C++ 类别名不编译为相同类型

Question

我不明白为什么下面的代码不能编译。我在使用 GCC 和 Clang 时遇到了同样的错误。有人可以解释或指出标准的一部分来解释为什么 p1 和 p2 不是同一类型吗？

struct TypeT {};

struct TypeU {};

template<typename T, typename U = TypeU>
struct Foo {};


template<typename T, typename U>
struct Bar
{
};

template<typename T, template <typename> class U>
struct FooBar
{
};

template<typename T>
using FooAlias1 = Foo<T>;

template<typename T>
using FooAlias2 = Foo<T>;

template<typename T>
void DoStuff(const T& p1, const T& p2)
{
}

int main(void)
{
    FooBar<TypeT, FooAlias1> p1;
    FooBar<TypeT, FooAlias2> p2;
    DoStuff(p1, p2);
}

这是 gcc 的输出:

$ gcc --version
gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4

$ gcc -std=c++11 test.cpp
test.cpp: In function ‘int main()’:
test.cpp:34:19: error: no matching function for call to ‘DoStuff(FooBar<TypeT, FooAlias1>&, FooBar<TypeT, FooAlias2>&)’
     DoStuff(p1, p2);
                   ^
test.cpp:34:19: note: candidate is:
test.cpp:26:6: note: template<class T> void DoStuff(const T&, const T&)
 void DoStuff(const T& p1, const T& p2)
      ^
test.cpp:26:6: note:   template argument deduction/substitution failed:
test.cpp:34:19: note:   deduced conflicting types for parameter ‘const T’ (‘FooBar<TypeT, FooAlias1>’ and ‘FooBar<TypeT, FooAlias2>’)
     DoStuff(p1, p2);

和 clang :

$ clang --version
Ubuntu clang version 3.4-1ubuntu3 (tags/RELEASE_34/final) (based on LLVM 3.4)

$ clang -std=c++11 test.cpp
test.cpp:34:5: error: no matching function for call to 'DoStuff'
    DoStuff(p1, p2);
    ^~~~~~~
test.cpp:26:6: note: candidate template ignored: deduced conflicting types for parameter 'T' ('FooBar<[...], template FooAlias1>'
      vs. 'FooBar<[...], template FooAlias2>')
void DoStuff(const T& p1, const T& p2)
     ^
1 error generated.

Answer 1

根据规范，§14.4，如果...，两种类型是等价的

— their corresponding template template-arguments refer to the same template.

但是您有两个不同的别名模板(§14.5.7)。它们不是类型别名。

1. A template-declaration in which the declaration is an alias-declaration (Clause 7) declares the identifier to be a alias template. An alias template is a name for a family of types. The name of the alias template is a template-name.