpp:1899-1907
     // Canonicalize the type. This (for instance) replaces references to
     // typedef members of the current instantiations with the definitions of
     // those typedefs, avoiding triggering instantiation of the deduced type
     // during deduction.
     // FIXME: It would be preferable to retain type sugar and source
     // information here (and handle this in substitution instead).
     NewDI = SemaRef.Context.getTrivialTypeSourceInfo(
----------------
This is really the problem: we shouldn't be doing a full canonicalization step here. I expect that even with your patch applied we'll still see problems for cases like:
```
template<unsigned long> struct X {};
template<typename T> struct A {
  A(T t, X<sizeof(t)>);
};
A a(0, {});
template<typename U> struct B {
  B(U u, X<sizeof(u)>);
};
B b(0, {});
```
... because we'll canonicalize the second parameter of `B`'s deduction guide to have type `X<sizeof(t)>` (where that's the `t` from `A`'s deduction guide).

So I think we should look at fixing the FIXME here properly. There seem to be at least two viable options:

1) don't canonicalize the type; instead, extend template instantiation to be able to cope with one template referring to members of another template directly, without instantiating the class containing those members, or
2) add a custom `TreeTransform` to do the canonicalization we want to do here, and to avoid the canonicalization we don't want to do

Both of these seem pretty tricky to get right, though, which is why we currently use the canonicalization hack :(


Repository:
  rC Clang

https://reviews.llvm.org/D49439