Only enforce Member/NotNullWhen on analyzable returned expressions (#48425)

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -611,18 +611,50 @@ void enforceMemberNotNullWhenForPendingReturn(PendingBranch pendingReturn, Bound
             {
                 if (pendingReturn.IsConditionalState)
                 {
-                    enforceMemberNotNullWhen(returnStatement.Syntax, sense: true, pendingReturn.StateWhenTrue);
-                    enforceMemberNotNullWhen(returnStatement.Syntax, sense: false, pendingReturn.StateWhenFalse);
+                    if (returnStatement.ExpressionOpt is { ConstantValue: { IsBoolean: true, BooleanValue: bool value } })
+                    {
+                        enforceMemberNotNullWhen(returnStatement.Syntax, sense: value, pendingReturn.State);
+                        return;
+                    }
+
+                    if (!pendingReturn.StateWhenTrue.Reachable || !pendingReturn.StateWhenFalse.Reachable)
+                    {
+                        return;
+                    }
+
+                    if (_symbol is MethodSymbol method)
+                    {
+                        foreach (var memberName in method.NotNullWhenTrueMembers)
+                        {
+                            enforceMemberNotNullWhenIfAffected(returnStatement.Syntax, sense: true, method.ContainingType.GetMembers(memberName), pendingReturn.StateWhenTrue, pendingReturn.StateWhenFalse);
+                        }
+
+                        foreach (var memberName in method.NotNullWhenFalseMembers)
+                        {
+                            enforceMemberNotNullWhenIfAffected(returnStatement.Syntax, sense: false, method.ContainingType.GetMembers(memberName), pendingReturn.StateWhenFalse, pendingReturn.StateWhenTrue);
+                        }
+                    }
+                }
+                else if (returnStatement.ExpressionOpt is { ConstantValue: { IsBoolean: true, BooleanValue: bool value } })
+                {
+                    enforceMemberNotNullWhen(returnStatement.Syntax, sense: value, pendingReturn.State);
                 }
             }
 
-            void enforceMemberNotNullWhen(SyntaxNode? syntaxOpt, bool sense, LocalState state)
+            void enforceMemberNotNullWhenIfAffected(SyntaxNode? syntaxOpt, bool sense, ImmutableArray<Symbol> members, LocalState state, LocalState otherState)
             {
-                if (!state.Reachable)
+                foreach (var member in members)
                 {
-                    return;
+                    // For non-constant values, only complain if we were able to analyze a difference for this member between two branches
+                    if (memberHasBadState(member, state) != memberHasBadState(member, otherState))
+                    {
+                        reportMemberIfBadConditionalState(syntaxOpt, sense, member, state);
+                    }
                 }
+            }
 
+            void enforceMemberNotNullWhen(SyntaxNode? syntaxOpt, bool sense, LocalState state)
+            {
                 if (_symbol is MethodSymbol method)
                 {
                     var notNullMembers = sense ? method.NotNullWhenTrueMembers : method.NotNullWhenFalseMembers;
@@ -630,16 +662,21 @@ void enforceMemberNotNullWhen(SyntaxNode? syntaxOpt, bool sense, LocalState stat
                     {
                         foreach (var member in method.ContainingType.GetMembers(memberName))
                         {
-                            if (memberHasBadState(member, state))
-                            {
-                                // Member '{name}' must have a non-null value when exiting with '{sense}'.
-                                Diagnostics.Add(ErrorCode.WRN_MemberNotNullWhen, syntaxOpt?.GetLocation() ?? methodMainNode.Syntax.GetLastToken().GetLocation(), member.Name, sense ? "true" : "false");
-                            }
+                            reportMemberIfBadConditionalState(syntaxOpt, sense, member, state);
                         }
                     }
                 }
             }
 
