Proposed resolution (April, 2016):

Change 9.5.1 [dcl.fct.def.general] paragraph 2 as follows:

The declarator in In a function-definition shall have the form,

D1 parameters-and-qualifiers trailing-return-type_opt

either void declarator ; or declarator ; shall be a well-formed function declarator as described in 9.3.4.6 [dcl.fct]. A function shall be defined only in namespace or class scope.

[Adopted at the June, 2016 meeting.]

According to 9.5.1 [dcl.fct.def.general] paragraph 2,

The declarator in a function-definition shall have the form

D1 ( parameter-declaration-clause ) cv-qualifier-seq_opt
ref-qualifier_opt exception-specification_opt attribute-specifier-seq_opt trailing-return-type_opt

However, in practice implementations accept a parenthesized declarator in a function definition.