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编译器bug ?

news 2025/7/3 15:00:56

## 问题描述

两个结构几乎相同的模板实现，一个能正常工作，另一个在 VS2019 和 GCC 中都会报错。

## 最小化测试代码

// bug_report.cpp
#include <type_traits>
#include <string>

template<typename T> struct Type2Type { using type = T; };
template<typename T1, typename T2> struct IsSameType : std::false_type {};
template<typename T> struct IsSameType<T, T> : std::true_type {};

// 工作正常的版本
template<size_t idx, typename TList> struct FindNthTypeInTypeList;
template<typename T, template <typename ...> class TList, typename ... Types>
struct FindNthTypeInTypeList<0,TList<T, Types...>> : public Type2Type<T> {};
template<size_t idx, template <typename ...> class TList, typename T, typename ... Types>
struct FindNthTypeInTypeList<idx, TList<T, Types ...>> : public FindNthTypeInTypeList<idx-1, TList<Types ...>> {};
template<size_t idx, typename TList> using FindNthTypeInTypeList_t = typename FindNthTypeInTypeList<idx, TList>::type;

// 不能工作的版本
template<size_t idx, typename TList> struct GetAllTypesFromIdx;
template<template <typename ...> class TList, typename ... Types>
struct GetAllTypesFromIdx<0, TList<Types...>> : public Type2Type<TList<Types...>> {};
template<size_t idx, typename T, template <typename ...> class TList, typename ... Types>
struct GetAllTypesFromIdx<idx, TList<T, Types...>> : public GetAllTypesFromIdx<idx-1, TList<Types...>> {};
template<size_t idx, typename TList> using GetAllTypesFromIdx_t = typename GetAllTypesFromIdx<idx, TList>::type;

// 测试类型列表
template<typename...> struct TypeList {};
using tc_typeList = TypeList<int, char, std::string, char*>;

int main()
{
    // FindNthTypeInTypeList 测试 - 这些都能工作
    static_assert(IsSameType<FindNthTypeInTypeList_t<0, tc_typeList>, int>::value);
    static_assert(IsSameType<FindNthTypeInTypeList_t<1, tc_typeList>, char>::value);
    static_assert(IsSameType<FindNthTypeInTypeList_t<2, tc_typeList>, std::string>::value);
    static_assert(IsSameType<FindNthTypeInTypeList_t<3, tc_typeList>, char*>::value);
    static_assert(!IsSameType<FindNthTypeInTypeList_t<0, tc_typeList>, char>::value);

    // GetAllTypesFromIdx 测试 - 这些会报错
    static_assert(IsSameType<GetAllTypesFromIdx_t<0, tc_typeList>, TypeList<int, char, std::string, char*>>::value);
    // static_assert(IsSameType<GetAllTypesFromIdx_t<1, tc_typeList>, TypeList<char, std::string, char*>>::value);
    // static_assert(IsSameType<GetAllTypesFromIdx_t<2, tc_typeList>, TypeList<std::string, char*>>::value);
    // static_assert(IsSameType<GetAllTypesFromIdx_t<3, tc_typeList>, TypeList<char*>>::value);
}

## 错误信息

在 VS2019 和 GCC 中都会报错：

1>C:\...\MiniMPL\sources\UnitTest\UT_MiniMPL\src\tc_typeList_cpp11.cpp(135,9): error C2752: 'MiniMPL::GetAllTypesFromIdx<0,UnitTest::tc_typeList>': more than one partial specialization matches the template argument list
1>C:\...\MiniMPL\sources\MiniMPL\include\MiniMPL\typeList_cpp11.hpp(56,105): message : could be 'MiniMPL::GetAllTypesFromIdx<0,TList<Types...>>'
1>C:\...\MiniMPL\sources\MiniMPL\include\MiniMPL\typeList_cpp11.hpp(58,105): message : or       'MiniMPL::GetAllTypesFromIdx<idx,TList<T,Types...>>'
1>C:\...\MiniMPL\sources\UnitTest\UT_MiniMPL\src\tc_typeList_cpp11.cpp(135): message : see reference to alias template instantiation 'MiniMPL::GetAllTypesFromIdx_t<0,UnitTest::tc_typeList>' being compiled
1>C:\...\MiniMPL\sources\MiniMPL\include\MiniMPL\typeList_cpp11.hpp(59,176): error C2794: 'type': is not a member of any direct or indirect base class of 'MiniMPL::GetAllTypesFromIdx<0,UnitTest::tc_typeList>'
1>C:\...\MiniMPL\sources\UnitTest\UT_MiniMPL\src\tc_typeList_cpp11.cpp(135,9): error C2938: 'MiniMPL::GetAllTypesFromIdx_t' : Failed to specialize alias template

## 问题分析

1. 两个模板实现 `FindNthTypeInTypeList` 和 `GetAllTypesFromIdx` 的结构几乎完全相同

2. 它们都有：

- 一个 `idx=0` 的特化

- 一个通用的递归特化

- 相同的类型列表处理方式

3. `FindNthTypeInTypeList` 能正常工作，所有测试都通过

4. `GetAllTypesFromIdx` 在 VS2019 和 GCC 中都会报错

## 预期行为

- 两个模板都应该能正常工作

- `FindNthTypeInTypeList` 应该返回指定索引的类型

- `GetAllTypesFromIdx` 应该返回从指定索引开始的所有类型