+            void reportMemberIfBadConditionalState(SyntaxNode? syntaxOpt, bool sense, Symbol member, LocalState state)
+            {
+                if (memberHasBadState(member, state))
+                {
+                    // Member '{name}' must have a non-null value when exiting with '{sense}'.
+                    Diagnostics.Add(ErrorCode.WRN_MemberNotNullWhen, syntaxOpt?.GetLocation() ?? methodMainNode.Syntax.GetLastToken().GetLocation(), member.Name, sense ? "true" : "false");
+                }
+            }
+
             bool memberHasBadState(Symbol member, LocalState state)
             {
                 switch (member.Kind)
@@ -767,12 +804,45 @@ void enforceNotNullWhenForPendingReturn(PendingBranch pendingReturn, BoundReturn
                 {
                     if (pendingReturn.IsConditionalState)
                     {
-                        enforceParameterNotNullWhen(returnStatement.Syntax, parameters, sense: true, stateWhen: pendingReturn.StateWhenTrue);
-                        enforceParameterNotNullWhen(returnStatement.Syntax, parameters, sense: false, stateWhen: pendingReturn.StateWhenFalse);
+                        if (returnStatement.ExpressionOpt is { ConstantValue: { IsBoolean: true, BooleanValue: bool value } })
+                        {
+                            enforceParameterNotNullWhen(returnStatement.Syntax, parameters, sense: value, stateWhen: pendingReturn.State);
+                            return;
+                        }
+
+                        if (!pendingReturn.StateWhenTrue.Reachable || !pendingReturn.StateWhenFalse.Reachable)
+                        {
+                            return;
+                        }
+
+                        foreach (var parameter in parameters)
+                        {
+                            // For non-constant values, only complain if we were able to analyze a difference for this parameter between two branches
+                            if (GetOrCreateSlot(parameter) is > 0 and var slot && pendingReturn.StateWhenTrue[slot] != pendingReturn.StateWhenFalse[slot])
+                            {
+                                reportParameterIfBadConditionalState(returnStatement.Syntax, parameter, sense: true, stateWhen: pendingReturn.StateWhenTrue);
+                                reportParameterIfBadConditionalState(returnStatement.Syntax, parameter, sense: false, stateWhen: pendingReturn.StateWhenFalse);
+                            }
+                        }
+                    }
+                    else if (returnStatement.ExpressionOpt is { ConstantValue: { IsBoolean: true, BooleanValue: bool value } })
+                    {
+                        // example: return (bool)true;
+                        enforceParameterNotNullWhen(returnStatement.Syntax, parameters, sense: value, stateWhen: pendingReturn.State);
+                        return;
                     }
                 }
             }
 
+            void reportParameterIfBadConditionalState(SyntaxNode syntax, ParameterSymbol parameter, bool sense, LocalState stateWhen)
+            {
+                if (parameterHasBadConditionalState(parameter, sense, stateWhen))
+                {
+                    // Parameter '{name}' must have a non-null value when exiting with '{sense}'.
+                    Diagnostics.Add(ErrorCode.WRN_ParameterConditionallyDisallowsNull, syntax.Location, parameter.Name, sense ? "true" : "false");
+                }
+            }
+
             void enforceNotNull(SyntaxNode? syntaxOpt, LocalState state)
             {
                 if (!state.Reachable)
@@ -812,11 +882,7 @@ void enforceParameterNotNullWhen(SyntaxNode syntax, ImmutableArray<ParameterSymb
 
                 foreach (var parameter in parameters)
                 {
-                    if (parameterHasBadConditionalState(parameter, sense, stateWhen))
-                    {
-                        // Parameter '{name}' must have a non-null value when exiting with '{sense}'.
-                        Diagnostics.Add(ErrorCode.WRN_ParameterConditionallyDisallowsNull, syntax.Location, parameter.Name, sense ? "true" : "false");
-                    }
+                    reportParameterIfBadConditionalState(syntax, parameter, sense, stateWhen);
                 }
             }
 
@@ -2428,6 +2494,7 @@ private bool TryGetReturnType(out TypeWithAnnotations type, out FlowAnalysisAnno
             }
 
             SetResult(node, GetAdjustedResult(type.ToTypeWithState(), slot), type);
+            SplitIfBooleanConstant(node);
             return null;
         }
 
@@ -8152,9 +8219,27 @@ private TypeWithAnnotations GetDeclaredParameterResult(ParameterSymbol parameter
             return null;
         }
 
