================
@@ -1876,11 +1901,25 @@ namespace {
TemplateParameterList *OrigTPL) {
if (!OrigTPL || !OrigTPL->size()) return OrigTPL;
+ std::optional<MultiLevelTemplateArgumentList> OldMLTAL;
+ // We need to preserve the lambda depth in parameter mapping.
+ // Otherwise the template argument deduction would fail, if we reduced
the
+ // depth too early.
+ if (SemaRef.inParameterMappingSubstitution() &&
+ getDepthAndIndex(OrigTPL->getParam(0)).first >=
+ TemplateArgs.getNumSubstitutedLevels())
+ OldMLTAL = ForgetSubstitution();
----------------
zyn0217 wrote:
> Also, I don't see how the level check is necessary.
@mizvekov
The fact is that we will transform the lambda expression in both building and
substitution of parameter mapping.
Say we have
```cpp
template<auto Pred, typename... Ts>
concept PredicateFor_bad = pass_a_concept_inside_a_lambda<[]<typename... Xs>
requires(__is_same(decltype(Pred.template operator()<Xs>()), bool) and ...)
{},
Ts...>;
```
When building, we only have outer template arguments `<Pred, <Ts...>>`, and
since the transform of parameter list will reduce the depth of <Xs...> anyway
if only partial template arguments are provided, depth of <Xs> will be
converted to 0.
And in the second transform where we started to substitute real template
arguments into parameter mapping (i.e. when we have `<Pred=lambda, Ts=<int,
int, int>>`), the depth of `Xs` is reduced before deduction happens. This
results in a -1 depth during argument deduction and thus we won't have a
correct template argument list when checking the inner constraint.
Note that we didn't transform the lambda expression in the second transfrom,
because the lambda was seen as non-dependent. I do want the lambda to have a
second transform, because otherwise we don't have a way to get substituted
`Pred` in constraint evaluation. (`Pred` is normalized and we have to evaluate
the lambda's inner constraint during parameter mapping substitution, when we
dont have any proper template arguments at that moment, which is an
egg-and-chicken problem though)
https://github.com/llvm/llvm-project/pull/195995
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