[C++23][Modules] imported function with auto return type causes lambda does not satisfy 'move_constructible' #130080

thbwd · 2025-03-06T11:50:12Z

A.ixx:

module;

#include <vector>
#include <ranges>

export module A;

export auto f(const std::vector<int> &vec) {
    return vec | std::views::transform([](int x) {
        return x + 1;
    });
}

/*void clang_bug_fix() {
    for (auto x : f(std::vector<int>()));
}*/

main.cpp:

#include <vector>

import A;

int main() {
    for (int ptr : f(std::vector<int>(5))) {
    }
}

Produces:

/opt/homebrew/Cellar/llvm/19.1.7_1/bin/clang++   -g -std=gnu++23 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -fcolor-diagnostics -MD -MT CMakeFiles/test.dir/main.cpp.o -MF CMakeFiles/test.dir/main.cpp.o.d @CMakeFiles/test.dir/main.cpp.o.modmap -o CMakeFiles/test.dir/main.cpp.o -c path/main.cpp
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:184:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view<const std::vector<int>>, _Fn = (lambda at path/A.ixx:9:40)]
  184 |   friend class transform_view<_View, _Fn>::__iterator;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:97:53: note: in instantiation of template class 'std::ranges::transform_view<std::ranges::ref_view<const std::vector<int>>, (lambda at path/A.ixx:9:40)>::__iterator<false>' requested here
   97 |   _LIBCPP_HIDE_FROM_ABI constexpr __iterator<false> begin() { return __iterator<false>{*this, ranges::begin(__base_)}; }
      |                                                     ^
path/main.cpp:6:18: note: in instantiation of member function 'std::ranges::transform_view<std::ranges::ref_view<const std::vector<int>>, (lambda at path/A.ixx:9:40)>::begin' requested here
    6 |     for (int ptr : f(std::vector<int>(5))) {
      |                  ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template <input_range _View, move_constructible _Fn>
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to<(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)>' evaluated to false
   39 | concept move_constructible = constructible_from<_Tp, _Tp> && convertible_to<_Tp, _Tp>;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast<_To>(std::declval<_From>())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v<_From, _To> && requires { static_cast<_To>(std::declval<_From>()); };
      |                                                                     ^
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:187:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view<const std::vector<int>>, _Fn = (lambda at path/A.ixx:9:40)]
  187 |   friend class transform_view<_View, _Fn>::__sentinel;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template <input_range _View, move_constructible _Fn>
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to<(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)>' evaluated to false
   39 | concept move_constructible = constructible_from<_Tp, _Tp> && convertible_to<_Tp, _Tp>;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast<_To>(std::declval<_From>())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v<_From, _To> && requires { static_cast<_To>(std::declval<_From>()); };
      |                                                                     ^
2 errors generated.

Even though the lambda is move constructible. The bug disappears when uncommenting the function clang_bug_fix.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)
project(test)

set(CMAKE_CXX_STANDARD 23)

add_executable(test main.cpp)

target_sources(test
        PUBLIC
        FILE_SET all_my_modules TYPE CXX_MODULES FILES
        A.ixx
)

The text was updated successfully, but these errors were encountered:

llvmbot · 2025-03-06T12:01:18Z

@llvm/issue-subscribers-clang-modules

Author: None (thbwd)

A.ixx: ```c++ module;

#include <vector>
#include <ranges>

export module A;

export auto f(const std::vector<int> &vec) {
return vec | std::views::transform([](int x) {
return x + 1;
});
}

