Check NotNullIfNotNull in implementation

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -5600,6 +5600,18 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="WRN_ParameterDisallowsNull_Title" xml:space="preserve">
     <value>Parameter must have a non-null value when exiting.</value>
   </data>
+  <data name="WRN_ParameterNotNullIfNotNull" xml:space="preserve">
+    <value>Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.</value>
+  </data>
+  <data name="WRN_ParameterNotNullIfNotNull_Title" xml:space="preserve">
+    <value>Parameter must have a non-null value when exiting because parameter referenced by NotNullIfNotNull is non-null.</value>
+  </data>
+  <data name="WRN_ReturnNotNullIfNotNull" xml:space="preserve">
+    <value>Return value must be non-null because parameter '{0}' is non-null.</value>
+  </data>
+  <data name="WRN_ReturnNotNullIfNotNull_Title" xml:space="preserve">
+    <value>Return value must be non-null because parameter is non-null.</value>
+  </data>
   <data name="WRN_MemberNotNull" xml:space="preserve">
     <value>Member '{0}' must have a non-null value when exiting.</value>
   </data>

src/Compilers/CSharp/Portable/Errors/ErrorCode.cs

@@ -1835,6 +1835,8 @@ internal enum ErrorCode
         ERR_OverrideDefaultConstraintNotSatisfied = 8822,
         ERR_DefaultConstraintOverrideOnly = 8823,
         WRN_PartialMethodTypeDifference = 8824,
+        WRN_ParameterNotNullIfNotNull = 8825,
+        WRN_ReturnNotNullIfNotNull = 8826,
 
         ERR_RuntimeDoesNotSupportCovariantReturnsOfClasses = 8830,
         ERR_RuntimeDoesNotSupportCovariantPropertiesOfClasses = 8831,

src/Compilers/CSharp/Portable/Errors/ErrorFacts.cs

@@ -77,6 +77,8 @@ static ErrorFacts()
             nullableWarnings.Add(getId(ErrorCode.WRN_MemberNotNullBadMember));
             nullableWarnings.Add(getId(ErrorCode.WRN_MemberNotNullWhen));
             nullableWarnings.Add(getId(ErrorCode.WRN_ParameterDisallowsNull));
+            nullableWarnings.Add(getId(ErrorCode.WRN_ParameterNotNullIfNotNull));
+            nullableWarnings.Add(getId(ErrorCode.WRN_ReturnNotNullIfNotNull));
 
             NullableWarnings = nullableWarnings.ToImmutable();
 
@@ -480,6 +482,8 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_SwitchExpressionNotExhaustiveWithWhen:
                 case ErrorCode.WRN_SwitchExpressionNotExhaustiveForNullWithWhen:
                 case ErrorCode.WRN_RecordNamedDisallowed:
+                case ErrorCode.WRN_ParameterNotNullIfNotNull:
+                case ErrorCode.WRN_ReturnNotNullIfNotNull:
                     return 1;
                 default:
                     return 0;

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

@@ -469,10 +469,10 @@ protected override ImmutableArray<PendingBranch> Scan(ref bool badRegion)
             {
                 enforceMemberNotNull(syntaxOpt: pendingReturn.Branch.Syntax, pendingReturn.State);
 
-                if (pendingReturn.Branch is BoundReturnStatement { ExpressionOpt: BoundExpression expr } returnStatement)
+                if (pendingReturn.Branch is BoundReturnStatement returnStatement)
                 {
                     enforceNotNull(returnStatement.Syntax, pendingReturn.State);
-                    enforceNotNullWhenForPendingReturn(pendingReturn, expr, returnStatement);
+                    enforceNotNullWhenForPendingReturn(pendingReturn, returnStatement);
                     enforceMemberNotNullWhenForPendingReturn(pendingReturn, returnStatement);
                 }
             }
@@ -757,7 +757,7 @@ void makeMemberMaybeNull(MethodSymbol method, string memberName)
                 }
             }
 
-            void enforceNotNullWhenForPendingReturn(PendingBranch pendingReturn, BoundExpression expr, BoundReturnStatement returnStatement)
+            void enforceNotNullWhenForPendingReturn(PendingBranch pendingReturn, BoundReturnStatement returnStatement)
             {
                 var parameters = this.MethodParameters;
 
