Clang frontend failed on parsing concept with packed parameters

[llvmbot]

Bugzilla Link	45770
Version	10.0
OS	All
Attachments	Clang generated bug description files
Reporter	LLVM Bugzilla Contributor
CC	@zygoloid,@saarraz

Extended Description

Clang frontend failed on parsing following minimized case:

template <typename T, typename U>
concept C1 = false;

template <typename T, typename ... Args>
concept C2 = sizeof...(Args) == 1 && C1<T, Args>;

template requires C2<T, int>
void f(T t) {}

int main() {
f(1);
return 0;
}

Steps to Reproduce:

Compile this snippet with command:

clang++-10 -std=c++20 main.cpp -o main

will cause the frontend to crash.

Expected Results:
It's obvious to find that code line concept C2 = sizeof...(Args) == 1 && C1<T, Args>; doesn't correctly unpacked the parameter pack, which should be corrected to:
concept C2 = sizeof...(Args) == 1 && (... && C1<T, Args>);
which will then compile correctly. It seems that the frontend has some defects preventing itself from handling such syntax error.

Additional Information:
The environment tested is WSL1 on Windows 10 2004 with Ubuntu-18.04, and the clang version is 10.0.0-++20200412073436+50d7e5d5e7d-1exp120200412054917.132. I've also tested on the Compiler Explorer website with clang 10 & clang (trunk) which all give the same result. G++ (trunk) can handle the error correctly.

[xgupta]

Working fine on the trunk, godbolt link - https://godbolt.org/z/33Gd7GGEn. Possibly fixed with https://reviews.llvm.org/rG7fb562c1ab373a3d4e14003e40556791ec032bab.

[llvmbot]

@llvm/issue-subscribers-clang-frontend