Fix false positive for Equals/CompareTo with nullable reference types

csharp/ql/lib/semmle/code/csharp/frameworks/System.qll

@@ -144,7 +144,6 @@ class SystemIComparableInterface extends SystemInterface {
     result.getDeclaringType() = this and
     result.hasName("CompareTo") and
     result.getNumberOfParameters() = 1 and
-    result.getParameter(0).getType() instanceof ObjectType and
     result.getReturnType() instanceof IntType
   }
 }
@@ -263,7 +262,6 @@ class SystemObjectClass extends SystemClass instanceof ObjectType {
     result.getDeclaringType() = this and
     result.hasName("Equals") and
     result.getNumberOfParameters() = 1 and
-    result.getParameter(0).getType() instanceof ObjectType and
     result.getReturnType() instanceof BoolType
   }
 
@@ -273,8 +271,6 @@ class SystemObjectClass extends SystemClass instanceof ObjectType {
     result.getDeclaringType() = this and
     result.hasName("Equals") and
     result.getNumberOfParameters() = 2 and
-    result.getParameter(0).getType() instanceof ObjectType and
-    result.getParameter(1).getType() instanceof ObjectType and
     result.getReturnType() instanceof BoolType
   }
 
@@ -284,8 +280,6 @@ class SystemObjectClass extends SystemClass instanceof ObjectType {
     result.getDeclaringType() = this and
     result.hasName("ReferenceEquals") and
     result.getNumberOfParameters() = 2 and
-    result.getParameter(0).getType() instanceof ObjectType and
-    result.getParameter(1).getType() instanceof ObjectType and
     result.getReturnType() instanceof BoolType
   }
 

csharp/ql/test/library-tests/frameworks/system/Equals/Equals.cs

@@ -24,3 +24,16 @@ struct Equals1Struct
 {
     public override bool Equals(object other) => false;
 }
+
+#nullable enable
+
+class NullableEquals1
+{
+    public override bool Equals(object? other) => false;
+}
+
+class NullableEquals2 : IEquatable<NullableEquals2>
+{
+    public bool Equals(NullableEquals2? other) => other != null;
+    public override bool Equals(object? other) => other is NullableEquals2 n && Equals(n);
+}

csharp/ql/test/library-tests/frameworks/system/Equals/Equals.expected

@@ -5,3 +5,5 @@
 | Equals.cs:16:7:16:13 | Equals3 | Equals3.Equals(Equals3) | true |
 | Equals.cs:21:8:21:21 | NoEqualsStruct | System.ValueType.Equals(object) | false |
 | Equals.cs:23:8:23:20 | Equals1Struct | Equals1Struct.Equals(object) | true |
+| Equals.cs:31:7:31:21 | NullableEquals1 | NullableEquals1.Equals(object) | true |
+| Equals.cs:36:7:36:21 | NullableEquals2 | NullableEquals2.Equals(NullableEquals2) | true |

csharp/ql/test/query-tests/API Abuse/ClassDoesNotImplementEquals/NullableTest.cs

@@ -0,0 +1,46 @@
+using System;
+
+#nullable enable
+
+namespace Test
+{
+    class TestClass1 : IEquatable<TestClass1>
+    {
+        private int field1;
+
+        public bool Equals(TestClass1? param1)
+        {
+            return param1 != null && field1 == param1.field1;
+        }
+
+        public override bool Equals(object? param2)
+        {
+            return param2 is TestClass1 tc && Equals(tc);
+        }
+
+        public override int GetHashCode()
+        {
+            return field1;
+        }
+    }
+
+    class TestClass2
+    {
+        private string field2;
+
+        public TestClass2(string s)
+        {
+            field2 = s;
+        }
+
+        public override bool Equals(object? param3)
+        {
+            return param3 is TestClass2 tc && field2 == tc.field2;
+        }
+
+        public override int GetHashCode()
+        {
+            return field2?.GetHashCode() ?? 0;
+        }
+    }
+}