typedef decltype(true ? (long&&)0 : (long&&)0) T;

应该是什么？

根据gcc(4.7),它很长.根据clang(主干),它是长&&amp ;.这种差异导致clang无法编译使用gcc 4.7的libstdc的代码.谁是对的？

更新：正如ildjarn指出的那样,Clang是对的,正如理查德史密斯指出的那样,错误libstdc是由于标准中的错误引起的.这是relevant GCC bug和relevant Defect Report.

解决方法:

Clang是对的. N3337§7.1.6.2/ 4：

The type denoted by decltype(e) is defined as follows:

• if e is an unparenthesized id-expression or an unparenthesized class member access, decltype(e) is the type of the entity named by e. If there is no such entity, or if e names a set of overloaded functions, the program is ill-formed;
• otherwise, if e is an xvalue, decltype(e) is T&&, where T is the type of e;
• otherwise, if e is an lvalue, decltype(e) is T&, where T is the type of e;
• otherwise, decltype(e) is the type of e.

The operand of the decltype specifier is an unevaluated operand.

§5/ 6：

[ Note: An expression is an xvalue if it is:

• the result of calling a function, whether implicitly or explicitly, whose return type is an rvalue reference to object type,
• a cast to an rvalue reference to object type,
• a class member access expression designating a non-static data member of non-reference type in which the object expression is an xvalue, or
• a .* pointer-to-member expression in which the first operand is an xvalue and the second operand is a pointer to data member.

In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not. —end note ]

我之前很谨慎,文字0可能会以某种方式阻止它在此上下文中作为对象类型进行限定,但§3.9/ 8澄清了一些事情：

An object type is a (possibly cv-qualified) type that is not a function type, not a reference type, and not a void type.

条件运算符不会影响这里的任何内容 – §5.16/ 4：

If the second and third operands are glvalues of the same value category and have the same type, the result is of that type and value category and it is a bit-field if the second or the third operand is a bit-field, or if both are bit-fields.

在这种情况下,两者具有相同的值类别(xvalue),并且xvalues是glvalues.