[clang] Fix non-deterministic infinite recursion... (#118288)

...in `ASTContext::getAutoTypeInternal`

Given

```cpp
template < typename >
concept C1 = true;

template < typename , auto >
concept C2 = true;

template < C1 auto V, C2< V > auto>
struct S;
```

Both `C1 auto V` and `C2<V> auto` end on the set `AutoType`, the former
being a template parameter for the latter.

Since the hashing is not deterministic (i.e., pointers are hashed),
every now and then, both will end on the same bucket. Given that
`FoldingSet` recomputes the `FoldingSetID` for each node in the target
bucket on lookup, this triggers an infinite recursion:

1. Look for `X` in `AutoTypes`
2. Let's assume it would be in bucket N, so it iterates over nodes in
that bucket. Let's assume the first is `C2<V> auto`.
3. Computes the `FoldingSetID` for this one, which requires the profile
of its template parameters, so they are visited.
4. In some frames below, we end on the same `FoldingSet`, and, by
chance, `C1 auto V` would be in bucket N too.
5. But the first node in the bucket is `C2<V> auto` for which we need to
profile `C1 auto V`
6. ... stack overflow!

No step individually does anything wrong, but in general, `FoldingSet`
seems not to be re-entrant, and this fact is hidden behind many nested
calls.

With this change, we store the `AutoType`s inside a `DenseMap` instead.
The `FoldingSetID` is computed once only and then kept as the map's key,
avoiding the need to do recursive lookups.

We also now make sure the key for the inserted `AutoType` is the same as
the key used for lookup. Before, this was not the case, and it caused
also non-deterministic parsing errors.

Fixes https://github.com/llvm/llvm-project/issues/110231

0 files changed, 0 insertions, 0 deletions