Rationale (May, 2015):

An explicit specialization with a deduced return type can only match a template declared with a deduced return type, so the actual return type is not needed in order to match the explicit specialization with the template being specialized.

The Standard does not indicate whether an explicit specialization of a function template can have a deduced return type. It seems a bit too much to require parsing the entire function body in order to tell which template is being specialized. In extreme cases, that could mean deferring access checks for the entire body of the function.