I think this is a defect in [expr.context]/2

[Alexander]

[expr.context]/2:

2    

In some contexts, an expression only appears for its side effects.

 

Such an expression is called a discarded-value expression.

 

The array-to-pointer and function-to-pointer standard conversions are not applied.

 

The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:

• (2.1)
  (2.1)   ( expression ), where expression is one of these expressions,
• (2.2)
  (2.2)       id-expression,
• (2.3)
  (2.3)      subscripting,
• (2.4)
  (2.4)      class member access,
• (2.5)
  (2.5)      indirection,
• (2.6)
  (2.6)      pointer-to-member operation,
• (2.7)
  (2.7)     conditional expression where both the second and the third operands are one of these expressions, or
• (2.8)
  (2.8)     comma expression where the right operand is one of these expressions.
• 
  

[ Note

: 

Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.

 — end note

 ]

 

If the (possibly converted) expression is a prvalue, the temporary materialization conversion is applied.

 

[ Note

: 

If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the lvalue-to-rvalue conversion.

 — end note

 ]

 

The glvalue expression is evaluated and its value is discarded.

The paragraph defines a discarded-value expression, only considering expressions of the type ( expression ), where any expression of the type expression; satisfying (2.2) to (2.8) is also a discarded-value expression.  To correct this, it's enough to change (2.1) to this:

(2.1)       expression;  or  (expression), where expression is one of these expressions, 

[Tam S. B.]

I don't think this is a defect, but an editorial change could be applied to [expr.context]/2 to make it more clear.

[expr.context]/2 only specifies some special handlings of discard-value expressions. The answer to "exactly which expressions are discard-value expressions" is not in [expr.context]/2, but in [expr.static.cast]/6, [expr.comma]/1, and [stmt.expr]/1. Thus, it makes sense to at least add cross references to these places in [expr.context]/2.

________________________________________
From: Alexander <alexandreter...@gmail.com>
Sent: Thursday, January 11, 2018 1:09:51 PM
To: ISO C++ Standard - Discussion
Subject: [std-discussion] I think this is a defect in [expr.context]/2

[expr.context]/2<http://eel.is/c++draft/expr#context-2>:

2
In some contexts, an expression only appears for its side effects.<http://eel.is/c++draft/expr#context-2.sentence-1>

Such an expression is called a discarded-value expression<http://eel.is/c++draft/expr#def:discarded-value_expression>.<http://eel.is/c++draft/expr#context-2.sentence-2>

The array-to-pointer<http://eel.is/c++draft/conv.array> and function-to-pointer<http://eel.is/c++draft/conv.func> standard conversions are not applied.<http://eel.is/c++draft/expr#context-2.sentence-3>

The lvalue-to-rvalue<http://eel.is/c++draft/conv.lval> conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:


*
(2.1)<http://eel.is/c++draft/expr#context-2.1>
(2.1) ( expression<http://eel.is/c++draft/expr.comma#nt:expression> ), where expression<http://eel.is/c++draft/expr.comma#nt:expression> is one of these expressions,
*
(2.2)<http://eel.is/c++draft/expr#context-2.2>
(2.2) id-expression<http://eel.is/c++draft/expr.prim.id#nt:id-expression>,
*
(2.3)<http://eel.is/c++draft/expr#context-2.3>
(2.3) subscripting<http://eel.is/c++draft/expr.sub>,
*
(2.4)<http://eel.is/c++draft/expr#context-2.4>
(2.4) class member access<http://eel.is/c++draft/expr.ref>,
*
(2.5)<http://eel.is/c++draft/expr#context-2.5>
(2.5) indirection<http://eel.is/c++draft/expr.unary.op#def:indirection>,
*
(2.6)<http://eel.is/c++draft/expr#context-2.6>
(2.6) pointer-to-member operation<http://eel.is/c++draft/expr.mptr.oper>,
*
(2.7)<http://eel.is/c++draft/expr#context-2.7>
(2.7) conditional expression<http://eel.is/c++draft/expr.cond> where both the second and the third operands are one of these expressions, or
*
(2.8)<http://eel.is/c++draft/expr#context-2.8>
(2.8) comma expression<http://eel.is/c++draft/expr.comma> where the right operand is one of these expressions.<http://eel.is/c++draft/expr#context-2.8.sentence-1>
*

[ Note<http://eel.is/c++draft/expr#context-2.note-1>
:
Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.<http://eel.is/c++draft/expr#context-2.sentence-8>
— end note
 ]

If the (possibly converted) expression is a prvalue, the temporary materialization conversion<http://eel.is/c++draft/conv.rval> is applied.<http://eel.is/c++draft/expr#context-2.sentence-6>

[ Note<http://eel.is/c++draft/expr#context-2.note-2>
:
If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the lvalue-to-rvalue conversion.<http://eel.is/c++draft/expr#context-2.sentence-9>
— end note
 ]

The glvalue expression is evaluated and its value is discarded.<http://eel.is/c++draft/expr#context-2.sentence-7>

The paragraph defines a discarded-value expression, only considering expressions of the type ( expression ), where any expression of the type expression; satisfying (2.2) to (2.8) is also a discarded-value expression. To correct this, it's enough to change (2.1) to this:

(2.1) expression; or (expression), where expression is one of these expressions,

--

---
You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Discussion" group.
To unsubscribe from this group and stop receiving emails from it, send an email to std-discussio...@isocpp.org<mailto:std-discussio...@isocpp.org>.
To post to this group, send email to std-dis...@isocpp.org<mailto:std-dis...@isocpp.org>.
Visit this group at https://groups.google.com/a/isocpp.org/group/std-discussion/.

[Alexander]

On Thursday, January 11, 2018 at 11:57:48 AM UTC-2, Tam S. B. wrote:

I don't think this is a defect, but an editorial change could be applied to [expr.context]/2 to make it more clear.

[expr.context]/2 only specifies some special handlings of discard-value expressions. The answer to "exactly which expressions are discard-value expressions" is not in [expr.context]/2, but in [expr.static.cast]/6, [expr.comma]/1, and [stmt.expr]/1. Thus, it makes sense to at least add cross references to these places in [expr.context]/2.

Sorry for the late reply, but I was busy with something else not related to C++. I have to agree with you, after reading the paragraphs you mentioned above, that there is no defect in [expr.context]/2. Thanks for your help.