Proposed resolution (September, 2019):

Change the grammar in 13.7.2.3 [temp.deduct.guide] paragraph 1 as follows:

deduction-guide:
explicit explicit-specifier_opt template-name ( parameter-declaration-clause ) -> simple-template-id ;

[Adopted as a DR at the November, 2019 meeting.]

According to 13.7.2.3 [temp.deduct.guide] paragraph 1, the syntax of a deduction-guide is:

deduction-guide:
explicit_opt template-name ( parameter-declaration-clause ) -> simple-template-id ;

Instead of explicit, this production should use the explicit-specifier nonterminal. (The wording of 12.2.2.9 [over.match.class.deduct] has references to the explicit-specifier of a deduction guide, for example.)