[clang][concepts] Clang doesn't properly diagnose concept return types with insufficient template arg count

[erichkeane]

Given:

template<typename T> concept C = true;

We properly diagnose:

void foo(C<int> auto);

as:
<source>:7:10: error: too many template arguments for concept 'C'

This happens as a part of the call to InventTemplateParameter.

However, we don't do the same for return types or trailing return types:

C<int> auto Foo();
auto Foo2() -> C<int> auto;

Should both diagnose, in Block, File, or Record scope.

However, calls to the InventTemplateParameter all need the InventedParameterInfos to have something in it, which it obviously doesn't, since there are no parameters to be filled in. Likely we need to do something similar with return types, though it isn't clear where that could come from.

@zygoloid did something similar in https://github.com/llvm/llvm-project/commit/9cf98d26e7b1204478cc13ae3df44a6843965c11 for trailing return types, but only applied it to inside a lambda or prototype context, and not File/Block/Record context, so it is incomplete.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Erich Keane (erichkeane)

Given:

template<typename T> concept C = true;

We properly diagnose:

void foo(C<int> auto);

as:
<source>:7:10: error: too many template arguments for concept 'C'

This happens as a part of the call to InventTemplateParameter.

However, we don't do the same for return types or trailing return types:

C<int> auto Foo();
auto Foo2() -> C<int> auto;

Should both diagnose, in Block, File, or Record scope.

However, calls to the InventTemplateParameter all need the InventedParameterInfos to have something in it, which it obviously doesn't, since there are no parameters to be filled in. Likely we need to do something similar with return types, though it isn't clear where that could come from.

@zygoloid did something similar in https://github.com/llvm/llvm-project/commit/9cf98d26e7b1204478cc13ae3df44a6843965c11 for trailing return types, but only applied it to inside a lambda or prototype context, and not File/Block/Record context, so it is incomplete.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Erich Keane (erichkeane)

Given:

template<typename T> concept C = true;

We properly diagnose:

void foo(C<int> auto);

as:
<source>:7:10: error: too many template arguments for concept 'C'

This happens as a part of the call to InventTemplateParameter.

However, we don't do the same for return types or trailing return types:

C<int> auto Foo();
auto Foo2() -> C<int> auto;

Should both diagnose, in Block, File, or Record scope.

However, calls to the InventTemplateParameter all need the InventedParameterInfos to have something in it, which it obviously doesn't, since there are no parameters to be filled in. Likely we need to do something similar with return types, though it isn't clear where that could come from.

@zygoloid did something similar in https://github.com/llvm/llvm-project/commit/9cf98d26e7b1204478cc13ae3df44a6843965c11 for trailing return types, but only applied it to inside a lambda or prototype context, and not File/Block/Record context, so it is incomplete.

[shafik]

Interesting MSVC catches none of them: https://godbolt.org/z/s3PfxWz43