Address various minor param-nullchecking issues

src/Compilers/CSharp/Portable/CSharpResources.resx

@@ -6187,11 +6187,11 @@ To remove the warning, you can use /reference instead (set the Embed Interop Typ
   <data name="ERR_NullCheckingOnByRefParameter" xml:space="preserve">
     <value>By-reference parameter '{0}' cannot be null-checked.</value>
   </data>
-  <data name="WRN_NullCheckingOnNullableValueType" xml:space="preserve">
-    <value>Nullable value type '{0}' is null-checked and will throw if null.</value>
+  <data name="WRN_NullCheckingOnNullableType" xml:space="preserve">
+    <value>Nullable type '{0}' is null-checked and will throw if null.</value>
   </data>
-  <data name="WRN_NullCheckingOnNullableValueType_Title" xml:space="preserve">
-    <value>Nullable value type is null-checked and will throw if null.</value>
+  <data name="WRN_NullCheckingOnNullableType_Title" xml:space="preserve">
+    <value>Nullable type is null-checked and will throw if null.</value>
   </data>
   <data name="ERR_ReAbstractionInNoPIAType" xml:space="preserve">
     <value>Type '{0}' cannot be embedded because it has a re-abstraction of a member from base interface. Consider setting the 'Embed Interop Types' property to false.</value>

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

@@ -2012,7 +2012,7 @@ internal enum ErrorCode
         ERR_NonNullableValueTypeIsNullChecked = 8992,
         WRN_NullCheckedHasDefaultNull = 8993,
         ERR_NullCheckingOnByRefParameter = 8994,
-        WRN_NullCheckingOnNullableValueType = 8995,
+        WRN_NullCheckingOnNullableType = 8995,
 
         #endregion
 

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

@@ -456,7 +456,7 @@ internal static int GetWarningLevel(ErrorCode code)
                 case ErrorCode.WRN_NullabilityMismatchInTypeParameterNotNullConstraint:
                 case ErrorCode.WRN_DisallowNullAttributeForbidsMaybeNullAssignment:
                 case ErrorCode.WRN_NullCheckedHasDefaultNull:
-                case ErrorCode.WRN_NullCheckingOnNullableValueType:
+                case ErrorCode.WRN_NullCheckingOnNullableType:
                 case ErrorCode.WRN_ParameterConditionallyDisallowsNull:
                 case ErrorCode.WRN_NullReferenceInitializer:
                 case ErrorCode.WRN_ShouldNotReturn:

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

@@ -1626,7 +1626,7 @@ private NullableFlowState GetDefaultState(ref LocalState state, int slot)
                         {
                             parameterType = parameter.TypeWithAnnotations;
                         }
-                        return GetParameterState(parameterType, parameter.FlowAnalysisAnnotations).State;
+                        return GetParameterState(parameterType, parameter.FlowAnalysisAnnotations, parameter.IsNullChecked).State;
                     }
                 case SymbolKind.Field:
                 case SymbolKind.Property:
@@ -2044,7 +2044,11 @@ internal static bool AreParameterAnnotationsCompatible(
             {
                 // pre-condition attributes
                 // Check whether we can assign a value from overridden parameter to overriding
-                var valueState = GetParameterState(overriddenType, overriddenAnnotations);
+                var valueState = GetParameterState(
+                    overriddenType,
+                    overriddenAnnotations,
+                    // We don't consider '!!' when deciding whether 'overridden' is compatible with 'override'
+                    isNullChecked: false);
                 if (isBadAssignment(valueState, overridingType, overridingAnnotations))
                 {
                     return false;
@@ -2470,7 +2474,7 @@ private void EnterParameter(ParameterSymbol parameter, TypeWithAnnotations param
                 Debug.Assert(!IsConditionalState);
                 if (slot > 0)
                 {
-                    var state = GetParameterState(parameterType, parameter.FlowAnalysisAnnotations).State;
+                    var state = GetParameterState(parameterType, parameter.FlowAnalysisAnnotations, parameter.IsNullChecked).State;
                     this.State[slot] = state;
                     if (EmptyStructTypeCache.IsTrackableStructType(parameterType.Type))
                     {
@@ -2503,8 +2507,13 @@ private void EnterParameter(ParameterSymbol parameter, TypeWithAnnotations param
             return null;
         }
 
