During template instantiation involving templated lambdas, clang
could hit an assertion in `TemplateDeclInstantiator::SubstFunctionType`
since the functions are not associated with any `TypeSourceInfo`:
`assert(OldTInfo && "substituting function without type source info");`
This path is triggered when using templated lambdas like the one added as
a test to this patch. To fix this:
- Create `TypeSourceInfo`s for special members and make sure the template
instantiator can get through all patterns.
- Introduce a `SpecialMemberTypeInfoRebuilder` tree transform to rewrite
such member function arguments. Without this, we get errors like:
`error: only special member functions and comparison operators may be defaulted`
since `getDefaultedFunctionKind` can't properly recognize these functions
as special members as part of `SetDeclDefaulted`.
Fixes PR45828 and PR44848
Differential Revision: https://reviews.llvm.org/D88327
David Stone