Fix crash for Equals analysis of types with bad base clauses (#42968)

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -3958,12 +3958,14 @@ private void ReinferMethodAndVisitArguments(BoundCall node, TypeWithState receiv
             SetUpdatedSymbol(node, node.Method, method);
         }
 
+#nullable enable
         private void LearnFromEqualsMethod(MethodSymbol method, BoundCall node, TypeWithState receiverType, ImmutableArray<VisitArgumentResult> results)
         {
             // easy out
             var parameterCount = method.ParameterCount;
             var arguments = node.Arguments;
-            if ((parameterCount != 1 && parameterCount != 2)
+            if (node.HasErrors
+                || (parameterCount != 1 && parameterCount != 2)
                 || parameterCount != arguments.Length
                 || method.MethodKind != MethodKind.Ordinary
                 || method.ReturnType.SpecialType != SpecialType.System_Boolean
@@ -3986,11 +3988,12 @@ private void LearnFromEqualsMethod(MethodSymbol method, BoundCall node, TypeWith
 
             var isObjectEqualsMethodOrOverride = method.GetLeastOverriddenMethod(accessingTypeOpt: null)
                 .Equals(compilation.GetSpecialTypeMember(SpecialMember.System_Object__Equals));
-            if (isObjectEqualsMethodOrOverride ||
-                isWellKnownEqualityMethodOrImplementation(compilation, method, receiverType.Type, WellKnownMember.System_IEquatable_T__Equals))
+            if (node.ReceiverOpt is BoundExpression receiver &&
+                    (isObjectEqualsMethodOrOverride ||
+                     isWellKnownEqualityMethodOrImplementation(compilation, method, receiverType.Type, WellKnownMember.System_IEquatable_T__Equals)))
             {
                 Debug.Assert(arguments.Length == 1);
-                learnFromEqualsMethodArguments(node.ReceiverOpt, receiverType, arguments[0], results[0].RValueType);
+                learnFromEqualsMethodArguments(receiver, receiverType, arguments[0], results[0].RValueType);
                 return;
             }
 
@@ -4007,9 +4010,9 @@ static bool anyOverriddenMethodHasExplicitImplementation(MethodSymbol method)
                 return false;
             }
 
-            static bool isWellKnownEqualityMethodOrImplementation(CSharpCompilation compilation, MethodSymbol method, TypeSymbol receiverType, WellKnownMember wellKnownMember)
+            static bool isWellKnownEqualityMethodOrImplementation(CSharpCompilation compilation, MethodSymbol method, TypeSymbol? receiverType, WellKnownMember wellKnownMember)
             {
-                var wellKnownMethod = (MethodSymbol)compilation.GetWellKnownTypeMember(wellKnownMember);
+                var wellKnownMethod = (MethodSymbol?)compilation.GetWellKnownTypeMember(wellKnownMember);
                 if (wellKnownMethod is null || receiverType is null)
                 {
                     return false;
@@ -4120,6 +4123,7 @@ void learnFromEqualsMethodArguments(BoundExpression left, TypeWithState leftType
                 }
             }
         }
+#nullable restore
 
         private bool IsCompareExchangeMethod(MethodSymbol method)
         {

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -123946,6 +123946,58 @@ static void M1(C c, string? s)
                     Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "s").WithLocation(18, 15));
         }
 
+        [Fact, WorkItem(42960, "https://github.com/dotnet/roslyn/issues/42960")]
+        public void EqualsMethod_IncompleteBaseClause()
+        {
+            var source = @"
+class C :
+{
+    void M(C? other)
+    {
+        if (Equals(other)) { other.ToString(); }
+        if (this.Equals(other)) { other.ToString(); }
+    }
+}
+";
+            var comp = CreateNullableCompilation(source);
+            comp.VerifyDiagnostics(
+                    // (2,10): error CS1031: Type expected
+                    // class C :
+                    Diagnostic(ErrorCode.ERR_TypeExpected, "").WithLocation(2, 10),
+                    // (6,13): error CS0120: An object reference is required for the non-static field, method, or property 'object.Equals(object)'
+                    //         if (Equals(other)) { other.ToString(); }
+                    Diagnostic(ErrorCode.ERR_ObjectRequired, "Equals").WithArguments("object.Equals(object)").WithLocation(6, 13),
+                    // (6,30): warning CS8602: Dereference of a possibly null reference.
+                    //         if (Equals(other)) { other.ToString(); }
+                    Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "other").WithLocation(6, 30));
+        }
+
+        [Fact, WorkItem(42960, "https://github.com/dotnet/roslyn/issues/42960")]
+        public void EqualsMethod_ErrorBaseClause()
+        {
+            var source = @"
+class C : ERROR
+{
+    void M(C? other)
+    {
+        if (Equals(other)) { other.ToString(); }
+        if (this.Equals(other)) { other.ToString(); }
+    }
+}
+";
+            var comp = CreateNullableCompilation(source);
+            comp.VerifyDiagnostics(
+                // (2,11): error CS0246: The type or namespace name 'ERROR' could not be found (are you missing a using directive or an assembly reference?)
+                // class C : ERROR
+                Diagnostic(ErrorCode.ERR_SingleTypeNameNotFound, "ERROR").WithArguments("ERROR").WithLocation(2, 11),
+                // (6,13): error CS0120: An object reference is required for the non-static field, method, or property 'object.Equals(object)'
+                //         if (Equals(other)) { other.ToString(); }
+                Diagnostic(ErrorCode.ERR_ObjectRequired, "Equals").WithArguments("object.Equals(object)").WithLocation(6, 13),
+                // (6,30): warning CS8602: Dereference of a possibly null reference.
+                //         if (Equals(other)) { other.ToString(); }
+                Diagnostic(ErrorCode.WRN_NullReferenceReceiver, "other").WithLocation(6, 30));
+        }
+
         [Fact]
         [WorkItem(36131, "https://github.com/dotnet/roslyn/issues/36131")]
         public void ShouldRunNullableWalker_GlobalDirective()