fix(mocks): Is<T>(predicate) passes T, not T?, for non-nullable T

.editorconfig

@@ -148,3 +148,7 @@ indent_style = unset
 insert_final_newline = false
 tab_width = unset
 trim_trailing_whitespace = false
+
+# Keep the predicate nullability regression a compile-time failure (#7304).
+[tests/TUnit.Mocks.Tests/PredicateNullabilityTests.cs]
+dotnet_diagnostic.CS8602.severity = error

docs/docs/writing-tests/mocking/argument-matchers.md

@@ -111,6 +111,10 @@ mock.GetUser(Is<int>(id => id > 0)).Returns(new User("Valid"));
 mock.GetUser(Is<int>(id => id <= 0)).Throws<ArgumentException>();
 ```
 
+For a non-nullable reference type, the predicate receives `T`, not `T?`, so `Is<User>(u => u.Name == "Alice")` needs no null guard. Use `Is<User?>(u => ...)` when the predicate should handle null itself, including when matching a parameter declared as `User?`.
+
+Nullability annotations do not distinguish `Is<User>` from `Is<User?>` at runtime: both pass null to the predicate. A predicate that explicitly accepts null can match it in either form. All predicate exceptions propagate, including `NullReferenceException` when a null argument is dereferenced. Use an explicit null guard, such as `Is<User?>(u => u is not null && u.Name == "Alice")`, when null should be a non-match.
+
 ### Inline Lambda Predicates
 
 You can write lambda predicates directly in mock setup and verification calls — no `Is<T>()` wrapper needed:

src/TUnit.Mocks/Arguments/Arg.cs

@@ -26,7 +26,14 @@ public static class Arg
     public static Arg<T> Is<T>(T value) => new(new ExactMatcher<T>(value));
 
     /// <summary>Matches when the predicate returns true for the actual argument.</summary>
-    public static Arg<T> Is<T>(Func<T?, bool> predicate) => new(new PredicateMatcher<T>(predicate));
+    /// <remarks>
+    /// For a non-nullable reference type, the predicate receives <typeparamref name="T"/>, so it can
+    /// dereference its argument without a null guard. Use <c>Is&lt;T?&gt;(...)</c> when the predicate
+    /// should handle a null argument itself. Both forms pass null to the predicate at runtime.
+    /// Predicate exceptions propagate, including when the argument is null. Use an explicit null
+    /// guard if null should be a non-match.
+    /// </remarks>
+    public static Arg<T> Is<T>(Func<T, bool> predicate) => new(new PredicateMatcher<T>(predicate));
 
     /// <summary>Matches only when the argument is null. Supports reference types and nullable value types.</summary>
     public static Arg<T> IsNull<T>() => new(NullMatcher<T>.Instance);

src/TUnit.Mocks/Arguments/ArgOfT.cs

@@ -41,7 +41,7 @@ public Arg(IArgumentMatcher matcher)
 
     /// <summary>Implicitly converts a predicate to an <see cref="Arg{T}"/> using predicate matching.</summary>
     /// <param name="predicate">The predicate that determines whether an argument matches.</param>
-    public static implicit operator Arg<T>(Func<T, bool> predicate) => new(new PredicateMatcher<T>(predicate!));
+    public static implicit operator Arg<T>(Func<T, bool> predicate) => new(new PredicateMatcher<T>(predicate));
 
     public static implicit operator Arg<T>(AnyArg _) => new(AnyMatcher<T>.Instance);
 

src/TUnit.Mocks/Matchers/PredicateMatcher.cs

@@ -7,14 +7,14 @@ namespace TUnit.Mocks.Matchers;
 /// </summary>
 internal sealed class PredicateMatcher<T> : IArgumentMatcher<T>
 {
-    private readonly Func<T?, bool> _predicate;
+    private readonly Func<T, bool> _predicate;
 
-    public PredicateMatcher(Func<T?, bool> predicate)