+        private void SplitIfBooleanConstant(BoundExpression node)
+        {
+            if (node.ConstantValue is { IsBoolean: true, BooleanValue: bool booleanValue })
+            {
+                Split();
+                if (booleanValue)
+                {
+                    StateWhenFalse = UnreachableState();
+                }
+                else
+                {
+                    StateWhenTrue = UnreachableState();
+                }
+            }
+        }
+
         public override BoundNode? VisitFieldAccess(BoundFieldAccess node)
         {
             var updatedSymbol = VisitMemberAccess(node, node.ReceiverOpt, node.FieldSymbol);
+
+            SplitIfBooleanConstant(node);
             SetUpdatedSymbol(node, node.FieldSymbol, updatedSymbol);
             return null;
         }
@@ -8614,8 +8699,9 @@ public override void VisitForEachIterationVariables(BoundForEachStatement node)
             switch (node.OperatorKind)
             {
                 case UnaryOperatorKind.BoolLogicalNegation:
-                    VisitCondition(node.Operand);
-                    SetConditionalState(StateWhenFalse, StateWhenTrue);
+                    Visit(node.Operand);
+                    if (IsConditionalState)
+                        SetConditionalState(StateWhenFalse, StateWhenTrue);
                     resultType = adjustForLifting(ResultType);
                     break;
                 case UnaryOperatorKind.DynamicTrue:
@@ -8968,19 +9054,7 @@ private TypeWithState InferResultNullabilityOfBinaryLogicalOperator(BoundExpress
             Debug.Assert(!IsConditionalState);
             SetResultType(node, TypeWithState.Create(node.Type, node.Type?.CanContainNull() != false && node.ConstantValue?.IsNull == true ? NullableFlowState.MaybeDefault : NullableFlowState.NotNull));
 
-            if (node.ConstantValue?.IsBoolean == true)
-            {
-                Split();
-                if (node.ConstantValue.BooleanValue)
-                {
-                    StateWhenFalse = UnreachableState();
-                }
-                else
-                {
-                    StateWhenTrue = UnreachableState();
-                }
-            }
-
+            SplitIfBooleanConstant(node);
             return result;
         }
 