/void clang_bug_fix() {
for (auto x : f(std::vector<int>()));
}/

main.cpp:
```c++
#include &lt;vector&gt;

import A;

int main() {
    for (int ptr : f(std::vector&lt;int&gt;(5))) {
    }
}

Produces:

/opt/homebrew/Cellar/llvm/19.1.7_1/bin/clang++   -g -std=gnu++23 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -fcolor-diagnostics -MD -MT CMakeFiles/test.dir/main.cpp.o -MF CMakeFiles/test.dir/main.cpp.o.d @<!-- -->CMakeFiles/test.dir/main.cpp.o.modmap -o CMakeFiles/test.dir/main.cpp.o -c path/main.cpp
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:184:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, _Fn = (lambda at path/A.ixx:9:40)]
  184 |   friend class transform_view&lt;_View, _Fn&gt;::__iterator;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:97:53: note: in instantiation of template class 'std::ranges::transform_view&lt;std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, (lambda at path/A.ixx:9:40)&gt;::__iterator&lt;false&gt;' requested here
   97 |   _LIBCPP_HIDE_FROM_ABI constexpr __iterator&lt;false&gt; begin() { return __iterator&lt;false&gt;{*this, ranges::begin(__base_)}; }
      |                                                     ^
path/main.cpp:6:18: note: in instantiation of member function 'std::ranges::transform_view&lt;std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, (lambda at path/A.ixx:9:40)&gt;::begin' requested here
    6 |     for (int ptr : f(std::vector&lt;int&gt;(5))) {
      |                  ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template &lt;input_range _View, move_constructible _Fn&gt;
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to&lt;(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)&gt;' evaluated to false
   39 | concept move_constructible = constructible_from&lt;_Tp, _Tp&gt; &amp;&amp; convertible_to&lt;_Tp, _Tp&gt;;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast&lt;_To&gt;(std::declval&lt;_From&gt;())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v&lt;_From, _To&gt; &amp;&amp; requires { static_cast&lt;_To&gt;(std::declval&lt;_From&gt;()); };
      |                                                                     ^
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:187:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, _Fn = (lambda at path/A.ixx:9:40)]
  187 |   friend class transform_view&lt;_View, _Fn&gt;::__sentinel;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template &lt;input_range _View, move_constructible _Fn&gt;
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to&lt;(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)&gt;' evaluated to false
   39 | concept move_constructible = constructible_from&lt;_Tp, _Tp&gt; &amp;&amp; convertible_to&lt;_Tp, _Tp&gt;;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast&lt;_To&gt;(std::declval&lt;_From&gt;())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v&lt;_From, _To&gt; &amp;&amp; requires { static_cast&lt;_To&gt;(std::declval&lt;_From&gt;()); };
      |                                                                     ^
2 errors generated.

Even though the lambda is move constructible. The bug disappears when uncommenting the function clang_bug_fix.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)
project(test)

set(CMAKE_CXX_STANDARD 23)

add_executable(test main.cpp)

target_sources(test
        PUBLIC
        FILE_SET all_my_modules TYPE CXX_MODULES FILES
        A.ixx
)

llvmbot · 2025-03-06T12:01:40Z

@llvm/issue-subscribers-clang-frontend

Author: None (thbwd)

A.ixx: ```c++ module;

#include <vector>
#include <ranges>

export module A;

export auto f(const std::vector<int> &vec) {
return vec | std::views::transform([](int x) {
return x + 1;
});
}

/void clang_bug_fix() {
for (auto x : f(std::vector<int>()));
}/

main.cpp:
```c++
#include &lt;vector&gt;

import A;

int main() {
    for (int ptr : f(std::vector&lt;int&gt;(5))) {
    }
}

Produces:

