Proposed resolution (February, 2019):

Change 13.7.5 [temp.friend] paragraph 8 as follows:

When a friend declaration refers to a specialization of a function template, the function parameter declarations shall not include default arguments, nor shall the inline specifier inline, constexpr, or consteval specifiers be used in such a declaration.

Notes from the December, 2018 teleconference:

This should also cover the newly-added consteval specifier.

[Accepted as a DR at the February, 2019 meeting.]

According to 13.7.5 [temp.friend] paragraph 8,

When a friend declaration refers to a specialization of a function template, the function parameter declarations shall not include default arguments, nor shall the inline specifier be used in such a declaration.

Presumably this should also include the constexpr specifier.