[ADT] Conversion from ArrayRef<S:T> to ArrayRef<T> does not work

[tgymnich]

Conversion of an ArrayRef of some class to an ArrayRef of the base class of that class fails with the following error message:

ArrayRef.h(94, 28): Candidate template ignored:
requirement 'std::conjunction_v<std::is_convertible<const mlir::StringAttr **, const mlir::Attribute *const *>, std::is_integral<unsigned long>>' was not satisfied [with C = mlir::ArrayRef<mlir::StringAttr>]

Example

ArrayRef<mlir::StringAttr> x;
ArrayRef<mlir::Attribute> y(x);

[zwuis]

In pointer arithmetic, base class object can be treated as an array with only one element.

https://en.cppreference.com/w/cpp/language/operator_arithmetic.html#Pointer_arithmetic

struct Base {};
struct Derived {};

Derived D[2];

void f() {
  // ok, B1 binds to the subobject of D[1]
  Base &B1 = *(D + 1);
  // undefined behaviour
  Base &B2 = *(static_cast<Base *>(D) + 1);
}