Update SA1131 to treat methods as constants

#3677

Author: bjornhellander

StyleCop.Analyzers/StyleCop.Analyzers.Test/ReadabilityRules/SA1131UnitTests.cs

@@ -508,5 +508,81 @@ public void Test()
             };
             await VerifyCSharpFixAsync(testCode, expected, fixedCode, CancellationToken.None).ConfigureAwait(false);
         }
+
+        [Theory]
+        [InlineData("Method1", "arg", true)]
+        [InlineData("Method2", "arg", true)]
+        [InlineData("Method1", "field1", true)]
+        [InlineData("Method2", "field1", true)]
+        [InlineData("Method1", "field2", true)]
+        [InlineData("Method2", "field2", true)]
+        [InlineData("Const1", "Method1", false)]
+        [InlineData("Const1", "Method2", false)]
+        [InlineData("Method1", "Const1", false)]
+        [InlineData("Method2", "Const1", false)]
+        [WorkItem(3677, "https://github.com/DotNetAnalyzers/StyleCopAnalyzers/issues/3677")]
+        public async Task TestMethodsAsync(string expr1, string expr2, bool shouldTrigger)
+        {
+            var testExpr = $"{expr1} == {expr2}";
+            var testCode = $@"
+using System;
+
+public class TestClass
+{{
+    private static readonly Action Const1 = Method1;
+
+    private Action field1 = Method1;
+    private readonly Action field2 = Method1;
+
+    public bool TestMethod(Action arg)
+    {{
+        return {(shouldTrigger ? $"[|{testExpr}|]" : testExpr)};
+    }}
+
+    private static void Method1()
+    {{
+    }}
+
+    private void Method2()
+    {{
+    }}
+}}
+";
+
+            var fixedExpr = $"{expr2} == {expr1}";
+            var fixedCode = $@"
+using System;
+
+public class TestClass
+{{
+    private static readonly Action Const1 = Method1;
+
+    private Action field1 = Method1;
+    private readonly Action field2 = Method1;
+
+    public bool TestMethod(Action arg)
+    {{
+        return {fixedExpr};
+    }}
+
+    private static void Method1()
+    {{
+    }}
+
+    private void Method2()
+    {{
+    }}
+}}
+";
+
+            if (shouldTrigger)
+            {
+                await VerifyCSharpFixAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, fixedCode, CancellationToken.None).ConfigureAwait(false);
+            }
+            else
+            {
+                await VerifyCSharpDiagnosticAsync(testCode, DiagnosticResult.EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);
+            }
+        }
     }
 }

StyleCop.Analyzers/StyleCop.Analyzers/ReadabilityRules/SA1131UseReadableConditions.cs

@@ -82,12 +82,16 @@ private static bool IsLiteral(ExpressionSyntax expression, SemanticModel semanti
                 return true;
             }
 
-            if (semanticModel.GetSymbolInfo(expression).Symbol is IFieldSymbol fieldSymbol)
+            var symbol = semanticModel.GetSymbolInfo(expression).Symbol;
+            switch (symbol)
             {
-                return fieldSymbol.IsStatic && fieldSymbol.IsReadOnly;
-            }
+            case IFieldSymbol fieldSymbol when fieldSymbol.IsStatic && fieldSymbol.IsReadOnly:
+            case IMethodSymbol:
+                return true;
 
-            return false;
+            default:
+                return false;
+            }
         }
     }
 }