/opt/homebrew/Cellar/llvm/19.1.7_1/bin/clang++   -g -std=gnu++23 -arch arm64 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX15.2.sdk -fcolor-diagnostics -MD -MT CMakeFiles/test.dir/main.cpp.o -MF CMakeFiles/test.dir/main.cpp.o.d @<!-- -->CMakeFiles/test.dir/main.cpp.o.modmap -o CMakeFiles/test.dir/main.cpp.o -c path/main.cpp
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:184:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, _Fn = (lambda at path/A.ixx:9:40)]
  184 |   friend class transform_view&lt;_View, _Fn&gt;::__iterator;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:97:53: note: in instantiation of template class 'std::ranges::transform_view&lt;std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, (lambda at path/A.ixx:9:40)&gt;::__iterator&lt;false&gt;' requested here
   97 |   _LIBCPP_HIDE_FROM_ABI constexpr __iterator&lt;false&gt; begin() { return __iterator&lt;false&gt;{*this, ranges::begin(__base_)}; }
      |                                                     ^
path/main.cpp:6:18: note: in instantiation of member function 'std::ranges::transform_view&lt;std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, (lambda at path/A.ixx:9:40)&gt;::begin' requested here
    6 |     for (int ptr : f(std::vector&lt;int&gt;(5))) {
      |                  ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template &lt;input_range _View, move_constructible _Fn&gt;
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to&lt;(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)&gt;' evaluated to false
   39 | concept move_constructible = constructible_from&lt;_Tp, _Tp&gt; &amp;&amp; convertible_to&lt;_Tp, _Tp&gt;;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast&lt;_To&gt;(std::declval&lt;_From&gt;())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v&lt;_From, _To&gt; &amp;&amp; requires { static_cast&lt;_To&gt;(std::declval&lt;_From&gt;()); };
      |                                                                     ^
In module 'A' imported from path/main.cpp:3:
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:187:16: error: constraints not satisfied for class template 'transform_view' [with _View = std::ranges::ref_view&lt;const std::vector&lt;int&gt;&gt;, _Fn = (lambda at path/A.ixx:9:40)]
  187 |   friend class transform_view&lt;_View, _Fn&gt;::__sentinel;
      |                ^~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__ranges/transform_view.h:68:30: note: because '(lambda at path/A.ixx:9:40)' does not satisfy 'move_constructible'
   68 | template &lt;input_range _View, move_constructible _Fn&gt;
      |                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/constructible.h:39:62: note: because 'convertible_to&lt;(lambda at path/A.ixx:9:40), (lambda at path/A.ixx:9:40)&gt;' evaluated to false
   39 | concept move_constructible = constructible_from&lt;_Tp, _Tp&gt; &amp;&amp; convertible_to&lt;_Tp, _Tp&gt;;
      |                                                              ^
/opt/homebrew/Cellar/llvm/19.1.7_1/bin/../include/c++/v1/__concepts/convertible_to.h:27:69: note: because 'static_cast&lt;_To&gt;(std::declval&lt;_From&gt;())' would be invalid: no matching conversion for static_cast from '(lambda at path/A.ixx:9:40)' to '(lambda at path/A.ixx:9:40)'
   27 | concept convertible_to = is_convertible_v&lt;_From, _To&gt; &amp;&amp; requires { static_cast&lt;_To&gt;(std::declval&lt;_From&gt;()); };
      |                                                                     ^
2 errors generated.

Even though the lambda is move constructible. The bug disappears when uncommenting the function clang_bug_fix.

CMakeLists.txt:

cmake_minimum_required(VERSION 3.30)
project(test)

set(CMAKE_CXX_STANDARD 23)

add_executable(test main.cpp)

target_sources(test
        PUBLIC
        FILE_SET all_my_modules TYPE CXX_MODULES FILES
        A.ixx
)

frederick-vs-ja · 2025-03-06T12:02:24Z

Is this reproducible when replacing the lambda with an equivalent hand-write function object?

thbwd · 2025-03-06T12:05:06Z

I didn't specifically test a separate function but just converting the lambda to a function pointer with the unary + makes the bug go away:

export auto f(const std::vector<int> &vec) {
    return vec | std::views::transform(+[](int x) {
        return x + 1;
    });
}

thbwd · 2025-03-06T12:07:38Z

Ah, I think I misunderstood. I haven't tried writing an equivalent "functor" so far.

ChuanqiXu9 · 2025-03-06T12:13:57Z

I am not sure if this relates to #116087

Can you verify by adding an inline to f?

And also -fmodules-reduced-bmi may be helpful.

llvmbot added the new issue label Mar 6, 2025

frederick-vs-ja added clang:frontend Language frontend issues, e.g. anything involving "Sema" clang:modules C++20 modules and Clang Header Modules and removed new issue clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Mar 6, 2025

frederick-vs-ja added the lambda C++11 lambda expressions label Mar 6, 2025

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[C++23][Modules] imported function with auto return type causes lambda does not satisfy 'move_constructible' #130080

[C++23][Modules] imported function with auto return type causes lambda does not satisfy 'move_constructible' #130080

thbwd commented Mar 6, 2025 •

edited

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llvmbot commented Mar 6, 2025

llvmbot commented Mar 6, 2025

frederick-vs-ja commented Mar 6, 2025

thbwd commented Mar 6, 2025

thbwd commented Mar 6, 2025 •

edited

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ChuanqiXu9 commented Mar 6, 2025

[C++23][Modules] imported function with auto return type causes lambda does not satisfy 'move_constructible' #130080

[C++23][Modules] imported function with auto return type causes lambda does not satisfy 'move_constructible' #130080

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thbwd commented Mar 6, 2025 • edited Loading

llvmbot commented Mar 6, 2025

llvmbot commented Mar 6, 2025

frederick-vs-ja commented Mar 6, 2025

thbwd commented Mar 6, 2025

thbwd commented Mar 6, 2025 • edited Loading

ChuanqiXu9 commented Mar 6, 2025

thbwd commented Mar 6, 2025 •

edited

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thbwd commented Mar 6, 2025 •

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