Similar to the approach of handling nested class templates when building
a CTAD guide, we substitute the template parameters of a type alias
declaration with the instantiating template arguments in order to ensure
the guide eventually doesn't reference any outer template parameters.
For example,
```cpp
template <class T> struct Outer {
using Alias = S<T>;
template <class U> struct Inner {
Inner(Alias);
};
};
```
we used to retain the reference to T accidentally because the
TreeTransform does nothing on type alias Decls by default.
Fixes https://github.com/llvm/llvm-project/issues/94614
Reland of f418319730341e9d41ce8ead6fbfe5603c343985 with proper handling
of template constructors
When a nested template is instantiated, the template pattern of the
inner class is not copied into the outer class
ClassTemplateSpecializationDecl. The specialization contains a
ClassTemplateDecl with an empty record that points to the original
template pattern instead.
As a result, when looking up the constructors of the inner class, no
results are returned. This patch finds the original template pattern and
uses that for the lookup instead.
Based on CWG2471 we must also substitute the known outer template
arguments when creating deduction guides for the inner class.
Changes from last iteration:
1. In template constructors, arguments are first rewritten to depth - 1
relative to the constructor as compared to depth 0 originally. These
arguments are needed for substitution into constraint expressions.
2. Outer arguments are then applied with the template instantiator to
produce a template argument at depth zero for use in the deduction
guide. This substitution does not evaluate constraints, which preserves
constraint arguments at the correct depth for later evaluation.
3. Tests are added that cover template constructors within nested
deduction guides for all special substitution cases.
4. Computation of the template pattern and outer instantiation arguments
are pulled into the constructor of
`ConvertConstructorToDeductionGuideTransform`.
Reland of dd0fba11690f9fef304d5f48cde646e5eca8d3c0
When a nested template is instantiated, the template pattern of the
inner class is not copied into the outer class
ClassTemplateSpecializationDecl. The specialization contains a
ClassTemplateDecl with an empty record that points to the original
template pattern instead.
As a result, when looking up the constructors of the inner class, no
results are returned. This patch finds the original template pattern and
uses that for the lookup instead.
Based on CWG2471 we must also substitute the known outer template
arguments when creating deduction guides for the inner class.
Changes from the last iteration:
1. The outer retained levels from the outer template are always added to
the `MultiLevelTemplateArgumentList` for rewriting
`FunctionTemplateDecl` arguments, even if there is no FTD and the
arguments are empty.
2. When building implicit deduction guides, the template pattern
underlying decl is pushed as the current context. This resolves the
issue where `FindInstantiatedDecl` is unable to find the inner template
class.
3. Tests are updated to cover the failing case, and to assert that the
type is correct after argument deduction in the implicit case.
