The following sentence frequently appears in several paragraphs in subclause [temp.deduct.general]

If the substitution results in an invalid type, as described above, type deduction fails.

However, we do not see the relevant contents referred to by "as described above". AFAIK, the invalid type or expression in the immediate context of the function type or something else stated in [temp.deduct.general] p8 can result in "deduction fails" rather than a program ill-formed. Does "as described above" intend to refer to [temp.deduct.general] p8? Should we rearrange the position of [temp.deduct.general] p8 to make "as described above" have a corresponding expound.

Since [temp.deduct.general] p7 says

The substitution occurs in all types and expressions that are used in the function type function type outside of the exception specification and in template parameter declarations.

[temp.deduct.general] p8 says

Only invalid types and expressions in the immediate context of the function type outside of the exception specification, its template parameters, and its explicit-specifier can result in a deduction failure.

template parameter seems to refer to T rather than typename T = type-id_opt(if it is the template parameter declaration), deduction failure can also occur in type-id.