var should infer nullable type (#40755)

Author: jcouv

docs/features/nullable-reference-types.md

@@ -146,11 +146,11 @@ Nullablilty follows from assignment above. Assigning `?` to `!` is a W warning.
 ```c#
 string notNull = maybeNull; // assigns ?, warning
 ```
-Nullability of `var` declarations is determined from flow analysis.
+The nullability of `var` declarations is the nullable version of the inferred type.
 ```c#
 var s = maybeNull; // s is ?, no warning
 if (maybeNull == null) return;
-var t = maybeNull; // t is !
+var t = maybeNull; // t is ? too
 ```
 
 ### Suppression operator (`!`)

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

@@ -438,7 +438,7 @@ private void BindPatternDesignation(
 
                         variableSymbol = localSymbol;
                         variableAccess = new BoundLocal(
-                            syntax: designation, localSymbol: localSymbol, constantValueOpt: null, type: declType.Type);
+                            syntax: designation, localSymbol: localSymbol, localSymbol.IsVar ? BoundLocalDeclarationKind.WithInferredType : BoundLocalDeclarationKind.WithExplicitType, constantValueOpt: null, isNullableUnknown: false, type: declType.Type);
                         return;
                     }
                     else

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

@@ -1824,12 +1824,12 @@ public override BoundNode VisitLocalDeclaration(BoundLocalDeclaration node)
                     valueType = type.ToTypeWithState();
                 }
 
-                type = valueType.ToTypeWithAnnotations();
+                type = valueType.ToAnnotatedTypeWithAnnotations();
                 _variableTypes[local] = type;
 
                 if (node.DeclaredTypeOpt != null)
                 {
-                    SetAnalyzedNullability(node.DeclaredTypeOpt, new VisitResult(valueType, type), true);
+                    SetAnalyzedNullability(node.DeclaredTypeOpt, new VisitResult(type.ToTypeWithState(), type), true);
                 }
             }
 
@@ -4122,7 +4122,7 @@ private void VisitArgumentOutboundAssignmentsAndPostConditions(
                         var lValueType = ApplyLValueAnnotations(declaredType, leftAnnotations);
                         if (argument is BoundLocal local && local.DeclarationKind == BoundLocalDeclarationKind.WithInferredType)
                         {
-                            var varType = worstCaseParameterWithState.ToTypeWithAnnotations();
+                            var varType = worstCaseParameterWithState.ToAnnotatedTypeWithAnnotations();
                             _variableTypes[local.LocalSymbol] = varType;
                             lValueType = varType;
                         }
@@ -6440,7 +6440,7 @@ private void VisitTupleDeconstructionArguments(ArrayBuilder<DeconstructionVariab
                         {
                             // when the LHS is a var declaration, we can just visit the right part to infer the type
                             valueType = operandType = VisitRvalueWithState(rightPart);
-                            _variableTypes[variable.Expression.ExpressionSymbol] = operandType.ToTypeWithAnnotations();
+                            _variableTypes[variable.Expression.ExpressionSymbol] = operandType.ToAnnotatedTypeWithAnnotations();
                         }
                         else
                         {
@@ -7153,6 +7153,7 @@ public override void VisitForEachIterationVariables(BoundForEachStatement node)
                     {
                         TypeWithAnnotations destinationType = iterationVariable.TypeWithAnnotations;
                         TypeWithState result = sourceState;
+                        TypeWithState resultForType = sourceState;
                         if (iterationVariable.IsRef)
                         {
                             // foreach (ref DestinationType variable in collection)
@@ -7165,7 +7166,9 @@ public override void VisitForEachIterationVariables(BoundForEachStatement node)
                         else if (node.Syntax is ForEachStatementSyntax { Type: { IsVar: true } })
                         {
                             // foreach (var variable in collection)
-                            _variableTypes[iterationVariable] = sourceState.ToTypeWithAnnotations();
+                            destinationType = sourceState.ToAnnotatedTypeWithAnnotations();
+                            _variableTypes[iterationVariable] = destinationType;
+                            resultForType = destinationType.ToTypeWithState();
                         }
                         else
                         {
@@ -7191,7 +7194,7 @@ public override void VisitForEachIterationVariables(BoundForEachStatement node)
                         }
 
                         // In non-error cases we'll only run this loop a single time. In error cases we'll set the nullability of the VariableType multiple times, but at least end up with something
-                        SetAnalyzedNullability(node.IterationVariableType, new VisitResult(result, destinationType), isLvalue: true);
+                        SetAnalyzedNullability(node.IterationVariableType, new VisitResult(resultForType, destinationType), isLvalue: true);
                         state = result.State;
                     }
 

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

@@ -402,17 +402,18 @@ protected override void VisitSwitchSection(BoundSwitchSection node, bool isLastS
                             Debug.Assert(foundTemp);
                             var (tempSlot, tempType) = tempSlotAndType;
                             var tempState = this.State[tempSlot];
-                            if (variableAccess is BoundLocal { LocalSymbol: SourceLocalSymbol local })
+                            if (variableAccess is BoundLocal { LocalSymbol: SourceLocalSymbol local } boundLocal)
                             {
-                                var inferredType = TypeWithState.Create(tempType, tempState).ToTypeWithAnnotations();
+                                var value = TypeWithState.Create(tempType, tempState);
+                                var type = boundLocal.DeclarationKind == BoundLocalDeclarationKind.WithInferredType ? value.ToAnnotatedTypeWithAnnotations() : value.ToTypeWithAnnotations();
                                 if (_variableTypes.TryGetValue(local, out var existingType))
                                 {
                                     // merge inferred nullable annotation from different branches of the decision tree
-                                    _variableTypes[local] = TypeWithAnnotations.Create(existingType.Type, existingType.NullableAnnotation.Join(inferredType.NullableAnnotation));
+                                    _variableTypes[local] = TypeWithAnnotations.Create(existingType.Type, existingType.NullableAnnotation.Join(type.NullableAnnotation));
                                 }
                                 else
                                 {
-                                    _variableTypes[local] = inferredType;
+                                    _variableTypes[local] = type;
                                 }
 
                                 int localSlot = GetOrCreateSlot(local, forceSlotEvenIfEmpty: true);

src/Compilers/CSharp/Portable/Symbols/TypeWithState.cs

@@ -86,5 +86,12 @@ public TypeWithAnnotations ToTypeWithAnnotations()
                 ? NullableAnnotation.NotAnnotated : NullableAnnotation.Annotated;
             return TypeWithAnnotations.Create(this.Type, annotation);
         }
+
+        public TypeWithAnnotations ToAnnotatedTypeWithAnnotations()
+        {
+            NullableAnnotation annotation = Type?.IsTypeParameterDisallowingAnnotation() == true
+                ? NullableAnnotation.NotAnnotated : NullableAnnotation.Annotated;
+            return TypeWithAnnotations.Create(this.Type, annotation);
+        }
     }
 }