P0012R1
revises N4533 and P0012R0
Jens Maurer <Jens.Maurer@gmx.net>
2015-10-22

P0012R1: Make exception specifications be part of the type system, version 5

Introduction

The exception specification of a function was never considered a part of its type. Instead, 15.4 [except.spec] paragraphs 5 and 6 only restrict assignments and initializations of pointers and references to (member) functions. This leads to odd holes in the type system that have resulted in a number of core issues.

Core issue 92 asks the fundamental question "Should exception-specifications be part of the type system?" and gives this example: 
    void (*p)() throw(int);
    void (**pp)() throw() = &p;   // not currently an error

This issue was closed as NAD in February 2014, because "EWG determined that no action should be taken on this issue" (quote from the core issues list). Little technical argument seems to have been presented; closing the issue was rather seen as warding off work.

Paper N1664 "Toward Improved Optimization Opportunities in C++0x" by Walter Brown and Marc Paterno suggested a nothrow qualifier to enhance optimization opportunities when using non-throwing functions. Section 7.2 of that paper described the type effects of the nothrow qualifier essentially identical to those of the noexcept qualifier as proposed in the present paper.

Core issue 1946 "exception-specifications vs pointer dereference" highlighted that clarifications for determining potential exceptions of an expression brought forward for core issue 1351 caused this example to be ill-formed, which was arguably well-formed under the earlier imprecise wording: 

  void (*p)() throw(int);
  void (&r)() throw(int) = *p;  // ill-formed
A similar example involving templates is impossible to fix without changes to the type system: 
  template<typename T> T& f(T* p);
  void (*p)() throw(int);
  void (&r)() throw(int) = f(p);  // ill-formed

Core issue 2010 "exception-specifications and conversion operators" adds these examples: 

  struct S { typedef void (*p)(); operator p(); };
  void (*q)() noexcept = S();

  void (*r)() noexcept = []() noexcept {};

The recent work on a specification for transactional memory support (see N4265 "Transactional Memory Support for C++: Wording (revision 3)") required integrating the transaction_safe specifier into the types of functions, among other things considering it for implicit conversions and overload resolution. This paper clones that approach to integrate the presence or absence of a non-throwing exception-specification (called noexcept) into the types of functions, too.

The predecessor paper N4518 was presented to the WG21 plenary in Lenexa, but straw polls were postponed to the following meeting to give all interested parties a chance to review the effects, in particular on existing code.

Changes compared to P0012R0

Changes compared to N4533

Changes compared to N4518

Changes compared to N4320

Library impact

The result_of template is unaffected, because it does not inspect the function or function object "Fn" in detail; similar for the INVOKE meta-operation (20.9.2 [func.require]).

The function call operator for reference_wrapper could be noexcept if "T" is a noexcept function (see wording below) and all conversions for arguments are noexcept. This is already implied by the existing wording for mem_fn (20.9.11 [func.memfn]).

There should be a second partial specialization for packaged_task (30.6.9 [futures.task]), namely packaged_task<R(Args...) noexcept> (not proposed in the present paper).

It is an open issue how to propagate "noexcept" through std::function.

Feature-test macro

The recommended feature-test macro is __cpp_noexcept_function_type.

Wording

Clause 4 [conv]

Change section 4 [conv] paragraph 1:
No change in section 4.3 [conv.func] paragraph 1:
An lvalue of function type T can be converted to a prvalue of type "pointer to T". The result is a pointer to the function. [ Footnote: ... ]
Add a new section after section 4.11 [conv.mem]:
4.12 [conv.fctptr] Function pointer conversions

A prvalue of type "pointer to noexcept function" can be converted to a prvalue of type "pointer to function". The result is a pointer to the function. A prvalue of type "pointer to member of type noexcept function" can be converted to a prvalue of type "pointer to member of type function". The result points to the member function.

[ Example: 

  void (*p)() throw(int);
  void (**pp)() noexcept = &p;   // error: cannot convert to pointer to noexcept function

  struct S { typedef void (*p)(); operator p(); };
  void (*q)() noexcept = S();   // error: cannot convert to pointer to noexcept function
-- end example ]

