https://cplusplus.github.io/LWG/issue3810

LWG would like design guidance here. Should CTAD work? It's currently sort-of implied it should, but only if you assume that the exposition-only types and constructors are implemented exactly as shown. If the format-arg-store-type is defined as a nested type of basic_format_args (as in libstdc++) then it doesn't work.

There's an example from Tomasz in the issue showing a case where CTAD might be useful, but Victor suggested we should explicitly disable CTAD here, because it makes it easier to create dangling references.