+    public PredicateMatcher(Func<T, bool> predicate)
     {
         _predicate = predicate ?? throw new ArgumentNullException(nameof(predicate));
     }
 
-    public bool Matches(T? value) => _predicate(value);
+    public bool Matches(T? value) => _predicate(value!);
 
     public bool Matches(object? value)
     {

tests/TUnit.Mocks.Tests/PredicateNullabilityTests.cs

@@ -0,0 +1,92 @@
+using TUnit.Mocks;
+using TUnit.Mocks.Arguments;
+
+namespace TUnit.Mocks.Tests;
+
+public record PredicateUser(string Name);
+
+public interface IPredicateUserRepo
+{
+    string Save(PredicateUser user);
+}
+
+public interface INullablePredicateUserRepo
+{
+    string Save(PredicateUser? user);
+}
+
+/// <summary>
+/// Regression tests for #7304: <c>Is&lt;T&gt;(predicate)</c> passes <c>T</c> (not <c>T?</c>) for a
+/// non-nullable reference type, so predicates can dereference their argument without a null guard.
+/// </summary>
+public class PredicateNullabilityTests
+{
+    [Test]
+    public async Task Predicate_For_NonNullable_Reference_Type_Dereferences_Without_Null_Guard()
+    {
+        var mock = IPredicateUserRepo.Mock();
+        mock.Save(Is<PredicateUser>(u => u.Name == "Alice")).Returns("alice");
+
+        IPredicateUserRepo repo = mock.Object;
+
+        await Assert.That(repo.Save(new PredicateUser("Alice"))).IsEqualTo("alice");
+        await Assert.That(repo.Save(new PredicateUser("Bob"))).IsNotEqualTo("alice");
+    }
+
+    [Test]
+    public async Task Null_Argument_Dereference_Propagates_Exception()
+    {
+        var mock = IPredicateUserRepo.Mock();
+        mock.Save(Is<PredicateUser>(u => u.Name == "Alice")).Returns("alice");
+
+        IPredicateUserRepo repo = mock.Object;
+
+        await Assert.That(() => repo.Save(null!)).Throws<NullReferenceException>();
+    }
+
+    [Test]
+    public async Task Null_Argument_Does_Not_Match_Explicit_Null_Guard()
+    {
+        var mock = INullablePredicateUserRepo.Mock();
+        mock.Save(Is<PredicateUser?>(u => u is not null && u.Name == "Alice")).Returns("alice");
+
+        INullablePredicateUserRepo repo = mock.Object;
+
+        await Assert.That(repo.Save(null)).IsEqualTo(string.Empty);
+    }
+
+    [Test]
+    public async Task Null_Argument_Preserves_Unrelated_Predicate_Exception()
+    {
+        var mock = INullablePredicateUserRepo.Mock();
+        var failure = new InvalidOperationException("from predicate");
+        mock.Save(Is<PredicateUser?>(u => throw failure)).Returns("never");
+
+        INullablePredicateUserRepo repo = mock.Object;
+
+        await Assert.That(() => repo.Save(null)).Throws<InvalidOperationException>();
+    }
+
+    [Test]
+    public async Task Nullable_Predicate_Can_Match_Null()
+    {
+        var mock = INullablePredicateUserRepo.Mock();
+        mock.Save(Is<PredicateUser?>(u => u is null)).Returns("null user");
+
+        INullablePredicateUserRepo repo = mock.Object;
+
+        await Assert.That(repo.Save(null)).IsEqualTo("null user");
+        await Assert.That(repo.Save(new PredicateUser("Alice"))).IsNotEqualTo("null user");
+    }
+
+    [Test]
+    public async Task Predicate_Exception_For_NonNull_Argument_Still_Propagates()
+    {
+        var mock = IPredicateUserRepo.Mock();
+        mock.Save(Is<PredicateUser>(u => throw new NullReferenceException("from predicate"))).Returns("never");
+
+        IPredicateUserRepo repo = mock.Object;
+
+        await Assert.That(() => repo.Save(new PredicateUser("Alice"))).Throws<NullReferenceException>();
+    }
+}