[Clang] Ensure initialized NTTP expressions when building CTAD for type aliases (#161035)

We missed calling CheckTemplateArgument when building CTAD deduction
guides. That ensures some InitExprs are correctly initialized, as in the
test that crashed due to incorrect NTTP initialization.

I don't use CheckTemplateArguments because, in CTAD synthesis, template
parameter packs don't always appear at the end of the parameter list,
unlike user-written ones mandated by the standard. This makes it
difficult for CheckTemplateArguments to determine how many arguments a
pack in middle should match, leading to unnecessary complexity.

On the other hand, since we substitute non-deduced template parameters
with deduced ones, we need to fold the packs midway through
substitution, where CheckTemplateArgument is more convenient.

As a drive-by this also removes some dead code in SemaInit.

Fixes #131408

0 files changed, 0 insertions, 0 deletions