New Function Declarator Syntax Wording

Jason Merrill
2008-02-28
Document Number N2541=08-0051
Revision 11

Proposed wording

7.1.6.4p2+

Otherwise (auto appearing with no type specifiers other than cv-qualifiers), the auto type-specifier signifies that the type of an object being declared shall be deduced from its initializer or specified explicitly at the end of a function declarator. The name of the object being declared shall not appear in the initializer expression.

auto can appear with a function declarator with a late-specified return type (8.3.5) in any context where such a declarator is valid, and the use of auto is replaced by the type specified at the end of the declarator.

Otherwise, the type of the object is deduced from its initializer. The name of the object being declared shall not appear in the initializer expression. This use of auto is allowed when declaring objects in a block (6.3), in namespace scope (3.3.5), and in a for-init-statement (6.5.3). [...]

8p4

direct-declarator:
	declarator-id
	direct-declarator ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt
	direct-declarator ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt -> type-id
	direct-declarator [ constant-expressionopt ]
	( declarator )

8.1p1

direct-abstract-declarator:
	direct-abstract-declaratoropt ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt
	direct-abstract-declaratoropt ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt -> type-id
	direct-abstract-declaratoropt [ constant-expressionopt ]
	( abstract-declarator )

8.3.5p1

In a declaration T D where D has the form 

D1 ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt
and the type of the contained declarator-id in the declaration T D1 is "derived-declarator-type-list T," the type of the declarator-id in D is "derived-declarator-type-list function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning T"; a type of this form is a function type*.

In a declaration T D where D has the form 

D1 ( parameter-declaration-clause ) cv-qualifier-seqopt ref-qualifieropt exception-specificationopt -> type-id
and the type of the contained declarator-id in the declaration T D1 is "derived-declarator-type-list T," T shall be the single type-specifier auto and the derived-declarator-type-list shall be empty. Then the type of the declarator-id in D is "function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning type-id". Such a function type has a late-specified return type.

The type-id in this form includes the longest possible sequence of abstract-declarators. [Note: this resolves the ambiguous binding of array and function declarators. [Example: 

    auto f()->int(*)[4]; // function returning a pointer to array[4] of int
                         // not function returning array[4] of pointer to int
--end example] --end note]

A type of either form is a function type*.

[Footnote: As indicated by the syntax, cv-qualifiers are a significant component in function return types. --- end foonote]

8.3.5p9

...[Note: typedefs and late-specified return types are sometimes convenient when the return type of a function is complex. For example, the function fpif above could have been declared 
  typedef int IFUNC(int);
  IFUNC* fpif(int);
or 
  auto fpif(int)->int(*)(int)
A late-specified return type is most useful for a type that would be more complicated to specify before the declarator-id: 
  template <class T, class U> auto add(T t, U u) -> decltype(t+u);
rather than 
  template <class T, class U> decltype((*(T*)0)+(*(U*)0)) add(T t, U u);
-- end note ]