@@ -780,11 +780,24 @@ void enforceNotNull(SyntaxNode? syntaxOpt, LocalState state)
 
                 foreach (var parameter in this.MethodParameters)
                 {
-                    if (parameterHasBadState(parameter, state))
+                    var slot = GetOrCreateSlot(parameter);
+                    if (slot <= 0)
+                    {
+                        continue;
+                    }
+
+                    var annotations = parameter.FlowAnalysisAnnotations;
+                    var hasNotNull = (annotations & FlowAnalysisAnnotations.NotNull) == FlowAnalysisAnnotations.NotNull;
+                    var parameterState = state[slot];
+                    if (hasNotNull && parameterState.MayBeNull())
                     {
                         // Parameter '{name}' must have a non-null value when exiting.
                         Diagnostics.Add(ErrorCode.WRN_ParameterDisallowsNull, syntaxOpt?.GetLocation() ?? methodMainNode.Syntax.GetLastToken().GetLocation(), parameter.Name);
                     }
+                    else
+                    {
+                        EnforceNotNullIfNotNull(syntaxOpt, state, this.MethodParameters, parameter.NotNullIfParameterNotNull, parameterState, parameter);
+                    }
                 }
             }
 
@@ -805,19 +818,6 @@ void enforceParameterNotNullWhen(SyntaxNode syntax, ImmutableArray<ParameterSymb
                 }
             }
 
-            bool parameterHasBadState(ParameterSymbol parameter, LocalState state)
-            {
-                var slot = GetOrCreateSlot(parameter);
-                if (slot > 0)
-                {
-                    var annotations = parameter.FlowAnalysisAnnotations;
-                    bool hasNotNull = (annotations & FlowAnalysisAnnotations.NotNull) == FlowAnalysisAnnotations.NotNull;
-                    return hasNotNull && state[slot].MayBeNull();
-                }
-
-                return false;
-            }
-
             bool parameterHasBadConditionalState(ParameterSymbol parameter, bool sense, LocalState stateWhen)
             {
                 var refKind = parameter.RefKind;
@@ -883,6 +883,35 @@ int getSlotForFieldOrPropertyOrEvent(Symbol member)
             }
         }
 
+        private void EnforceNotNullIfNotNull(SyntaxNode? syntaxOpt, LocalState state, ImmutableArray<ParameterSymbol> parameters, ImmutableHashSet<string> inputParamNames, NullableFlowState outputState, ParameterSymbol? outputParam)
+        {
+            if (inputParamNames.IsEmpty || outputState.IsNotNull())
+            {
+                return;
+            }
+
+            foreach (var inputParam in parameters)
+            {
+                if (inputParamNames.Contains(inputParam.Name)
+                    && GetOrCreateSlot(inputParam) is > 0 and int inputSlot
+                    && state[inputSlot].IsNotNull())
+                {
+                    var location = syntaxOpt?.GetLocation() ?? methodMainNode.Syntax.GetLastToken().GetLocation();
+                    if (outputParam is object)
+                    {
+                        // Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.
+                        Diagnostics.Add(ErrorCode.WRN_ParameterNotNullIfNotNull, location, outputParam.Name, inputParam.Name);
+                    }
+                    else
+                    {
+                        // Return value must be non-null because parameter '{0}' is non-null.
+                        Diagnostics.Add(ErrorCode.WRN_ReturnNotNullIfNotNull, location, inputParam.Name);
+                    }
+                    break;
+                }
+            }
+        }
+
         private void EnforceDoesNotReturn(SyntaxNode? syntaxOpt)
         {
             // DoesNotReturn is only supported in member methods
@@ -2309,6 +2338,11 @@ private static TypeWithState GetParameterState(TypeWithAnnotations parameterType
             }
 
             Unsplit();
+            if (CurrentSymbol is MethodSymbol method)
+            {
+                EnforceNotNullIfNotNull(node.Syntax, this.State, method.Parameters, method.ReturnNotNullIfParameterNotNull, ResultType.State, outputParam: null);
+            }
+
             SetUnreachable();
 
             return null;

src/Compilers/CSharp/Portable/Lowering/LocalRewriter/LocalRewriter.cs

@@ -180,7 +180,8 @@ private PEModuleBuilder? EmitModule
             }
             Debug.Assert(!node.HasErrors, "nodes with errors should not be lowered");
 
-            return VisitExpressionImpl(node);
+            // https://github.com/dotnet/roslyn/issues/47682
+            return VisitExpressionImpl(node)!;
         }
 
         private BoundStatement? VisitStatement(BoundStatement? node)

src/Compilers/CSharp/Portable/Syntax/CSharpSyntaxRewriter.cs

@@ -43,7 +43,8 @@ public virtual bool VisitIntoStructuredTrivia
                 var result = ((CSharpSyntaxNode)node).Accept(this);
 
