NET-1323 Fix S3267 AD0001: Support empty return and lambdas

Author: sebastien-marichal

analyzers/src/SonarAnalyzer.CSharp/Rules/LoopsAndLinq.cs

@@ -61,13 +61,17 @@ private static bool CanBeSimplifiedUsingWhere(SyntaxNode statement, SonarSyntaxN
 
     private static bool RequiresNullableConversion(SyntaxNode returnOrAssignment, SonarSyntaxNodeReportingContext context)
     {
-        var typeInfo = context.Model.GetTypeInfo(returnOrAssignment switch
+        var expression = returnOrAssignment switch
         {
             ReturnStatementSyntax returnStatement => returnStatement.Expression,
             AssignmentExpressionSyntax assignment => assignment.Right,
             _ => throw new InvalidOperationException("Unreachable")
-        });
-        return context.Compilation.ClassifyConversion(typeInfo.Type, typeInfo.ConvertedType).IsNullable;
+        };
+
+        // expression can be null if the return statement is empty
+        return expression is not null
+            && context.Model.GetTypeInfo(expression) is { Type: { } type, ConvertedType: { } convertedType }
+            && context.Compilation.ClassifyConversion(type, convertedType).IsNullable;
     }
 
     private static SyntaxNode SingleReturnOrBreakingAssignment(IfStatementSyntax ifStatementSyntax)

analyzers/tests/SonarAnalyzer.Test/TestCases/LoopsAndLinq.Latest.cs

@@ -263,4 +263,43 @@ void IndexTest(List<string> strings)
             }
         }
     }
+
+    class ReturnNull
+    {
+        interface IInterface { }
+
+        private IInterface? ReturnNullInterface(IEnumerable<int> enumerable, Predicate<int> predicate)
+        {
+            foreach (var element in enumerable) // Noncompliant
+            {
+                if (predicate(element))         // Secondary
+                {
+                    return null;
+                }
+            }
+
+            return null;
+        }
+
+        private int? ReturnDefault(IEnumerable<int> enumerable, Predicate<int> predicate)
+        {
+            foreach (var element in enumerable) // Noncompliant
+            {
+                if (predicate(element))         // Secondary
+                {
+                    return default;
+                }
+            }
+
+            foreach (var element in enumerable) // Noncompliant
+            {
+                if (predicate(element))         // Secondary
+                {
+                    return null;
+                }
+            }
+
+            return null;
+        }
+    }
 }

analyzers/tests/SonarAnalyzer.Test/TestCases/LoopsAndLinq.cs

@@ -548,4 +548,36 @@ public int Integer
             }
         }
     }
+
+    class EmptyReturn
+    {
+        public static void MyMethod<T>(IEnumerable<T> enumerable, Predicate<T> predicate) where T : struct
+        {
+            foreach (var element in enumerable) // Noncompliant
+            {
+                if (predicate(element))         // Secondary
+                {
+                    return;
+                }
+            }
+        }
+    }
+
+    class Lambda
+    {
+        private Predicate<Object> LambdaMethod(IEnumerable<int> enumerable, Predicate<int> predicate)
+        {
+            Predicate<Object> lambda = null;
+            foreach (var element in enumerable) // Noncompliant
+            {
+                if (predicate(element))         // Secondary
+                {
+                    lambda = instance => false;
+                    break;
+                }
+            }
+
+            return lambda;
+        }
+    }
 }