[clang][temp.explicit] Explicit template instantiation leads to incorrect checking of constraints

[ddvamp]

Clang trunk x86-64 c++20 (godbolt.org)

template <typename T>
struct S {
  void foo() requires (sizeof(T) == 1); // declaration
};

struct I;

template struct S<I>; // explicit
S<I> s;

struct I {};

template <typename T>
void S<T>::foo() requires (sizeof(T) == 1) { // definition
  static_assert(sizeof(T) == 2);
}

void fn() {
    S<I>{}.foo(); // CE: constraints not satisfied (ill-formed, incomplete type)
}

It looks like the constraints are checked at the point of declaration, not definition. Correct me please if this behavior is determined by the standard

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Artyom Kolpakov (ddvamp)

Clang trunk x86-64 c++20 ([godbolt.org](https://godbolt.org/z/x88Exz6Tn))

template <typename T>
struct S {
  void foo() requires (sizeof(T) == 1); // declaration
};

struct I;

template struct S<I>; // explicit
S<I> s;

struct I {};

template <typename T>
void S<T>::foo() requires (sizeof(T) == 1) { // definition
  static_assert(sizeof(T) == 2);
}

void fn() {
    S<I>{}.foo(); // CE: constraints not satisfied (ill-formed, incomplete type)
}

It looks like the constraints are checked at the point of declaration, not definition. Correct me please if this behavior is determined by the standard