Clause 5 [expr] Expressions

Change in 5 [expr] paragraph 13:
[ Note: ... ] The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer to member type or std::nullptr_t, is:
Change in 5.1.2 [expr.prim.lambda] paragraph 6:
The closure type for a non-generic lambda-expression with no lambda-capture has a public non-virtual non-explicit const conversion function to pointer to function with C++ language linkage (7.5 [dcl.link]) having the same parameter and return types as the closure type's function call operator. The conversion is to "pointer to noexcept function" if the function call operator has a non-throwing exception specification.
Change in 5.2.2 [expr.call] paragraph 1:
The postfix expression shall have function type or pointer to function function pointer type. For a call to a non-member function or to a static member function, the postfix expression shall be either an lvalue that refers to a function (in which case the function-to-pointer standard conversion (4.3) is suppressed on the postfix expression), or it shall have pointer to function function pointer type.
Change in 8.3.1 [dcl.ptr] paragraph 4:
[ Note: Forming a pointer to reference type is ill-formed; see 8.3.2. Forming a pointer to function function pointer type is ill-formed if the function type has cv-qualifiers or a ref-qualifier; see 8.3.5. Since the address of a bit-field (9.6) cannot be taken, a pointer can never point to a bit-field. — end note ]
Change in 5.2.9 [expr.static.cast] paragraph 7:
The inverse of any standard conversion sequence (Clause 4 [conv]) not containing an lvalue-to-rvalue (4.1 [conv.lval]), array-to-pointer (4.2 [conv.array]), function-to-pointer (4.3), null pointer (4.10), null member pointer (4.11), or boolean (4.12), or function pointer (4.12 [conv.fctptr]) conversion, can be performed explicitly using static_cast. ...
Change in 5.10 [expr.eq] paragraph 2:
If at least one of the operands is a pointer, pointer conversions (4.10 [conv.ptr]), function pointer conversions (4.12 [conv.fctptr]), and qualification conversions (4.4 [conv.qual]) are performed on both operands to bring them to their composite pointer type (clause 5 [expr]).
Change in 5.16 [expr.cond] paragraph 6:
Change in 5.17 [expr.throw]:
Evaluating a throw-expression with an operand throws an exception (15.1 [except.throw]); the type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand and adjusting the type from "array of T" or "function returning T" function type T to "pointer to T" or "pointer to "function returning T," respectively.
Change in 5.20 [expr.const] paragraph 3:
... A converted constant expression of type T is an expression, implicitly converted to a prvalue of type T, where the converted expression is a core constant expression and the implicit conversion sequence contains only user-defined conversions, lvalue-to-rvalue conversions (4.1 [conv.lval]), integral promotions (4.5 [conv.prom]), and integral conversions (4.7 [conv.integral]) other than narrowing conversions (8.5.4 [dcl.init.list]), and function pointer conversions (4.12 [conv.fctptr]). [ Note: ... ]

Section 8 [dcl.decl] Declarators

Change in 8.3.5 [dcl.fct] paragraphs 1 and 2:
In a declaration T D where D has the form 
    D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
         ref-qualifieropt exception-specificationopt attribute-specifier-seqopt
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 noexceptopt function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning T", where the optional noexcept is present if and only if the exception-specification is non-throwing. The optional attribute-specifier-seq appertains to the function type.

In a declaration T D where D has the form 

    D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
         ref-qualifieropt exception-specificationopt attribute-specifier-seqopt trailing-return-type
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. The type of the declarator-id in D is "derived-declarator-type-list noexceptopt function of (parameter-declaration-clause) cv-qualifier-seqopt ref-qualifieropt returning trailing-return-type", where the optional noexcept is present if and only if the exception-specification is non-throwing. The optional attribute-specifier-seq appertains to the function type.
Change in 8.3.5 [dcl.fct] paragraph 5:
... After determining the type of each parameter, any parameter of type "array of T" or "function returning T" of function type T is adjusted to be "pointer to T" or "pointer to function returning T," respectively. ...
Change in 8.3.5 [dcl.fct] paragraph 6:
... The return type, the parameter-type-list, the ref-qualifier, and the cv-qualifier-seq, and whether the function has a non-throwing exception-specification, but not the default arguments (8.3.6 [dcl.fct.default]) or the exception specification (15.4 [except.spec]), are part of the function type. ...
Change in section 8.4.1 [dcl.fct.def.general] paragraph 2:
The declarator in a function-definition shall have the form 
         D1 ( parameter-declaration-clause ) cv-qualifier-seqopt
               ref-qualifieropt exception-specificationopt attribute-specifier-seqopt 
               parameters-and-qualifiers trailing-return-typeopt

[ Drafting note: This is intended to reduce the grammar redundancies around function declarators. ]

Change in 8.5.3 [dcl.init.ref] paragraph 1:
A variable whose declared type is to be a T& or T&&, that is, "reference to type T" (8.3.2), shall be initialized by an object, or function, of type T or by an object that can be converted into a T. [ Example: 
  int g(int) noexcept;
  ...
]
Change in 8.5.3 [dcl.init.ref] paragraph 4:
Given types "cv1 T1" and "cv2 T2", "cv1 T1" is reference-related to "cv2 T2" if T1 is the same type as T2, or T1 is a base class of T2. "cv1 T1" is reference-compatible with "cv2 T2" if and cv1 is the same cv-qualification as, or greater cv-qualification than, cv2. In all cases where the reference-related or reference-compatible relationship of two types is used to establish the validity of a reference binding, and T1 is a base class of T2, a program that necessitates such a binding is ill-formed if T1 is an inaccessible (Clause 11) or ambiguous (10.2) base class of T2.