-        internal static TypeWithState GetParameterState(TypeWithAnnotations parameterType, FlowAnalysisAnnotations parameterAnnotations)
+        internal static TypeWithState GetParameterState(TypeWithAnnotations parameterType, FlowAnalysisAnnotations parameterAnnotations, bool isNullChecked)
         {
+            if (isNullChecked)
+            {
+                return TypeWithState.Create(parameterType.Type, NullableFlowState.NotNull);
+            }
+
             if ((parameterAnnotations & FlowAnalysisAnnotations.AllowNull) != 0)
             {
                 return TypeWithState.Create(parameterType.Type, NullableFlowState.MaybeDefault);
@@ -7977,7 +7986,7 @@ private void VisitThisOrBaseReference(BoundExpression node)
             var parameter = node.ParameterSymbol;
             int slot = GetOrCreateSlot(parameter);
             var parameterType = GetDeclaredParameterResult(parameter);
-            var typeWithState = GetParameterState(parameterType, parameter.FlowAnalysisAnnotations);
+            var typeWithState = GetParameterState(parameterType, parameter.FlowAnalysisAnnotations, parameter.IsNullChecked);
             SetResult(node, GetAdjustedResult(typeWithState, slot), parameterType);
             return null;
         }

src/Compilers/CSharp/Portable/Symbols/Source/ParameterHelpers.cs

@@ -802,16 +802,14 @@ internal static void ReportParameterNullCheckingErrors(DiagnosticBag diagnostics
             {
                 diagnostics.Add(useSiteInfo.DiagnosticInfo, location);
             }
-            if (parameter.Type.IsValueType && !parameter.Type.IsPointerOrFunctionPointer())
+            if (parameter.TypeWithAnnotations.NullableAnnotation.IsAnnotated()
+                || parameter.Type.IsNullableTypeOrTypeParameter())
             {
-                if (!parameter.Type.IsNullableTypeOrTypeParameter())
-                {
-                    diagnostics.Add(ErrorCode.ERR_NonNullableValueTypeIsNullChecked, location, parameter);
-                }
-                else
-                {
-                    diagnostics.Add(ErrorCode.WRN_NullCheckingOnNullableValueType, location, parameter);
-                }
+                diagnostics.Add(ErrorCode.WRN_NullCheckingOnNullableType, location, parameter);
+            }
+            else if (parameter.Type.IsValueType && !parameter.Type.IsPointerOrFunctionPointer())
+            {
+                diagnostics.Add(ErrorCode.ERR_NonNullableValueTypeIsNullChecked, location, parameter);
             }
         }
 

src/Compilers/CSharp/Portable/Symbols/Source/SourceMemberContainerSymbol_ImplementationChecks.cs

@@ -1293,7 +1293,7 @@ TypeWithAnnotations getNotNullIfNotNullOutputType(TypeWithAnnotations outputType
                     {
                         var overrideParam = overrideParameters[i + overrideParameterOffset];
                         var baseParam = baseParameters[i];
-                        if (notNullIfParameterNotNull.Contains(overrideParam.Name) && NullableWalker.GetParameterState(baseParam.TypeWithAnnotations, baseParam.FlowAnalysisAnnotations).IsNotNull)
+                        if (notNullIfParameterNotNull.Contains(overrideParam.Name) && NullableWalker.GetParameterState(baseParam.TypeWithAnnotations, baseParam.FlowAnalysisAnnotations, baseParam.IsNullChecked).IsNotNull)
                         {
                             return outputType.AsNotAnnotated();
                         }

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

@@ -143,6 +143,7 @@ public enum SyntaxKind : ushort
         PercentEqualsToken = 8283,
         /// <summary>Represents <c>??=</c> token.</summary>
         QuestionQuestionEqualsToken = 8284,
+        /// <summary>Represents <c>!!</c> token.</summary>
         ExclamationExclamationToken = 8285,
 
         // Keywords