This is an unofficial snapshot of the ISO/IEC JTC1 SC22 WG21 Core Issues List revision 112e. See http://www.open-std.org/jtc1/sc22/wg21/ for the official list.
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.
Change and add before 11.4.1 [class.mem.general] paragraph 1 as follows:
member-declarator: declarator virt-specifier-seqopt pure-specifieropt declarator brace-or-equal-initializer
The token sequence = 0 is treated as a pure-specifier if the type of the declarator-id (184.108.40.206 [dcl.meaning.general]) is a function type, and is otherwise treated as a brace-or-equal-initializer. [ Note: If the member declaration acquires a function type through template instantiation, the program is ill-formed; see 13.9.1 [temp.spec.general]. --end note ]