[Clang] Lambda with explicit template parameter and requires clause that uses a dependent variable template is incorrectly rejected

[brycelelbach]

Top of trunk Clang rejects the following code:

template <typename T> concept C = true;
template <typename T> constexpr bool I = true;

template <typename T> requires(C<T>) auto a(T&& t) { };
template <typename T> requires(I<T>) auto b(T&& t) { };

auto c = [] <typename T> requires(C<T>) (T&& t) { };
auto d = [] <typename T> requires(I<T>) (T&& t) { };

complaining that:

<source>:8:42: error: 'T' does not refer to a value
auto d = [] <typename T> requires(I<T>) (T&& t) { };
                                         ^
<source>:8:23: note: declared here
auto d = [] <typename T> requires(I<T>) (T&& t) { };
                      ^
<source>:8:46: error: use of undeclared identifier 't'
auto d = [] <typename T> requires(I<T>) (T&& t) { };
                                             ^
2 errors generated.
Compiler returned: 1

If a concept is used instead of an variable template, the code compiles fine. A function definition of similar construction (explicit template parameter and a use of a dependent variable template in the requires clause) is also fine.

GCC and MSVC both accept this code.

Godbolt reproducer.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

[llvmbot]

@llvm/issue-subscribers-c-20

[shafik]

This is #61278

CC @erichkeane