Clause 13 [over] Overloading

Change in 13.1 [over.load] paragraph 2:
Certain function declarations cannot be overloaded:
No change in 13.3.3.1.1 [over.ics.scs] paragraph 1:
... [ Note: These categories are orthogonal with respect to value category, cv-qualification, and data representation: the Lvalue Transformations do not change the cv-qualification or data representation of the type; the Qualification Adjustments do not change the value category or data representation of the type; and the Promotions and Conversions do not change the value category or cv-qualification of the type. -- end note ]
In 13.3.3.1.1 [over.ics.scs], add an entry to table 11 "Conversions":
Change in 13.3.3.1.1 [over.ics.ics] paragraph 2:
[ Note: ... At most one conversion from each category is allowed in a single standard conversion sequence. ... ]
Change in 13.4 [over.over] paragraph 1:
... The function selected is the one whose type is identical to the function type of the target type required in the context. A function with type F is selected for the function type FT of the target type required in the context if F (after possibly applying the function pointer conversion (4.12 [conv.fctptr])) is identical to FT. [ Note: ... ]
Change in 13.4 paragraph 3:
... Non-member functions and static member functions match targets of pointer to function function pointer type or reference to function type. ...
Change in 13.4 [over.over] paragraph 7:
[ Note: There are no standard conversions (Clause 4) of one pointer-to-function type into another. In particular, even Even if B is a public base of D, we have 
D* f();
B* (*p1)() = &f;     // error
void g(D*);
void (*p2)(B*) = &g; // error
]

Clause 14 [temp] Templates

Change in 14.1 [temp.param] paragraph 8:
A non-type template-parameter of type "array of T" or "function returning T" of function type T is adjusted to be of type "pointer to T" or "pointer to function returning T", respectively. [ Example: ... ]
Change in 14.8.2.1 [temp.deduct.call] paragraph 4:
... However, there are three cases that allow a difference:
Change in 14.8.2.1 [temp.deduct.call] paragraph 6:
When P is a function type, pointer to function function pointer type, or pointer to member function type: ...
Change in 14.8.2.3 temp.deduct.conv paragraph 5:
In general, the deduction process attempts to find template argument values that will make the deduced A identical to A. However, there are two four cases that allow a difference:

Clause 15 [except] Exception handling

Change in 15.3 except.handle paragraph 3:
A handler is a match for an exception object of type E if
Change in 15.4 [except.spec] paragraph 1:
A function declaration declarator lists exceptions that its function might directly or indirectly throw by using an exception-specification as a suffix of its declarator.
Change in 15.4 [except.spec] paragraph 2:
An exception-specification shall appear only on a function declarator for a function type, pointer to function type, reference to function type, or pointer to member function type that is the top-level type of a declaration or definition, or on such a type appearing as a parameter or return type in a function declarator. An exception-specification shall not appear in a typedef declaration or alias-declaration. [ Example: ... ] ...
Change in 15.4 [except.spec] paragraph 5 and delete paragraph 6:

[ Example: ... ] A similar restriction applies to assignment to and initialization of pointers to functions, pointers to member functions, and references to functions: the target entity shall allow at least the exceptions allowed by the source value in the assignment or initialization. [ Example: ... ]

In such an assignment or initialization, exception-specifications on return types and parameter types shall be compatible. In other assignments or initializations, exception-specifications shall be compatible.

Change in 15.4 [except.spec] paragraph 15 bullet 1:
Add a new paragraph after 15.4 [except.spec] paragraph 15:
A function with an implied non-throwing exception specification, where the function's type is declared to be T, is instead considered to be of type "noexcept T".
Change in 20.9.2 [func.require] paragraph 3 bullet 1:
Change in 20.9.4 [refwrap] paragraph 4 bullet 1:
Change in 20.9.4 [refwrap] paragraph 5 bullet 1:
Add a new section after C.4.1 [diff.cpp14.lex]:
C.4.2 Clause 8: Declarators [diff.cpp14.decl]
8.3.5 [dcl.fct]

Change: Make exception specifications be part of the type system.
Rationale: Improve type-safety.
Effect on original feature: Valid C++2014 code may fail to compile or change meaning in this International Standard: 

  void g1() noexcept;
  void g2();
  template<class T> int f(T *, T *);
  int x = f(g1, g2);    // ill-formed; previously well-formed