Issue description

We have an ambiguous grammar for member-declarator:

member-declarator:

• declarator virt-specifier-seq[opt] pure-specifier[opt]
• declarator brace-or-equal-initializer[opt]

pure-specifier:

• = 0

The primary issue here is that foo = 0 matches both member-declarator productions. Secondarily, a declarator by itself is also ambiguous, but that's easily fixed by removing the [opt] from the second production and doesn't lead to two different program interpretations.

We then have:

11. A brace-or-equal-initializer shall appear only in the declaration of a data member.
12. A pure-specifier shall be used only in the declaration of a virtual function that is not a friend declaration.

... which make the two mutually exclusive but isn't itself a disambiguation rule.

Utterances like virtual FunctionType f = 0; are valid, so we can't disambiguate based on the syntactic form of the declarator. In practice, implementations disambiguate based on whether decl-specifier-seq declarator declares a name with function type, and I believe that is the intent based on the rule in [temp.spec] that acquiring a function type through a dependent type is ill-formed.

Suggested resolution:

Option 1

Add a disambiguation rule, such as:

The token sequence = 0 is treated as a pure-specifier if the type of the declarator-id [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 [temp.spec.general].]

... and remove the other ambiguity with a grammar change:

member-declarator:

• declarator virt-specifier-seq[opt] pure-specifier[opt]
• declarator brace-or-equal-initializer[opt]

Option 2

Change the grammar to remove the ambiguity entirely:

member-declarator:

• declarator virt-specifier-seq[opt] pure-specifier[opt] brace-or-equal-initializer[opt]
• declarator brace-or-equal-initializer[opt]

pure-specifier:

• = 0

11. A brace-or-equal-initializer shall appear only in the declaration of a data member or a function. [...]
12. [...] . A brace-or-equal-initializer appearing in the declaration of a member function shall be of the form = 0, and is called a pure-specifier. A pure-specifier shall be used only in the declaration of a virtual function that is not a friend declaration.

Either way, this seems borderline editorial to me.