[Clang] Lambda in a `decltype()` in template parameters causing incorrect type mismatches

[Metadorius]

The following code (minimal repro. example)

template <typename T>
struct X {};

template <typename>
using DistinctCollector = X<decltype([]{})>;

DistinctCollector<int*> getCellSpreadItems()
{
	DistinctCollector<int*> set;
	return set;
}

int main() { getCellSpreadItems(); }

https://godbolt.org/z/obMWeKjKr

fails with

error: no viable conversion from returned value of type 'DistinctCollector<...>' to function return type 'DistinctCollector<...>'

The same error is not observed on GCC or MSVC (in conformance mode, iow /permissive-) compilers.

If you use

auto getCellSpreadItems()
{
	DistinctCollector<int*> set;
	return set;
}

https://godbolt.org/z/4fnocEr13

it works.

Per @Sirraide on Discord:

Looking at the AST, it seems like we’re instantiating that template twice; so it seems what it’s complaining about is that the two instances of DistinctCollector<T> are ... not the same type, which er, is not quite right
There are some things we try to diagnose even if the function is still dependent, but usually you need to instantiate it yes, and this also looks like a problem specifically w/ instantiation

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Kerbiter (Metadorius)

The following code (minimal repro. example) ```cpp template <typename T> struct X {};

template <typename>
using DistinctCollector = X<decltype([]{})>;

DistinctCollector<int*> getCellSpreadItems()
{
DistinctCollector<int*> set;
return set;
}

int main() { getCellSpreadItems(); }

https://godbolt.org/z/obMWeKjKr

fails with

`error: no viable conversion from returned value of type 'DistinctCollector<...>' to function return type 'DistinctCollector<...>'`

The same error is not observed on GCC or MSVC (in conformance mode, iow `/permissive-`) compilers.

If you use

```cpp
auto getCellSpreadItems()
{
	DistinctCollector<int*> set;
	return set;
}

https://godbolt.org/z/4fnocEr13

it works.

Per @Sirraide on Discord:
> Looking at the AST, it seems like we’re instantiating that template twice; so it seems what it’s complaining about is that the two instances of DistinctCollector<T> are ... not the same type, which er, is not quite right
> There are some things we try to diagnose even if the function is still dependent, but usually you need to instantiate it yes, and this also looks like a problem specifically w/ instantiation

[Sirraide]

Looks like this has been an issue since Clang 13, which is the earliest release that supports lambdas in unevaluated operands.

[cor3ntin]

This is the dreaded CWG2794