Parameter scope variance + UnscopedRef xref

| File | Change |
|------|--------|
| variables.md (new §parameter-scope-variance) | Overrides/implementations/delegate conversions may: add scoped to ref/in, add scoped to ref struct param, remove UnscopedRef from out/ref-of-ref-struct. Other differences = mismatch. No effect on hiding; overloads shall not differ only on scoped/UnscopedRef. |
| attributes.md §UnscopedRefAttribute | Add "(§scoped-modifier)" xref to opening paragraph. |

Author: BillWagner

standard/attributes.md

@@ -1226,7 +1226,7 @@ Specifies that a nullable argument will not be `null` when the method returns th
 
 ### §UnscopedRefAttribute The UnscopedRef attribute
 
-There are several cases in which a ref is treated as being implicitly scoped; that is, the ref is not allowed to escape a method. For example:
+There are several cases in which a ref is treated as being implicitly scoped (§scoped-modifier); that is, the ref is not allowed to escape a method. For example:
 
 - `this` for struct instance methods.
 - ref parameters that refer to ref struct types.

standard/variables.md

@@ -1472,3 +1472,18 @@ Consider the following declarations and their safe contexts:
 | `scoped ref Span<int> s`    | *function-member*  | *caller-context* |
 
 In this relationship the *ref-safe-context* of a value can never be wider than the *safe-context*.
+
+### §parameter-scope-variance Parameter scope variance
+
+The `scoped` modifier (§scoped-modifier) and `[UnscopedRef]` attribute (§UnscopedRefAttribute) on parameters affect overriding, interface implementation, and `delegate` conversion. The signature for an override, interface implementation, or `delegate` conversion may:
+
+- Add `scoped` to a `ref` or `in` parameter.
+- Add `scoped` to a parameter of a `ref struct` type.
+- Remove `[UnscopedRef]` from an `out` parameter.
+- Remove `[UnscopedRef]` from a `ref` parameter of a `ref struct` type.
+
+Any other difference with respect to `scoped` or `[UnscopedRef]` between the base and the overriding, implementing, or converting signature is a mismatch.
+
+The `scoped` modifier and `[UnscopedRef]` attribute do not affect hiding.
+
+Overloads shall not differ only on `scoped` or `[UnscopedRef]`.