src/Compilers/CSharp/Test/Semantic/Semantics/NullableReferenceTypesTests.cs

@@ -24076,6 +24076,149 @@ bool Init()
                 );
         }
 
+        [Fact]
+        public void MemberNotNullWhenTrue_EnforcedInMethodBody_NonConstant()
+        {
+            var c = CreateNullableCompilation(new[] { @"
+using System.Diagnostics.CodeAnalysis;
+public class C
+{
+    public string field1;
+    public string? field2;
+    public string field3;
+    public string? field4;
+
+    C() // 1
+    {
+        if (!Init()) throw null!;
+    }
+
+    [MemberNotNullWhen(true, nameof(field1), nameof(field2))]
+    bool Init()
+    {
+        bool b = true;
+        if (b)
+        {
+            return b;
+        }
+        if (b)
+        {
+            return !b;
+        }
+        return M(out _);
+    }
+
+    bool M([MaybeNullWhen(true)]out string s) => throw null!;
+}
+", MemberNotNullWhenAttributeDefinition, MaybeNullWhenAttributeDefinition });
+
+            c.VerifyDiagnostics(
+                // (10,5): warning CS8618: Non-nullable field 'field3' is uninitialized. Consider declaring the field as nullable.
+                //     C() // 1
+                Diagnostic(ErrorCode.WRN_UninitializedNonNullableField, "C").WithArguments("field", "field3").WithLocation(10, 5)
+                );
+        }
+
+        [Fact]
+        public void MemberNotNullWhenTrue_EnforcedInMethodBody_NonConstant_ButAnalyzable()
+        {
+            var c = CreateNullableCompilation(new[] { @"
+using System.Diagnostics.CodeAnalysis;
+public class C
+{
+    public string field1;
+    public string field3;
+
+    C()
+    {
+        if (!Init()) throw null!;
+    }
+
+    [MemberNotNullWhen(true, nameof(field1), nameof(field3))]
+    bool Init()
+    {
+        bool b = true;
+        if (b)
+        {
+            return field1 == null; // 1
+        }
+        if (b)
+        {
+            return field1 != null;
+        }
+        if (b)
+        {
+            return Init();
+        }
+        return !Init(); // 2, 3
+    }
+}
+", MemberNotNullWhenAttributeDefinition, MaybeNullWhenAttributeDefinition });
+
+            c.VerifyDiagnostics(
+                // (19,13): warning CS8775: Member 'field1' must have a non-null value when exiting with 'true'.
+                //             return field1 == null; // 1
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return field1 == null;").WithArguments("field1", "true").WithLocation(19, 13),
+                // (29,9): warning CS8775: Member 'field1' must have a non-null value when exiting with 'true'.
+                //         return !Init(); // 2, 3
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return !Init();").WithArguments("field1", "true").WithLocation(29, 9),
+                // (29,9): warning CS8775: Member 'field3' must have a non-null value when exiting with 'true'.
+                //         return !Init(); // 2, 3
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return !Init();").WithArguments("field3", "true").WithLocation(29, 9)
+                );
+        }
+
+        [Fact]
+        public void MemberNotNullWhenFalse_EnforcedInMethodBody_NonConstant_ButAnalyzable()
+        {
+            var c = CreateNullableCompilation(new[] { @"
+using System.Diagnostics.CodeAnalysis;
+public class C
+{
+    public string field1;
+    public string field3;
+
+    C()
+    {
+        if (Init()) throw null!;
+    }
+
+    [MemberNotNullWhen(false, nameof(field1), nameof(field3))]
+    bool Init()
+    {
+        bool b = true;
+        if (b)
+        {
+            return field1 == null;
+        }
+        if (b)
+        {
+            return field1 != null; // 1
+        }
+        if (b)
+        {
+            return Init();
+        }
+        return !Init(); // 2, 3
+    }
+
+    bool M([MaybeNullWhen(true)]out string s) => throw null!;
+}
+", MemberNotNullWhenAttributeDefinition, MaybeNullWhenAttributeDefinition });
+
+            c.VerifyDiagnostics(
+                // (23,13): warning CS8775: Member 'field1' must have a non-null value when exiting with 'false'.
+                //             return field1 != null; // 1
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return field1 != null;").WithArguments("field1", "false").WithLocation(23, 13),
+                // (29,9): warning CS8775: Member 'field1' must have a non-null value when exiting with 'false'.
+                //         return !Init(); // 2, 3
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return !Init();").WithArguments("field1", "false").WithLocation(29, 9),
+                // (29,9): warning CS8775: Member 'field3' must have a non-null value when exiting with 'false'.
+                //         return !Init(); // 2, 3
+                Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return !Init();").WithArguments("field3", "false").WithLocation(29, 9)
+                );
+        }
+
         [Fact]
         public void MemberNotNullWhenTrue_WithMemberNotNull()
         {
@@ -24315,7 +24458,7 @@ bool IsInit2
 ", MemberNotNullWhenAttributeDefinition }, parseOptions: TestOptions.Regular9);
 
             c.VerifyDiagnostics(
-                // (21,13): error CS8765: Member 'field2' must have a non-null value when exiting with 'true'.
+                // (21,13): warning CS8775: Member 'field2' must have a non-null value when exiting with 'true'.
                 //             return field2 == null; // 1
                 Diagnostic(ErrorCode.WRN_MemberNotNullWhen, "return field2 == null;").WithArguments("field2", "true").WithLocation(21, 13)
                 );
@@ -25084,6 +25227,23 @@ public static bool TryGetValue([NotNullWhen(true)] out string? s)
             comp.VerifyDiagnostics();
         }
 
