C#: Add implicit span conversion test case.

csharp/ql/test/library-tests/conversion/span/Span.cs

@@ -0,0 +1,54 @@
+using System;
+using System.Collections.Generic;
+
+public interface CovariantInterface<out T> { }
+
+public interface InvariantInterface<T> { }
+
+public interface Interface<out T1, T2> { }
+
+public class Base { }
+
+public class Derived : Base { }
+
+public class C
+{
+    public void M()
+    {
+        string[] stringArray = [];
+        string[][] stringArrayArray;
+        string[,] stringArray2D;
+
+        Span<string> stringSpan = stringArray; // string[] -> Span<string>;
+
+        // Covariant conversions to ReadOnlySpan
+        // Assignments are included to illustrate that this compiles.
+        // Only the use of the types matter in terms of test output.
+        ReadOnlySpan<CovariantInterface<Base>> covariantInterfaceBaseReadOnlySpan;
+        ReadOnlySpan<CovariantInterface<Derived>> covariantInterfaceDerivedReadOnlySpan = default;
+        Span<CovariantInterface<Derived>> covariantInterfaceDerivedSpan = default;
+        CovariantInterface<Derived>[] covariantInterfaceDerivedArray = [];
+        covariantInterfaceBaseReadOnlySpan = covariantInterfaceDerivedReadOnlySpan; // ReadOnlySpan<CovariantInterface<Derived>> -> ReadOnlySpan<CovariantInterface<Base>>
+        covariantInterfaceBaseReadOnlySpan = covariantInterfaceDerivedSpan; // Span<CovariantInterface<Derived>> -> ReadOnlySpan<CovariantInterface<Base>>
+        covariantInterfaceBaseReadOnlySpan = covariantInterfaceDerivedArray; // CovariantInterface<Derived>[] -> ReadOnlySpan<CovariantInterface<Base>>
+
+        // Identify conversions to ReadOnlySpan
+        ReadOnlySpan<string> stringReadOnlySpan;
+        stringReadOnlySpan = stringSpan; // Span<string> -> ReadOnlySpan<string>;
+        stringReadOnlySpan = stringArray; // string[] -> ReadOnlySpan<string>;
+
+        // Convert string to ReadOnlySpan<char>
+        string s = "";
+        ReadOnlySpan<char> charReadOnlySpan = s; // string -> ReadOnlySpan<char>
+
+        // Use the non-covariant interfaces to show that no conversion is possible.
+        ReadOnlySpan<InvariantInterface<Base>> invariantInterfaceBaseReadOnlySpan;
+        ReadOnlySpan<InvariantInterface<Derived>> invariantInterfaceDerivedReadOnlySpan;
+        Span<InvariantInterface<Derived>> invariantInterfaceDerivedSpan;
+        InvariantInterface<Derived>[] invariantInterfaceDerivedArray;
+        ReadOnlySpan<Interface<Base, string>> interfaceBaseReadOnlySpan;
+        ReadOnlySpan<Interface<Derived, string>> interfaceDerivedReadOnlySpan;
+        Span<Interface<Derived, string>> interfaceDerivedSpan;
+        Interface<Derived, string>[] interfaceDerivedArray;
+    }
+}

csharp/ql/test/library-tests/conversion/span/span.expected

@@ -0,0 +1,16 @@
+| CovariantInterface<Derived>[] | ReadOnlySpan<CovariantInterface<Base>> |
+| CovariantInterface<Derived>[] | ReadOnlySpan<CovariantInterface<Derived>> |
+| CovariantInterface<Derived>[] | Span<CovariantInterface<Derived>> |
+| Interface<Derived,String>[] | ReadOnlySpan<Interface<Derived, string>> |
+| Interface<Derived,String>[] | Span<Interface<Derived, string>> |
+| InvariantInterface<Derived>[] | ReadOnlySpan<InvariantInterface<Derived>> |
+| InvariantInterface<Derived>[] | Span<InvariantInterface<Derived>> |
+| ReadOnlySpan<CovariantInterface<Derived>> | ReadOnlySpan<CovariantInterface<Base>> |
+| Span<CovariantInterface<Derived>> | ReadOnlySpan<CovariantInterface<Base>> |
+| Span<CovariantInterface<Derived>> | ReadOnlySpan<CovariantInterface<Derived>> |
+| Span<Interface<Derived, string>> | ReadOnlySpan<Interface<Derived, string>> |
+| Span<InvariantInterface<Derived>> | ReadOnlySpan<InvariantInterface<Derived>> |
+| Span<string> | ReadOnlySpan<string> |
+| String[] | ReadOnlySpan<string> |
+| String[] | Span<string> |
+| string | ReadOnlySpan<char> |

csharp/ql/test/library-tests/conversion/span/span.ql

@@ -0,0 +1,9 @@
+import semmle.code.csharp.Conversion
+
+private class InterestingType extends Type {
+  InterestingType() { exists(LocalVariable lv | lv.getType() = this) }
+}
+
+from InterestingType sub, InterestingType sup
+where convSpan(sub, sup) and sub != sup
+select sub.toStringWithTypes() as s1, sup.toStringWithTypes() as s2 order by s1, s2