List patterns: Slice value is assumed to be never null

src/Compilers/CSharp/Portable/Binder/DecisionDagBuilder.cs

@@ -421,8 +421,12 @@ private static void MakeCheckNotNull(
             ArrayBuilder<Tests> tests)
         {
             // Add a null test if needed
-            if (input.Type.CanContainNull())
+            if (input.Type.CanContainNull() &&
+                // The slice value is assumed to be never null
+                input.Source is not BoundDagSliceEvaluation)
+            {
                 tests.Add(new Tests.One(new BoundDagNonNullTest(syntax, isExplicitTest, input)));
+            }
         }
 
         /// <summary>

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6757,9 +6757,6 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureListPattern" xml:space="preserve">
     <value>list pattern</value>
   </data>
-  <data name="IDS_FeatureLengthPattern" xml:space="preserve">
-    <value>length pattern</value>
-  </data>
   <data name="ERR_UnsupportedTypeForListPattern" xml:space="preserve">
     <value>List patterns may not be used for a value of type '{0}'.</value>
   </data>
@@ -6926,6 +6923,12 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="IDS_FeatureNewLinesInInterpolations" xml:space="preserve">
     <value>newlines in interpolations</value>
   </data>
+  <data name="WRN_AnnotationOnSliceReturnType" xml:space="preserve">
+    <value>The annotation on the output type '{0}' is ignored.</value>
+  </data>
+  <data name="WRN_AnnotationOnSliceReturnType_Title" xml:space="preserve">
+    <value>The annotation on the output type is ignored.</value>
+  </data>
   <data name="ERR_InterpolatedStringsReferencingInstanceCannotBeInObjectInitializers" xml:space="preserve">
     <value>Interpolated string handler conversions that reference the instance being indexed cannot be used in indexer member initializers.</value>
   </data>

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

@@ -2025,6 +2025,8 @@ internal enum ErrorCode
         WRN_LowerCaseTypeName = 8981,
         ERR_RecordStructConstructorCallsDefaultConstructor = 8982,
 
+        WRN_AnnotationOnSliceReturnType = 8983,
+
         #endregion
 
         // Note: you will need to re-generate compiler code after adding warnings (eng\generate-compiler-code.cmd)

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

@@ -80,6 +80,7 @@ static ErrorFacts()
             nullableWarnings.Add(GetId(ErrorCode.WRN_ParameterDisallowsNull));
             nullableWarnings.Add(GetId(ErrorCode.WRN_ParameterNotNullIfNotNull));
             nullableWarnings.Add(GetId(ErrorCode.WRN_ReturnNotNullIfNotNull));
+            nullableWarnings.Add(GetId(ErrorCode.WRN_AnnotationOnSliceReturnType));
 
             NullableWarnings = nullableWarnings.ToImmutable();
         }
@@ -494,6 +495,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_InterpolatedStringHandlerArgumentAttributeIgnoredOnLambdaParameters:
                 case ErrorCode.WRN_CompileTimeCheckedOverflow:
                 case ErrorCode.WRN_MethGrpToNonDel:
+                case ErrorCode.WRN_AnnotationOnSliceReturnType:
                     return 1;
                 default:
                     return 0;

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

@@ -513,6 +513,7 @@ public PossiblyConditionalState Clone()
                                         var output = new BoundDagTemp(e.Syntax, type.Type, e);
                                         var outputSlot = makeDagTempSlot(type, output);
                                         Debug.Assert(outputSlot > 0);
+                                        ignoreAnnotationAndReport(outputSlot, type, ref this.State, e);
                                         addToTempMap(output, outputSlot, type.Type);
                                         break;
                                     }
@@ -665,6 +666,18 @@ public PossiblyConditionalState Clone()
             nodeStateMap.Free();
             return labelStateMap;
 
+            void ignoreAnnotationAndReport(int outputSlot, TypeWithAnnotations outputType, ref LocalState state, BoundDagSliceEvaluation e)
+            {
+                if (PossiblyNullableType(outputType.Type))
+                {
+                    if (state[outputSlot] == NullableFlowState.MaybeNull)
+                    {
+                        ReportDiagnostic(ErrorCode.WRN_AnnotationOnSliceReturnType, e.Syntax, outputType);
+                        state[outputSlot] = NullableFlowState.NotNull;
+                    }
+                }
+            }
+
             void learnFromNonNullTest(int inputSlot, ref LocalState state)
             {
                 if (stateWhenNotNullOpt is { } stateWhenNotNull && inputSlot == originalInputSlot)