+        [Fact]
+        public void NotNullWhenTrue_EnforceInMethodBody_ConditionalWithThrow()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+public class C
+{
+    static bool M([NotNullWhen(true)] object? o)
+    {
+        return o == null ? true : throw null!;
+    }
+}
+";
+            var comp = CreateNullableCompilation(new[] { source, NotNullWhenAttributeDefinition });
+            comp.VerifyDiagnostics();
+        }
+
         [Fact]
         public void NotNullWhenTrue_EnforceInMethodBody_WithMaybeNull_CallingObliviousAPI()
         {
@@ -25489,35 +25649,39 @@ public void NotNullWhenTrue_EnforceInMethodBody_OnConversionToBool()
 using System.Diagnostics.CodeAnalysis;
 public class C
 {
+    public static implicit operator bool(C? c) => throw null!;
+
     public static bool TryGetValue(C? c, [NotNullWhen(true)] out string? s)
     {
         s = null;
-        return c; // 1
+        return c;
     }
 
     public static bool TryGetValue2(C c, [NotNullWhen(false)] out string? s)
     {
         s = null;
-        return c; // 2
+        return c;
     }
 
-    public static implicit operator bool(C? c) => throw null!;
-
     static bool TryGetValue3([MaybeNullWhen(false)]out string s)
     {
         s = null;
-        return (bool)true; // 3
+        return (bool)true; // 1
     }
 
     static bool TryGetValue4([MaybeNullWhen(false)]out string s)
     {
         s = null;
-        return (bool)false; // 4
+        return (bool)false;
     }
 }
 ";
             var comp = CreateNullableCompilation(new[] { source, NotNullWhenAttributeDefinition, MaybeNullWhenAttributeDefinition });
-            comp.VerifyDiagnostics();
+            comp.VerifyDiagnostics(
+                // (22,9): warning CS8762: Parameter 's' must have a non-null value when exiting with 'true'.
+                //         return (bool)true;
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return (bool)true;").WithArguments("s", "true").WithLocation(22, 9)
+                );
         }
 
         [Fact]
@@ -37294,10 +37458,10 @@ static bool TryGetValue4([AllowNull] TYPE x, [MaybeNullWhen(false)] out TYPE y)
 ";
             var comp = CreateNullableCompilation(new[] { AllowNullAttributeDefinition, MaybeNullAttributeDefinition, MaybeNullWhenAttributeDefinition, source.Replace("TYPE", type) });
             comp.VerifyDiagnostics(
-                // (24,9): error CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
+                // (24,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
                 //         return y != null; // 1
                 Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y != null;").WithArguments("y", "false").WithLocation(24, 9),
-                // (30,9): error CS8762: Parameter 'y' must have a non-null value when exiting with 'true'.
+                // (30,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'true'.
                 //         return y == null; // 2
                 Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y == null;").WithArguments("y", "true").WithLocation(30, 9)
                 );
@@ -37387,7 +37551,7 @@ static bool TryGetValue<T>([AllowNull]T x, [MaybeNullWhen(true)]out T y, [MaybeN
     {
         y = x;
         z = x;
-        return y != null || z != null; // 1, 2
+        return y != null || z != null;
     }
 
     static bool TryGetValue2<T>([AllowNull]T x, [MaybeNullWhen(false)]out T y, [MaybeNullWhen(false)]out T z)
@@ -37399,14 +37563,7 @@ static bool TryGetValue2<T>([AllowNull]T x, [MaybeNullWhen(false)]out T y, [Mayb
 }
 ";
             var comp = CreateNullableCompilation(new[] { AllowNullAttributeDefinition, MaybeNullWhenAttributeDefinition, source });
-            comp.VerifyDiagnostics(
-                // (12,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
-                //         return y != null || z != null; // 1, 2
-                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y != null || z != null;").WithArguments("y", "false").WithLocation(12, 9),
-                // (12,9): warning CS8762: Parameter 'z' must have a non-null value when exiting with 'false'.
-                //         return y != null || z != null; // 1, 2
-                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y != null || z != null;").WithArguments("z", "false").WithLocation(12, 9)
-                );
+            comp.VerifyDiagnostics();
         }
 
         [Theory]
@@ -37440,20 +37597,32 @@ static bool TryGetValue2([NotNullWhen(true)] out TYPE y)
         return y != null;
     }
 
