[Accepted as a DR at the March, 2024 meeting.]
Subclause 11.4.1 [class.mem.general] has this grammar:
member-declarator: declarator virt-specifier-seq[opt] pure-specifier[opt] declarator brace-or-equal-initializer[opt] pure-specifier: = 0
The primary issue is that foo = 0 matches both member-declarator productions. Secondarily, a declarator by itself is also ambiguous.
Code such as virtual FunctionType f = 0; can be valid, so disambiguation on the syntactic form of the declarator is not possible.