c++ - template-name 是推导的上下文吗？

Question

[temp.deduct.type] paragraph 8列出所有推断的上下文，但它似乎不包括 template-name <TT>其中 template-name 指的是类模板和 TT引用模板模板参数。这是推导上下文吗？

如果是，为什么？

如果不是，请考虑以下代码:

template<template<typename> class U, template<typename> class V>
struct foo {};

template<template<typename> class U>
struct foo<U, U> {}; 

int main() {}

此代码编译 under Clang 7.0.0和 GCC 8.0.1 ，这意味着编译器认为部分特化比主模板更特化，这意味着 U和 V在主模板中成功推导出foo<U, U> .这是编译器错误吗？

Answer 1

这一段充满了问题，包括你指出的那个。 Core issue 2328有一个不错的 list :

The presentation style of 17.9.2.5 [temp.deduct.type] paragraph 8 results in a specification that is unclear, needlessly verbose, and incomplete. Specific problems include:

• What does it mean for P and A to have one of a set of forms? Do they both have to have that form? (That doesn't happen; typically, only P contains template parameters)

• In the introductory sentence, aren't T, TT, and i supposed to be the names of template parameters rather than template arguments?

• In T[i], it appears we can deduce i, but not T (T can only be deduced in the form T[<i>integer-constant</i>])

• What is an integer-constant supposed to be?

• What is a cv-list?

• Why can we not deduce const T from T? (Apparently you only get to deduce if both or neither type have a cv-list, whatever a cv-list is.)

• We have extreme redundancy because, for instance, there is no way to say “in T (T::*)(T), you can deduce any of those Ts, and it's OK if some of the positions don't have a T”. So we have seven (!) forms of that construct, for all cases except the one where none of the three positions contain a T.

• We have special case rules for pointers to member functions, even though they're not a special case and should be covered by the rule for pointers to members and the rule for functions.

• We do not allow deducing a template template parameter's value from a template template argument — there is a TT<T> form, a TT<i> form, a <i>template-name</i><T> form, and a <i>template-name</i><i> form, but no TT<TT> form nor <i>template-name</i><TT> form.

看起来编辑设法摆脱了 cv-list ，至少，自提交问题以来。现在只是 cv 。 ( cv-list 有点可笑的错误，因为 [syntax] 说 -list 后缀用于逗号分隔的列表...)