fix(mutating): out var in lambdas triggering NRE (#3575)

* fix: out var in lambdas triggereing NRE

* chore: copilot fixes

---------

Co-authored-by: Rouke Broersma <mobrockers@gmail.com>

Author: dupdob

src/Stryker.Core/Stryker.Core.UnitTest/Mutants/CsharpMutantOrchestratorTests.cs

@@ -1029,11 +1029,46 @@ public void ShouldNotMutateDefaultValues()
     [TestMethod]
     public void ShouldInitializeOutVars()
     {
-        var source = @"public void SomeMethod(out int x, out string text) { x = 1; text = ""hello"";}";
-        var expected = @"public void SomeMethod(out int x, out string text) {{x= default(int);text= default(string);}if(StrykerNamespace.MutantControl.IsActive(0)){}else{ x = 1; text = (StrykerNamespace.MutantControl.IsActive(1)?"""":""hello"");
-        }}";
+        const string Source = """public void SomeMethod(out int x, out string text) { x = 1; text = "hello";}""";
+        const string Expected = """
+                                public void SomeMethod(out int x, out string text) {{x= default(int);text= default(string);}if(StrykerNamespace.MutantControl.IsActive(0)){}else{ x = 1; text = (StrykerNamespace.MutantControl.IsActive(1)?"":"hello");
+                                        }}
+                                """;
 
-        ShouldMutateSourceInClassToExpected(source, expected);
+        ShouldMutateSourceInClassToExpected(Source, Expected);
+    }
+
+    [TestMethod]
+    public void ShouldInitializeOutVarsForLambdas()
+    {
+        const string Source = """
+                              public static void Thing()
+                              {
+                                  var results = new List<ExceptionResultDelegate>();
+
+                                  results.Add((out result) =>
+                                  {
+                                      result = false;
+                                      return false;
+                                  });
+                              }
+                              """;
+        const string Expected = """
+                                public static void Thing()
+                                {if(StrykerNamespace.MutantControl.IsActive(0)){}
+                                else{
+                                    var results = new List<ExceptionResultDelegate>();
+
+                                    if(StrykerNamespace.MutantControl.IsActive(1)){;}else{results.Add((out result) =>
+                                {{result= default;}    if(StrykerNamespace.MutantControl.IsActive(2))    {}else{
+                                        result = (StrykerNamespace.MutantControl.IsActive(3)?true:false);
+                                        return (StrykerNamespace.MutantControl.IsActive(4)?true:false);
+                                    }return default;});}
+                                }
+                                }
+                                """;
+
+        ShouldMutateSourceInClassToExpected(Source, Expected);
     }
 
     [TestMethod]

src/Stryker.Core/Stryker.Core/Helpers/RoslynHelper.cs

@@ -141,16 +141,18 @@ public static AccessorDeclarationSyntax GetAccessor(this PropertyDeclarationSynt
     /// Return a default(type) expression.
     /// </summary>
     /// <param name="type">Type used in the resulting default expression.</param>
-    /// <returns>An expression representing `default(<paramref name="type"/>'.</returns>
-    public static ExpressionSyntax BuildDefaultExpression(this TypeSyntax type)
-        => SyntaxFactory.DefaultExpression(type.WithoutTrivia()).WithLeadingTrivia(SyntaxFactory.Space);
+    ///  <returns>An expression representing `default(<paramref name="type"/>`.</returns>
+    public static ExpressionSyntax BuildDefaultExpression(this TypeSyntax type) =>
+        (type == null
+            ? (ExpressionSyntax) SyntaxFactory.LiteralExpression(SyntaxKind.DefaultLiteralExpression)
+            : SyntaxFactory.DefaultExpression(type.WithoutTrivia())).WithLeadingTrivia(SyntaxFactory.Space);
 
     /// <summary>
     /// Check if a statements (or one of its child statements, if any) verifies some given predicate.
     /// </summary>
     /// <param name="syntax">initial statements</param>
     /// <param name="predicate">predicate to verify</param>
-    /// <param name="skipBlocks">does not parse block statements if true</param>
+    /// <param name="skipBlocks">true to skip syntax blocks</param>
     /// <returns>true if any of the child statements verify the predicate</returns>
     /// <remarks>scanning stops as soon as one child matches the predicate</remarks>
     public static bool ScanChildStatements(this StatementSyntax syntax, Func<StatementSyntax, bool> predicate, bool skipBlocks = false)

src/Stryker.Core/Stryker.Core/Instrumentation/DefaultInitializationEngine.cs

@@ -48,7 +48,7 @@ public BlockSyntax InjectOutParametersInitialization(BlockSyntax body, IEnumerab
         }
         else
         {
-            // this is the first initializer helper, no pre-existing ones
+            // this is the first initializer helper, no pre existing ones
             initializers = [];
             // keep all statements
             originalStatements = body.Statements;