Fix NullReferenceException in takeWhilePlusLast with single-element sequences

[Copilot]

Using Implies with Replay threw NullReferenceException when the condition evaluated to false, as Replay mode produces single-element sequences that exposed a bug in takeWhilePlusLast.

Root Cause

takeWhilePlusLast had independent conditionals for MoveNext() and the predicate check:

if en.MoveNext() then
    yield en.Current
if predicate en.Current then  // ❌ executes even when MoveNext() = false
    yield! loop en

After the sequence exhausts, MoveNext() returns false, but the code still evaluates predicate en.Current, accessing the enumerator in an undefined state.

Changes

• Internals.Common.fs: Nested predicate check inside MoveNext() conditional to ensure en.Current is only accessed when valid
• Runner.fs: Moved OnArguments invocation into Passed/Failed branches (rejected tests have no meaningful arguments to report)
• PropertyExamples.fs: Added regression test for Implies with Replay

The bug manifested specifically with Replay because it forces single-test execution, guaranteeing the sequence exhausts on the first iteration.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Implies leads to target invokation exception</issue_title>
<issue_description>Hi, i think there is a problem with Implies method.

When used together with the Replay functionality A.Implies(B) leads to a TargetInvokationException if A is false.

Steps to reproduce:

• Create an NUnit Test Project
• Create a Property:

   [Property(Replay="(123, 127, 123")]
   public Property MinimalExampleProperty1()
   {
       return false.Implies(true);
   }

System.Reflection.TargetInvocationException : Exception has been thrown by the target of an invocation.
----> System.NullReferenceException : Object reference not set to an instance of an object.
at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span1 copyOfArgs, BindingFlags invokeAttr) at System.Reflection.MethodBaseInvoker.InvokeWithFewArgs(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture) at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters) at FsCheck.Runner.checkMethod(Config config, MethodInfo m, FSharpOption1 target) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Runner.fs:line 600
at FsCheck.Check.Method(Config config, MethodInfo methodInfo, FSharpOption1 target) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Runner.fs:line 673 at FsCheck.NUnit.FsCheckTestMethod.RunTestMethod(TestExecutionContext context, TestResult testResult) in C:\Users\kurts\Projects\FsCheck\src\FsCheck.NUnit\FsCheckPropertyAttribute.fs:line 275 at FsCheck.NUnit.FsCheckTestMethod.RunTestCase(TestExecutionContext context, TestResult testResult) in C:\Users\kurts\Projects\FsCheck\src\FsCheck.NUnit\FsCheckPropertyAttribute.fs:line 229 --NullReferenceException at FsCheck.Runner.runner@432-1.Invoke(TestStep step) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Runner.fs:line 433 at FsCheck.Internals.Common.loop@51-1.GenerateNext(IEnumerable1& next) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Internals.Common.fs:line 54
at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase1.MoveNextImpl() in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\seqcore.fs:line 368 at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase1.System.Collections.IEnumerator.MoveNext() in D:\workspace_work\1\s\src\fsharp\FSharp.Core\seqcore.fs:line 400
at FsCheck.Internals.Common.takeWhilePlusLast@59.GenerateNext(IEnumerable1& next) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Internals.Common.fs:line 60 at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase1.MoveNextImpl() in D:\workspace_work\1\s\src\fsharp\FSharp.Core\seqcore.fs:line 368
at Microsoft.FSharp.Core.CompilerServices.GeneratedSequenceBase1.System.Collections.IEnumerator.MoveNext() in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\seqcore.fs:line 400 at Microsoft.FSharp.Collections.SeqModule.Fold[T,TState](FSharpFunc2 folder, TState state, IEnumerable1 source) in D:\workspace\_work\1\s\src\fsharp\FSharp.Core\seq.fs:line 730 at FsCheck.Runner.runner[a](Config config, a prop) in C:\Users\kurts\Projects\FsCheck\src\FsCheck\Runner.fs:line 415 at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor) at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span1 copyOfArgs, BindingFlags invokeAttr)

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One or more child tests had errors
Exception doesn't have a stacktrace

I have also created a minimal example

https://github.com/marha333/FsCheckTests

Any Help is appreciated,

Kind Regards

Harry

</issue_description>

<agent_instructions>Add a repro of this issue to FsCheck.NUnit, debug and fix it.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Implies leads to target invokation exception #726

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