rsmith added inline comments.
================
Comment at: clang/lib/AST/ItaniumMangle.cpp:4847
+ /// Do we need to mangle template arguments with exactly correct types?
+ bool needExactType(unsigned I) const {
+ // We need correct types when the template-name is unresolved or when it
----------------
rjmccall wrote:
> rsmith wrote:
> > rjmccall wrote:
> > > This comment should probably say explicitly that I is the parameter
> > > index. Do we need to worry about packs that haven't yet been packed? Is
> > > it true that if we have a resolved template then we should always have
> > > packed the matching arguments?
> > Parameter renamed to `ParamIdx`.
> >
> > Regarding packs, yes, that was my expectation (and it holds for all current
> > examples in Clang's test suite), but it doesn't appear to be true in full
> > generality. Here's a counterexample:
> >
> > ```
> > template<int A, int ...B> struct X {};
> >
> > // Everyone agrees that the template arguments here are I (D) J (C...) E E
> > template<int D, int ...C> void h(X<D, C...>) {}
> > void i() { h<1, 2, 3, 4>(X<1, 2, 3, 4>()); }
> >
> > // But everyone agrees that the template arguments here are I (C...) (D) E,
> > with no J...E in sight
> > template<int D, int ...C> void f(X<C..., D>) {}
> > void g() { f<4, 1, 2, 3>(X<1, 2, 3, 4>()); }
> > ```
> >
> > I think the above example demonstrates a pre-existing hole in the ABI rule,
> > which I would imagine we fix by following the implementations: if a
> > left-to-right traversal of the parameters and arguments results in a pack
> > expansion corresponding to a non-pack, then from that point onwards we
> > don't form any `J...E` manglings and template arguments in an unresolved
> > form (in particular, mangle non-type arguments as expressions).
> >
> > I've added a test for that case. We don't strictly need to do anything
> > special here to handle that, because it doesn't matter what `needExactType`
> > returns when mangling an unresolved template argument, but it seems better
> > to more precisely track whether we expect to see matching arguments and
> > parameters or not, so we now track that here too.
> >
> > Thanks for catching this!
> Ugh. I think the right rule in the abstract would be to always mangle
> template arguments in a dependent template argument list using their written
> structure, even when we can immediately resolve the target template and check
> arguments against parameters. That would certainly include not trying to
> collect arguments into template parameter packs. But that ship probably
> sailed a long time ago, and your left-to-right rule is presumably what we're
> left with. We should document that in the ABI, though; mind opening a PR?
Not sure if PR in this context is problem report or pull request, but:
https://github.com/itanium-cxx-abi/cxx-abi/issues/113
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D91488/new/
https://reviews.llvm.org/D91488
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