Concepts with parameter pack not at end silently ignores errors when used

[MitalAshok]

Bugzilla Link	48838
Version	trunk
OS	All
Attachments	Patch to reject concepts with parameter packs not at the end
CC	@zygoloid

Extended Description

There are no errors or warnings compiling the following with clang++ -std=c++20 -fsyntax-only:

template<typename, typename..., typename = int>
concept invalid = true;

template<typename> requires invalid<int>
no errors are printed
;

static_assert(invalid<int> also here ;

int main() {
    bool b;
    b = invalid<int> not just in declarations;
    return b;
}

This happens because this assumption is violated with concepts:

llvm-project/clang/lib/Sema/SemaTemplate.cpp

Lines 5756 to 5760 in 39239f9

	// A non-expanded parameter pack before the end of the parameter list
	// only occurs for an ill-formed template parameter list, unless we've
	// got a partial argument list for a function template, so just bail out.
	if (Param + 1 != ParamEnd)
	return true;

and no diagnosis is printed beforehand

I have attached a patch that makes concepts like invalid which have a parameter pack not at the end an error.

There is currently no wording in the standard that makes these concepts ill-formed (The current C++20 [temp.param]p14 only mentions "primary class template, primary variable template, or alias template"), but I think this is an oversight.

[luken-google]

Patch out for review at https://reviews.llvm.org/D138210

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[luken-google]

Fixed in afd800f