可以将嵌套两次的sizeof作为依赖表达式吗？

Question

I noticed that gcc 5.0 rejects the following code, while clang 3.6 accepts it.

template<int n>
struct I
{
    typedef int Type;
};

template<typename T>
struct A
{
    typedef I<sizeof(sizeof(T))>::Type Type;
};

The two compilers seem to differ on whether sizeof(sizeof(T)) is a type-dependent or value-dependent expression. If the expression were dependent, then it follows that I<sizeof(sizeof(T))> is a dependent type, meaning that typename should be required.

This is covered by the following wording in the C++11 standard:

[temp.dep.type]/8

A type is dependent if it is

• a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent

[temp.dep.expr]/4

Expressions of the following forms are never type-dependent (because the type of the expression cannot be dependent):

 sizeof unary-expression sizeof ( type-id ) 

[temp.dep.constexpr]/2

Expressions of the following form are value-dependent if the unary-expression or expression is typedependent or the type-id is dependent:

 sizeof unary-expression sizeof ( type-id ) 

My interpretation is that sizeof(T) can never be type-dependent, meaning sizeof(sizeof(T)) can never be type-dependent or value-dependent.

Is this a bug in gcc?

Answer 1

I'm using a post-N4296 draft.

typedef I<sizeof(sizeof(T))>::Type Type;

typename is required if the nested-name-specifier I<..> depends on a template parameter [temp.res]/5. So, is I<..> dependent?

[temp.dep.type]/9 A type is dependent if it is

• [...]
• (9.7) a simple-template-id in which either the template name is a template parameter or any of the template arguments is a dependent type or an expression that is type-dependent or value-dependent , or [...]

I<..> is a simple-template-id , the template argument is an expression. Is this expression sizeof(sizeof(T)) type-dependent or value-dependent?

The expression sizeof(sizeof(T)) can be broken down into the following expressions:

expression           form
===============================================
              T      type-id
       sizeof(T)     sizeof ( type-id )
      (sizeof(T))    ( expression )
sizeof(sizeof(T))    sizeof unary-expression

T is not an expression, but I'll leave it in the list for later. A note on the parentheses: A primary-expression can be a parenthesized (general) expression . A unary-expression can be a postfix-expression which can be a primary-expression , hence it can be parenthesized, too.

A parenthesized expression (X) is dependent if X is dependent:

[temp.dep.expr]/1 Except as described below, an expression is type-dependent if any subexpression is type-dependent.

[temp.dep.constexpr]/1 Except as described below, a constant expression is value-dependent if any subexpression is value-dependent.

In general, sizeof expressions are never type -dependent, because they always produce a value of type std::size_t :

[temp.dep.expr]/4 Expressions of the following forms are never type-dependent (because the type of the expression cannot be dependent):

 [...] sizeof unary-expression sizeof ( type-id ) 

However, the value they yield can be dependent on a template parameter:

[temp.dep.constexpr]/2 Expressions of the following form are value-dependent if the unary-expression or expression is type-dependent or the type-id is dependent :

 sizeof unary-expression sizeof ( type-id ) 

expression           form                       value-dep?   type-dep?
=======================================================================
              T      type-id                    no           yes
       sizeof(T)     sizeof ( type-id )         yes          no
      (sizeof(T))    ( expression )             yes          no
sizeof(sizeof(T))    sizeof unary-expression    no           no

Since T is type -dependent, sizeof(T) becomes value -dependent. However, since (sizeof(T)) is not type -dependent, sizeof(sizeof(T)) is not dependent at all.