+    static bool TryGetValue2B([NotNullWhen(true)] out TYPE y)
+    {
+        y = null;
+        return y == null; // 2
+    }
+
     static bool TryGetValue3([NotNullWhen(false)] out TYPE y)
     {
         y = null;
         return y == null;
     }
 
+    static bool TryGetValue3B([NotNullWhen(false)] out TYPE y)
+    {
+        y = null;
+        return y != null; // 3
+    }
+
     static bool TryGetValue4([NotNull] TYPE x, [NotNullWhen(false)] out TYPE y)
     {
         y = null;
         if (y != null)
         {
-            return true; // 2
+            return true; // 4
         }
-        return false; // 3, 4
+        return false; // 5, 6
     }
 }
 ";
@@ -37462,15 +37631,21 @@ static bool TryGetValue4([NotNull] TYPE x, [NotNullWhen(false)] out TYPE y)
                 // (15,13): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'true'.
                 //             return true; // 1
                 Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return true;").WithArguments("y", "true").WithLocation(15, 13),
-                // (37,13): warning CS8777: Parameter 'x' must have a non-null value when exiting.
-                //             return true; // 2
-                Diagnostic(ErrorCode.WRN_ParameterDisallowsNull, "return true;").WithArguments("x").WithLocation(37, 13),
-                // (39,9): warning CS8777: Parameter 'x' must have a non-null value when exiting.
-                //         return false; // 3, 4
-                Diagnostic(ErrorCode.WRN_ParameterDisallowsNull, "return false;").WithArguments("x").WithLocation(39, 9),
-                // (39,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
-                //         return false; // 3, 4
-                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return false;").WithArguments("y", "false").WithLocation(39, 9)
+                // (29,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'true'.
+                //         return y == null; // 2
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y == null;").WithArguments("y", "true").WithLocation(29, 9),
+                // (41,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
+                //         return y != null; // 3
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return y != null;").WithArguments("y", "false").WithLocation(41, 9),
+                // (49,13): warning CS8777: Parameter 'x' must have a non-null value when exiting.
+                //             return true; // 4
+                Diagnostic(ErrorCode.WRN_ParameterDisallowsNull, "return true;").WithArguments("x").WithLocation(49, 13),
+                // (51,9): warning CS8777: Parameter 'x' must have a non-null value when exiting.
+                //         return false; // 5, 6
+                Diagnostic(ErrorCode.WRN_ParameterDisallowsNull, "return false;").WithArguments("x").WithLocation(51, 9),
+                // (51,9): warning CS8762: Parameter 'y' must have a non-null value when exiting with 'false'.
+                //         return false; // 5, 6
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return false;").WithArguments("y", "false").WithLocation(51, 9)
                 );
         }
 
@@ -140237,6 +140412,183 @@ void M()
                 Diagnostic(ErrorCode.ERR_DiscardTypeInferenceFailed, "_").WithLocation(10, 13));
         }
 