                 _recursionDepth--;
-                return result;
+                // https://github.com/dotnet/roslyn/issues/47682
+                return result!;
             }
             else
             {

src/Compilers/CSharp/Portable/xlf/CSharpResources.cs.xlf

@@ -912,14 +912,14 @@
         <target state="new">discards</target>
         <note />
       </trans-unit>
-      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial">
-        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
-        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+      <trans-unit id="WRN_ParameterNotNullIfNotNull">
+        <source>Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.</source>
+        <target state="new">Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.</target>
         <note />
       </trans-unit>
-      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial_Title">
-        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
-        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+      <trans-unit id="WRN_ParameterNotNullIfNotNull_Title">
+        <source>Parameter must have a non-null value when exiting because parameter referenced by NotNullIfNotNull is non-null.</source>
+        <target state="new">Parameter must have a non-null value when exiting because parameter referenced by NotNullIfNotNull is non-null.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_PartialMethodTypeDifference">
@@ -942,6 +942,16 @@
         <target state="new">Types and aliases should not be named 'record'.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ReturnNotNullIfNotNull">
+        <source>Return value must be non-null because parameter '{0}' is non-null.</source>
+        <target state="new">Return value must be non-null because parameter '{0}' is non-null.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ReturnNotNullIfNotNull_Title">
+        <source>Return value must be non-null because parameter is non-null.</source>
+        <target state="new">Return value must be non-null because parameter is non-null.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_SwitchExpressionNotExhaustiveWithUnnamedEnumValue">
         <source>The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value. For example, the pattern '{0}' is not covered.</source>
         <target state="new">The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value. For example, the pattern '{0}' is not covered.</target>
@@ -2474,6 +2484,16 @@
         <target state="translated">Typ odkazu s možnou hodnotou null ve vráceném typu neodpovídá přepsanému členu.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial">
+        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
+        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial_Title">
+        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
+        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_NullabilityMismatchInTypeOnExplicitImplementation">
         <source>Nullability of reference types in type doesn't match implemented member '{0}'.</source>
         <target state="translated">Typ odkazu s možnou hodnotou null v typu neodpovídá implementovanému členu {0}.</target>

src/Compilers/CSharp/Portable/xlf/CSharpResources.de.xlf

@@ -912,14 +912,14 @@
         <target state="new">discards</target>
         <note />
       </trans-unit>
-      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial">
-        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
-        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+      <trans-unit id="WRN_ParameterNotNullIfNotNull">
+        <source>Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.</source>
+        <target state="new">Parameter '{0}' must have a non-null value when exiting because parameter '{1}' is non-null.</target>
         <note />
       </trans-unit>
-      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial_Title">
-        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
-        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+      <trans-unit id="WRN_ParameterNotNullIfNotNull_Title">
+        <source>Parameter must have a non-null value when exiting because parameter referenced by NotNullIfNotNull is non-null.</source>
+        <target state="new">Parameter must have a non-null value when exiting because parameter referenced by NotNullIfNotNull is non-null.</target>
         <note />
       </trans-unit>
       <trans-unit id="WRN_PartialMethodTypeDifference">
@@ -942,6 +942,16 @@
         <target state="new">Types and aliases should not be named 'record'.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_ReturnNotNullIfNotNull">
+        <source>Return value must be non-null because parameter '{0}' is non-null.</source>
+        <target state="new">Return value must be non-null because parameter '{0}' is non-null.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_ReturnNotNullIfNotNull_Title">
+        <source>Return value must be non-null because parameter is non-null.</source>
+        <target state="new">Return value must be non-null because parameter is non-null.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_SwitchExpressionNotExhaustiveWithUnnamedEnumValue">
         <source>The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value. For example, the pattern '{0}' is not covered.</source>
         <target state="new">The switch expression does not handle some values of its input type (it is not exhaustive) involving an unnamed enum value. For example, the pattern '{0}' is not covered.</target>
@@ -2474,6 +2484,16 @@
         <target state="translated">Die NULL-Zulässigkeit von Verweistypen im Rückgabetyp entspricht nicht dem außer Kraft gesetzten Member.</target>
         <note />
       </trans-unit>
+      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial">
+        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
+        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+        <note />
+      </trans-unit>
+      <trans-unit id="WRN_NullabilityMismatchInReturnTypeOnPartial_Title">
+        <source>Nullability of reference types in return type doesn't match partial method declaration.</source>
+        <target state="new">Nullability of reference types in return type doesn't match partial method declaration.</target>
+        <note />
+      </trans-unit>
       <trans-unit id="WRN_NullabilityMismatchInTypeOnExplicitImplementation">
         <source>Nullability of reference types in type doesn't match implemented member '{0}'.</source>
         <target state="translated">Die NULL-Zulässigkeit von Verweistypen im Typ entspricht nicht dem implementierten Member "{0}".</target>