Reject oversized lambda signatures before delegate creation

src/FastExpressionCompiler/FastExpressionCompiler.cs

@@ -482,8 +482,11 @@ internal static Type[] RentPooledOrNewClosureTypeToParamTypes(ParamExprs paramEx
             var count = paramExprs.GetParamCount();
             if (count == 0)
                 return _closureAsASingleParamType;
-
-            var pooledOrNew = count < 8 ? Interlocked.Exchange(ref _paramTypesPoolWithElem0OfLength1[count], null) ?? new Type[count + 1] : new Type[count + 1];
+            // The hidden closure argument also counts toward the CLI signature limit.
+            // The pooled small-signature path is already below that limit.
+            var pooledOrNew = count < 8 ? Interlocked.Exchange(ref _paramTypesPoolWithElem0OfLength1[count], null) ?? new Type[count + 1] :
+                count < ushort.MaxValue ? new Type[count + 1] :
+                throw new InvalidProgramException("The lambda and its closure exceed the maximum method parameter count.");
             pooledOrNew[0] = typeof(ArrayClosure);
             for (var i = 0; i < count; i++)
             {

test/FastExpressionCompiler.TestsRunner.Net472/Program.cs

@@ -34,6 +34,7 @@ static void RunLightExpressionTests(object state)
                 var justLightTestsStopwatch = Stopwatch.StartNew();
 
                 var t = (LightExpression.TestRun)state;
+                t.Run(new LightExpression.UnitTests.ExcessiveArityRegressionTests());
                 t.Run(new LightExpression.UnitTests.NestedStructFieldAddressRegressionTests());
                 t.Run(new LightExpression.IssueTests.Issue183_NullableDecimal());
                 t.Run(new LightExpression.IssueTests.Issue398_Optimize_Switch_with_OpCodes_Switch());
@@ -60,6 +61,7 @@ static void RunLightExpressionTests(object state)
             var fecTestsStopwatch = Stopwatch.StartNew();
 
             var fecTests = new TestRun();
+            fecTests.Run(new ExcessiveArityRegressionTests());
             fecTests.Run(new NestedStructFieldAddressRegressionTests());
             fecTests.Run(new Issue183_NullableDecimal());
             fecTests.Run(new Issue468_Optimize_the_delegate_access_to_the_Closure_object_for_the_modern_NET());

test/FastExpressionCompiler.TestsRunner/Program.cs

@@ -63,6 +63,7 @@ public static void Main(string[] args)
             var ltTask = Task.Run(() =>
             {
                 var ltStopwatch = Stopwatch.StartNew();
+                lt.Run(new LightExpression.UnitTests.ExcessiveArityRegressionTests());
 #if NET8_0_OR_GREATER
                 lt.Run(new LightExpression.UnitTests.NestedStructFieldAddressRegressionTests());
                 lt.Run(new LightExpression.IssueTests.Issue490_Regression_in_compiling_lambdas_with_ref_struct_parameters());
@@ -86,6 +87,7 @@ public static void Main(string[] args)
             });
 
             var stStopwatch = Stopwatch.StartNew();
+            st.Run(new ExcessiveArityRegressionTests());
             st.Run(new NestedStructFieldAddressRegressionTests());
             st.Run(new Issue499_InvalidProgramException_for_Sorting_and_comparison_function());
             st.Run(new Issue561_NonBooleanEqualityOperator());

test/FastExpressionCompiler.UnitTests/ExcessiveArityRegressionTests.cs

@@ -0,0 +1,41 @@
+using System;
+using System.Linq;
+using S = System.Linq.Expressions.Expression;
+#if LIGHT_EXPRESSION
+namespace FastExpressionCompiler.LightExpression.UnitTests;
+#else
+namespace FastExpressionCompiler.UnitTests;
+#endif
+
+public struct ExcessiveArityRegressionTests : ITestX
+{
+    public void Run(TestRun t)
+    {
+        Unrepresentable_method_signatures_are_rejected_during_compilation(t);
+        Large_supported_signatures_keep_argument_indices(t);
+    }
+
+    public void Unrepresentable_method_signatures_are_rejected_during_compilation(TestContext t)
+    {
+        var parameters = Enumerable.Range(0, ushort.MaxValue).Select(_ => S.Parameter(typeof(int))).ToArray();
+        var system = S.Lambda(parameters[parameters.Length - 1], parameters);
+        var expression = system.FromSysExpression();
+        t.Throws<InvalidProgramException>(() => expression.CompileFast(true));
+        t.Throws<InvalidProgramException>(() => expression.CompileFast(true, CompilerFlags.ThrowOnNotSupportedExpression));
+        t.Throws<InvalidProgramException>(() => system.Compile());
+    }
+
+    public void Large_supported_signatures_keep_argument_indices(TestContext t)
+    {
+        foreach (var count in new[] { 255, 256, 300 })
+        {
+            var parameters = Enumerable.Range(0, count).Select(_ => S.Parameter(typeof(int))).ToArray();
+            var system = S.Lambda(S.Add(parameters[0], parameters[count - 1]), parameters);
+            var fast = system.FromSysExpression().CompileFast(true);
+            if (!t.IsNotNull(fast)) return;
+            var arguments = Enumerable.Range(0, count).Select(i => (object)i).ToArray();
+            t.AreEqual(count - 1, fast.DynamicInvoke(arguments));
+            t.AreEqual(system.Compile().DynamicInvoke(arguments), fast.DynamicInvoke(arguments));
+        }
+    }
+}