"is not a structural type" should be a substitution failure

[ecatmur]

On CE trunk 20231108 18.0.0 (https://github.com/llvm/llvm-project.git fb07d9c):

template<auto> struct test {};
template<class T> concept structural = requires { []<T x>(test<x>) {}; };
class C { int i; };
static_assert(not structural<C>);

(reduced from https://stackoverflow.com/a/68856187/567292)

fails with hard error:

<source>:2:56: error: type 'C' of non-type template parameter is not a structural type
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                        ^
<source>:2:51: note: while substituting into a lambda expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^
<source>:2:51: note: in instantiation of requirement here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^~~~~~~~~~~~~~~~~~~
<source>:2:40: note: while substituting template arguments into constraint expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:4:19: note: while checking the satisfaction of concept 'structural<C>' requested here
    4 | static_assert(not structural<C>);
      |                   ^~~~~~~~~~~~~
<source>:3:15: note: 'C' is not a structural type because it has a non-static data member that is not public
    3 | class C { int i; };
      |               ^
1 error generated.
Compiler returned: 1

Previously (13.0.0 through 17.0.1) this was a substitution failure, which I think it should be.

[ecatmur]

Workaround in this specific case is to change the test to:

template<auto> struct test {};
template<class T, template<T> class> struct test2;
template<class T> concept structural = requires { typename test2<T, test>; };

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Ed Catmur (ecatmur)

On CE trunk 20231108 18.0.0 (https://github.com/llvm/llvm-project.git fb07d9c): ```c++ template<auto> struct test {}; template<class T> concept structural = requires { []<T x>(test<x>) {}; }; class C { int i; }; static_assert(not structural<C>); ``` (reduced from https://stackoverflow.com/a/68856187/567292)

fails with hard error:

<source>:2:56: error: type 'C' of non-type template parameter is not a structural type
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                        ^
<source>:2:51: note: while substituting into a lambda expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^
<source>:2:51: note: in instantiation of requirement here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^~~~~~~~~~~~~~~~~~~
<source>:2:40: note: while substituting template arguments into constraint expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:4:19: note: while checking the satisfaction of concept 'structural<C>' requested here
    4 | static_assert(not structural<C>);
      |                   ^~~~~~~~~~~~~
<source>:3:15: note: 'C' is not a structural type because it has a non-static data member that is not public
    3 | class C { int i; };
      |               ^
1 error generated.
Compiler returned: 1

Previously (13.0.0 through 17.0.1) this was a substitution failure, which I think it should be.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Ed Catmur (ecatmur)

On CE trunk 20231108 18.0.0 (https://github.com/llvm/llvm-project.git fb07d9c): ```c++ template<auto> struct test {}; template<class T> concept structural = requires { []<T x>(test<x>) {}; }; class C { int i; }; static_assert(not structural<C>); ``` (reduced from https://stackoverflow.com/a/68856187/567292)

fails with hard error:

<source>:2:56: error: type 'C' of non-type template parameter is not a structural type
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                        ^
<source>:2:51: note: while substituting into a lambda expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^
<source>:2:51: note: in instantiation of requirement here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                                   ^~~~~~~~~~~~~~~~~~~
<source>:2:40: note: while substituting template arguments into constraint expression here
    2 | template<class T> concept structural = requires { []<T x>(test<x>) {}; };
      |                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<source>:4:19: note: while checking the satisfaction of concept 'structural<C>' requested here
    4 | static_assert(not structural<C>);
      |                   ^~~~~~~~~~~~~
<source>:3:15: note: 'C' is not a structural type because it has a non-static data member that is not public
    3 | class C { int i; };
      |               ^
1 error generated.
Compiler returned: 1

Previously (13.0.0 through 17.0.1) this was a substitution failure, which I think it should be.

[zyn0217]

I think this is #64138.

[zyn0217]

Sorry, I closed the issue too quickly: As CWG2672 says, it is that

Substituting into the body of a lambda-expression is never in the immediate context of substitution into the lambda-expression

which, seems not to cover the substitution for the parameter part of a lambda expression.

@zygoloid Do you think we should regard the non-body parts for a lambda as SFINAE context?

(@cor3ntin also pointed that in https://reviews.llvm.org/D156993#4557292)

Another question is, shall we update the examples for [temp.deduct]p9 as well? e.g.,

template <class T>
  auto h(T) -> decltype([x = T::invalid]() { });
void h(...);
h(0);  // error: invalid expression not part of the immediate context

This should compile now IIUC. (I didn't see the related changes in the DR, but it could be I have overlooked something.)

[zyn0217]

Friendly ping @cor3ntin and @zygoloid. Also cc @AaronBallman for help.

[zygoloid]

Here's the resolution of CWG2672, which is in the middle of being merged into the standard right now: cplusplus/draft@389a869

Seems pretty clear that it's only the body of the lambda that's outside the immediate context, so SFINAE should apply here.

[zygoloid]

Another question is, shall we update the examples for [temp.deduct]p9 as well?

Filed an editorial issue to fix the example and hopefully move the wording somewhere more central: cplusplus/draft#6718

[zyn0217]

Thank you Richard for clarification (and editorial fixes). Filed #75001 for it.