CWG2826 [class.temporary] Missing definition of "temporary expression"

[t3nsor]

Full name of submitter: Brian Bi

Reference (section label): [class.temporary]

Issue description: The term "temporary expression" is currently used in two places in the standard: [class.base.init]/8 "A temporary expression bound to a reference member in a _mem-initializer is ill-formed." and [class.base.init]/11 "A temporary expression bound to a reference member from a default member initializer is ill-formed." However, this term has not been defined.

Suggested resolution:
Edit [class.temporary]/6:

The third context is when a reference binds to a temporary object. The temporary object to which the
reference is bound or the temporary object that is the complete object of a subobject to which the reference
is bound persists for the lifetime of the reference if the glvalue to which the reference is bound wasA temporary expression is a glvalue obtained through one of the following:

• a temporary materialization conversion (7.3.5),

• ( expression ), where expression is one of these expressions a temporary expression,

• subscripting (7.6.1.2) of an array operand, where that operand is one of these expressions a temporary expression,

• a class member access (7.6.1.5) using the . operator where the left operand is one of these expressions a temporary expression and the right operand designates a non-static data member of non-reference type,

• a pointer-to-member operation (7.6.4) using the .* operator where the left operand is one of these
  expressions a temporary expression and the right operand is a pointer to data member of non-reference type,

• a

  • const_cast (7.6.1.11),
  • static_cast (7.6.1.9),
  • dynamic_cast (7.6.1.7), or
  • reinterpret_cast (7.6.1.10)

  converting, without a user-defined conversion, a glvalue operand that is one of these expressions temporary expression to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof,

• a glvalue conditional expression (7.6.16) that is a glvalue designating either its second or third operand, where the selected operand is a temporary expression where the second or third operand is one of these expressions, or

• a glvalue comma expression (7.6.20) that is a glvalue where the right operand is one of these expressions a temporary expression.

If the reference is bound to a temporary expression, the complete object of the object designated by the temporary expression persists for the lifetime of the reference.

[jensmaurer]

Personally, I would prefer not having the term "reference bound to a temporary expression". References bind to objects, not to expressions.

If we instead of the above just replaced the two uses of "temporary expression" with something like "attempting to bind a temporary object to (context) is ill-formed", that should do the trick, I think.

[t3nsor]

I'm not sure what you mean by (context).

I think it's fine to not use the term "temporary expression", but I think we need some way to refer back to [class.temporary] because the ill-formed cases are only those in which the expression would normally be eligible for lifetime extension.

[jensmaurer]

(context) means "mem-initializer" or "from a default member initializer". Yes, the wording needs a bit of tweaking.

[t3nsor]

I can't think of a great way to refer to "the set of expressions described in [class.temporary]/6" other than to have a term for it. We should either keep "temporary expression" or coin some other term, such as "extension-eligible expression". With this latter term, for example, the suggested resolution would be

Edit [class.temporary]/6:

The third context is when a reference binds to a temporary object. The temporary object to which the
reference is bound or the temporary object that is the complete object of a subobject to which the reference
is bound persists for the lifetime of the reference if the glvalue to which the reference is bound was An expression is extension-eligible if it is a glvalue obtained through one of the following:

• a temporary materialization conversion (7.3.5),

• ( expression ), where expression is one of these expressions extension-eligible,

• subscripting (7.6.1.2) of an array operand, where that operand is one of these expressions extension-eligible,

• a class member access (7.6.1.5) using the . operator where the left operand is one of these expressions extension-eligible and the right operand designates a non-static data member of non-reference type,

• a pointer-to-member operation (7.6.4) using the .* operator where the left operand is one of these
  expressions extension-eligible and the right operand is a pointer to data member of non-reference type,

• a

  • const_cast (7.6.1.11),
  • static_cast (7.6.1.9),
  • dynamic_cast (7.6.1.7), or
  • reinterpret_cast (7.6.1.10)

  converting, without a user-defined conversion, a glvalue operand that is one of these expressions extension-eligible to a glvalue that refers to the object designated by the operand, or to its complete object or a subobject thereof,

• a conditional expression (7.6.16) that is a glvalue designating either its second or third operand, where the selected operand is extension-eligible where the second or third operand is one of these expressions, or

• a comma expression (7.6.20) that is a glvalue where the right operand is one of these expressions extension-eligible.

If the reference is initialized from an extension-eligible expression, the complete object of the object designated by the temporary expression persists for the lifetime of the reference.

Edit [class.base.init]/8 to read: "A mem-initializer is ill-formed if it initializes a reference member from an extension-eligible expression (6.7.7 [class.temporary])."

Edit [class.base.init]/11 likewise (see CWG 2669).

[jensmaurer]

CWG2826

[frederick-vs-ja]

Now the following also seems incorrect:

The third context is when a reference binds to a temporary object.

cplusplus/draft#6572 should have clarified that a temporary object is always a complete object created in one of two ways. In the reference binding case, it's always created in temporary materialization. Perhaps we should say "the complete object of O" is the temporary object in some way.

Personally, I would prefer not having the term "reference bound to a temporary expression". References bind to objects, not to expressions.

We're already saying "the reference binds to some expression" in [dcl.init.ref]. I guess we can clean this up later. If we want to avoid saying binding to expression, how about the following?

In [class.temporary] (on the top of current resolution of CWG2826):

The third context is when intializing a reference binds to a temporary object with a temporary expression. [...]
[...]
If a reference is bound to an object O that is the result of is initialized with a temporary expression, the complete object of O the result of the temporary expression persists for the lifetime of the reference.

In [class.base.init] (both places):

A temporary expression bound to Initializing a reference member with a temporary expression [...]

In [stmt.return]:

[...] a return statement that binds initializes the returned reference to with a temporary expression ([class.temporary]) is ill-formed.