Fixes issue in SA1313 where symbol is marked as override, but the overriden method field is null

Author: vweijsters

StyleCop.Analyzers/StyleCop.Analyzers.Test/NamingRules/SA1313UnitTests.cs

@@ -529,6 +529,42 @@ public TestClass(string text, bool flag)
             await this.VerifyCSharpFixAsync(testCode, fixedCode).ConfigureAwait(false);
         }
 
+        /// <summary>
+        /// Verify that an invalid method override will not produce a diagnostic nor crash the analyzer.
+        /// This is a regression test for #2189
+        /// </summary>
+        /// <returns>A <see cref="Task"/> representing the asynchronous unit test.</returns>
+        [Fact]
+        public async Task InvalidMethodOverrideShouldNotProduceDiagnosticAsync()
+        {
+            var testCode = @"
+namespace TestNamespace
+{
+    public abstract class BaseClass
+    {
+        public abstract void TestMethod(int p1, int p2);
+    }
+
+    public class TestClass : BaseClass
+    {
+        public override void TestMethod(int p1, X int P2)
+        {
+        }
+    }
+}
+";
+            DiagnosticResult[] expected =
+            {
+                this.CSharpCompilerError("CS0534").WithLocation(9, 18).WithMessage("'TestClass' does not implement inherited abstract member 'BaseClass.TestMethod(int, int)'"),
+                this.CSharpCompilerError("CS0115").WithLocation(11, 30).WithMessage("'TestClass.TestMethod(int, X, int)': no suitable method found to override"),
+                this.CSharpCompilerError("CS0246").WithLocation(11, 49).WithMessage("The type or namespace name 'X' could not be found (are you missing a using directive or an assembly reference?)"),
+                this.CSharpCompilerError("CS1001").WithLocation(11, 51).WithMessage("Identifier expected"),
+                this.CSharpCompilerError("CS1003").WithLocation(11, 51).WithMessage("Syntax error, ',' expected"),
+            };
+
+            await this.VerifyCSharpDiagnosticAsync(testCode, expected, CancellationToken.None).ConfigureAwait(false);
+        }
+
         protected override IEnumerable<DiagnosticAnalyzer> GetCSharpDiagnosticAnalyzers()
         {
             yield return new SA1313ParameterNamesMustBeginWithLowerCaseLetter();

StyleCop.Analyzers/StyleCop.Analyzers/NamingRules/SA1313ParameterNamesMustBeginWithLowerCaseLetter.cs

@@ -128,7 +128,8 @@ private static bool NameMatchesAbstraction(ParameterSyntax syntax, SemanticModel
 
             if (methodSymbol.IsOverride)
             {
-                if (methodSymbol.OverriddenMethod.Parameters[index].Name == syntax.Identifier.ValueText)
+                // OverridenMethod can be null in case of an invalid method declaration -> exit because there is no meaningful analysis to be done.
+                if ((methodSymbol.OverriddenMethod == null) || (methodSymbol.OverriddenMethod.Parameters[index].Name == syntax.Identifier.ValueText))
                 {
                     return true;
                 }