CTAD for alias templates: implement the "is_deducible" constrain

[hokein]

We have an initial support at #77890 now.

However, the feature is not fully implemented yet, the is_deducible constrain is missing, see the FIXME at https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaTemplate.cpp#L2909-L2914, and related testcase at https://github.com/llvm/llvm-project/blob/main/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp#L188-L200

We should have this fixed before the next clang 19 release.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

We have an initial support at #77890 now.

However, the feature is not fully implemented yet, the is_deducible constrain is missing, see the FIXME at https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaTemplate.cpp#L2909-L2914, and related testcase at https://github.com/llvm/llvm-project/blob/main/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp#L188-L200

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Haojian Wu (hokein)

We have an initial support at #77890 now.

However, the feature is not fully implemented yet, the is_deducible constrain is missing, see the FIXME at https://github.com/llvm/llvm-project/blob/main/clang/lib/Sema/SemaTemplate.cpp#L2909-L2914, and related testcase at https://github.com/llvm/llvm-project/blob/main/clang/test/SemaCXX/cxx20-ctad-type-alias.cpp#L188-L200

[ilya-biryukov]

Added to the LLVM 19 release milestone as we probably don't want to release without this. Not having this feature would allow writing code that will break in future releases of Clang.

We currently completely ignore constraints written in template aliases if the template arguments for it were deduced via CTAD.