[clang] Concept can access a private type in a dependent template

[sigasigasiga]

This code compiles just fine although it shouldn't

#include <concepts>

template<typename T>
class trait
{
    using type = int; // note that the type is private
};

template<typename T>
concept has_field = requires(T t) {
    { t.field } -> std::convertible_to<typename trait<T>::type>;
};

struct foo
{
    int field;
};

static_assert(has_field<foo>);

https://godbolt.org/z/WKzo7E7cK

[llvmbot]

@llvm/issue-subscribers-c-20

Author: Egor Bychin (sigasigasiga)

This code compiles just fine although it shouldn't ```cpp #include <concepts>

template<typename T>
class trait
{
using type = int; // note that the type is private
};

template<typename T>
concept has_field = requires(T t) {
{ t.field } -> std::convertible_to<typename trait<T>::type>;
};

struct foo
{
int field;
};

static_assert(has_field<foo>);

https://godbolt.org/z/WKzo7E7cK
</details>

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Egor Bychin (sigasigasiga)

This code compiles just fine although it shouldn't ```cpp #include <concepts>

template<typename T>
class trait
{
using type = int; // note that the type is private
};

template<typename T>
concept has_field = requires(T t) {
{ t.field } -> std::convertible_to<typename trait<T>::type>;
};

struct foo
{
int field;
};

static_assert(has_field<foo>);

https://godbolt.org/z/WKzo7E7cK
</details>

[shafik]

CC @erichkeane

[Backl1ght]

After some debugging I find out the root cause is that there is a SFINAE error not processed during TransformTemplateParameterList here

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

Lines 2737 to 2747 in 5021e6d

	TemplateParameterList *TPL = TransformTemplateParameterList(OrigTPL);
	if (!TPL)
	TransRetReq.emplace(createSubstDiag(SemaRef, Info,
	[&] (llvm::raw_ostream& OS) {
	RetReq.getTypeConstraint()->getImmediatelyDeclaredConstraint()
	->printPretty(OS, nullptr, SemaRef.getPrintingPolicy());
	}));
	else {
	TPLInst.Clear();
	TransRetReq.emplace(TPL);
	}

I will submit a fix later.