Most vexing parse with a qualified-id - or not?

问题

Consider:

struct Foo {
     enum { bar };
     explicit Foo(int){}
};

struct Baz { explicit Baz(Foo){} };

Baz b(Foo(Foo::bar)); // #1

Is line #1 the most vexing parse, even though Foo::bar is a qualified-id and can't possibly be a valid parameter name? Clang and GCC disagree; which compiler is correct?

回答1:

Clang is right.

Somewhat surprisingly, the grammar for parameter-declaration permits both qualified- and unqualified-ids, because it accepts all declarators:

parameter-declaration:
    attribute-specifier-seq_opt decl-specifier-seq declarator
    attribute-specifier-seq_opt decl-specifier-seq declarator = initializer-clause
    attribute-specifier-seq_opt decl-specifier-seq abstract-declarator_opt
    attribute-specifier-seq_opt decl-specifier-seq abstract-declarator_opt = initializer-clause

and the grammar for a declarator permits both qualified- and unqualified-ids. The "no qualified-id for function parameter names" rule, for better or worse, is a semantic rule, even though it is easily possible to write a grammar for parameter-declaration that excludes qualified-ids directly.

Just like the situation in this question, the disambiguation rule is purely syntatic, and since

Baz b(Foo(Foo::bar));

can syntatically be parsed as a function declaration, it is so parsed, even though the disambiguation in this case results in something that can never compile.