+        [Fact, WorkItem(48126, "https://github.com/dotnet/roslyn/issues/48126")]
+        public void EnforcedInMethodBody_MaybeNullWhen_Issue_48126()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
+class C
+{
+    static bool NullWhenFalseA(bool a, [MaybeNullWhen(false)] out string s1)
+    {
+        s1 = null;
+        return a;
+    }
+
+    static bool NullWhenFalseNotA(bool a, [MaybeNullWhen(false)] out string s1)
+    {
+        s1 = null;
+        return !a;
+    }
+
+    static bool NullWhenTrueA(bool a, [MaybeNullWhen(true)] out string s1)
+    {
+        s1 = null;
+        return a;
+    }
+
+    static bool NullWhenTrueNotA(bool a, [MaybeNullWhen(true)] out string s1)
+    {
+        s1 = null;
+        return !a;
+    }
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullWhenAttributeDefinition });
+            comp.VerifyEmitDiagnostics();
+        }
+
+        [Fact, WorkItem(48126, "https://github.com/dotnet/roslyn/issues/48126")]
+        public void EnforcedInMethodBody_MaybeNullWhen_Issue_48126_2()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
+class C
+{
+    static bool M1([MaybeNullWhen(false)] out string s1)
+    {
+        const bool b = true;
+        s1 = null;
+        return b; // 1
+    }
+
+    static bool M2([MaybeNullWhen(false)] out string s1)
+    {
+        s1 = null;
+        return (bool)true; // 2
+    }
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullWhenAttributeDefinition });
+            comp.VerifyEmitDiagnostics(
+                // (12,9): warning CS8762: Parameter 's1' must have a non-null value when exiting with 'true'.
+                //         return b; // 1
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return b;").WithArguments("s1", "true").WithLocation(12, 9),
+                // (18,9): warning CS8762: Parameter 's1' must have a non-null value when exiting with 'true'.
+                //         return (bool)true; // 2
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return (bool)true;").WithArguments("s1", "true").WithLocation(18, 9)
+                );
+        }
+
+        [Fact, WorkItem(48126, "https://github.com/dotnet/roslyn/issues/48126")]
+        public void EnforcedInMethodBody_MaybeNullWhen_Issue_48126_3()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
+class C
+{
+    static bool M1([MaybeNullWhen(false)] out string s1)
+    {
+        const bool b = true;
+        s1 = null;
+        return b; // 1
+    }
+
+    static bool M2([MaybeNullWhen(false)] out string s1)
+    {
+        const bool b = true;
+        s1 = null;
+        return b && true; // 2
+    }
+
+    static bool M3([MaybeNullWhen(false)] out string s1)
+    {
+        const bool b = false;
+        s1 = null;
+        return b;
+    }
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullWhenAttributeDefinition });
+            comp.VerifyEmitDiagnostics(
+                // (12,9): warning CS8762: Parameter 's1' must have a non-null value when exiting with 'true'.
+                //         return b; // 1
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return b;").WithArguments("s1", "true").WithLocation(12, 9),
+                // (19,9): warning CS8762: Parameter 's1' must have a non-null value when exiting with 'true'.
+                //         return b && true; // 2
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return b && true;").WithArguments("s1", "true").WithLocation(19, 9)
+                );
+        }
+
+        [Fact, WorkItem(48126, "https://github.com/dotnet/roslyn/issues/48126")]
+        public void EnforcedInMethodBody_MaybeNullWhen_Issue_48126_4()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
+class C
+{
+    static bool M1([MaybeNullWhen(false)] out string s1)
+    {
+        return M1(out s1);
+    }
+
+    static bool M2([MaybeNullWhen(false)] out string s1)
+    {
+        return !M1(out s1); // 1
+    }
+
+    static bool M3([MaybeNullWhen(true)] out string s1)
+    {
+        return !M1(out s1);
+    }
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullWhenAttributeDefinition });
+            comp.VerifyEmitDiagnostics(
+                // (15,9): warning CS8762: Parameter 's1' must have a non-null value when exiting with 'true'.
+                //         return !M1(out s1); // 1
+                Diagnostic(ErrorCode.WRN_ParameterConditionallyDisallowsNull, "return !M1(out s1);").WithArguments("s1", "true").WithLocation(15, 9)
+                );
+        }
+
+        [Fact, WorkItem(48126, "https://github.com/dotnet/roslyn/issues/48126")]
+        public void EnforcedInMethodBody_MaybeNullWhen_Issue_48126_5()
+        {
+            var source = @"
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
+internal static class Program
+{
+    public static bool M1([MaybeNullWhen(true)] out string s)
+    {
+        s = null;
+        return HasAnnotation(out _);
+    }
+
+    public static bool M2([MaybeNullWhen(true)] out string s)
+    {
+        s = null;
+        return NoAnnotations();
+    }
+
+    private static bool HasAnnotation([MaybeNullWhen(true)] out string s) { s = null; return true; }
+
+    private static bool NoAnnotations() => true;
+}";
+            var comp = CreateCompilation(new[] { source, MaybeNullWhenAttributeDefinition });
+            comp.VerifyEmitDiagnostics();
+        }
+
         [Fact]
         [WorkItem(47221, "https://github.com/dotnet/roslyn/issues/47221")]
         public void PropertyAccessorWithNullableContextAttribute_01()