Change narrowing rules for parameter initialisers

They don't narrow the parameter type, except to remove undefined,
and only if the initialiser type doesn't include undefined.

src/compiler/binder.ts

@@ -810,7 +810,7 @@ namespace ts {
             };
         }
 
-        function createFlowAssignment(antecedent: FlowNode, node: Expression | VariableDeclaration | BindingElement | ParameterDeclaration): FlowNode {
+        function createFlowAssignment(antecedent: FlowNode, node: Expression | VariableDeclaration | BindingElement): FlowNode {
             setFlowNodeReferenced(antecedent);
             return <FlowAssignment>{
                 flags: FlowFlags.Assignment,
@@ -819,6 +819,15 @@ namespace ts {
             };
         }
 
+        function createFlowInitializedParameter(antecedent: FlowNode, node: ParameterDeclaration): FlowNode {
+            setFlowNodeReferenced(antecedent);
+            return <FlowInitializedParameter>{
+                flags: FlowFlags.InitializedParameter,
+                antecedent,
+                node
+            };
+        }
+
         function createFlowArrayMutation(antecedent: FlowNode, node: CallExpression | BinaryExpression): FlowNode {
             setFlowNodeReferenced(antecedent);
             return <FlowArrayMutation>{
@@ -2312,7 +2321,7 @@ namespace ts {
             else {
                 declareSymbolAndAddToSymbolTable(node, SymbolFlags.FunctionScopedVariable, SymbolFlags.ParameterExcludes);
                 if (node.initializer) {
-                    currentFlow = createFlowAssignment(currentFlow, node);
+                    currentFlow = createFlowInitializedParameter(currentFlow, node);
                 }
             }
 

src/compiler/checker.ts

@@ -9060,8 +9060,8 @@ namespace ts {
             switch (source.kind) {
                 case SyntaxKind.Identifier:
                     return target.kind === SyntaxKind.Identifier && getResolvedSymbol(<Identifier>source) === getResolvedSymbol(<Identifier>target) ||
-                    (target.kind === SyntaxKind.VariableDeclaration || target.kind === SyntaxKind.BindingElement) && getExportSymbolOfValueSymbolIfExported(getResolvedSymbol(<Identifier>source)) === getSymbolOfNode(target) ||
-                    target.kind === SyntaxKind.Parameter && getExportSymbolOfValueSymbolIfExported(getResolvedSymbol(source as Identifier)) === getSymbolOfNode(target);
+                    (target.kind === SyntaxKind.VariableDeclaration || target.kind === SyntaxKind.BindingElement || target.kind === SyntaxKind.Parameter) &&
+                    getExportSymbolOfValueSymbolIfExported(getResolvedSymbol(source as Identifier)) === getSymbolOfNode(target);
                 case SyntaxKind.ThisKeyword:
                     return target.kind === SyntaxKind.ThisKeyword;
                 case SyntaxKind.PropertyAccessExpression:
@@ -9155,6 +9155,13 @@ namespace ts {
             return false;
         }
 
+        function getInitializedParameterReducedType(declaredType: UnionType, assignedType: Type) {
+            if (declaredType === assignedType || getFalsyFlags(assignedType) & TypeFlags.Undefined) {
+                return declaredType;
+            }
+            return getTypeWithFacts(declaredType, TypeFacts.NEUndefined);
+        }
+
         // Remove those constituent types of declaredType to which no constituent type of assignedType is assignable.
         // For example, when a variable of type number | string | boolean is assigned a value of type number | boolean,
         // we remove type string.
@@ -9351,7 +9358,7 @@ namespace ts {
                 getInitialTypeOfBindingElement(<BindingElement>node);
         }
 
-        function getInitialOrAssignedType(node: VariableDeclaration | BindingElement | Expression) {
+        function getInitialOrAssignedType(node: VariableDeclaration | BindingElement | Expression | ParameterDeclaration) {
             return node.kind === SyntaxKind.VariableDeclaration || node.kind === SyntaxKind.BindingElement || node.kind === SyntaxKind.Parameter ?
                 getInitialType(<VariableDeclaration | BindingElement | ParameterDeclaration>node) :
                 getAssignedType(<Expression>node);
@@ -9622,6 +9629,13 @@ namespace ts {
                             continue;
                         }
                     }
+                    else if (flow.flags & FlowFlags.InitializedParameter) {
+                        type = getTypeAtFlowInitializedParameter(flow as FlowInitializedParameter);
+                        if (!type) {
+                            flow = (flow as FlowInitializedParameter).antecedent;
+                            continue;
+                        }
+                    }
                     else if (flow.flags & FlowFlags.Condition) {
                         type = getTypeAtFlowCondition(<FlowCondition>flow);
                     }
@@ -9669,6 +9683,20 @@ namespace ts {
                 }
             }
 
+            function getTypeAtFlowInitializedParameter(flow: FlowInitializedParameter) {
+                const node = flow.node;
+                // Parameter initializers don't really narrow the declared type except to remove undefined.
+                // If the initializer includes undefined in the type, it doesn't even remove undefined.
+                if (isMatchingReference(reference, node)) {
+                    if (declaredType.flags & TypeFlags.Union) {
+                        return getInitializedParameterReducedType(<UnionType>declaredType, getInitialOrAssignedType(node));
+                    }
+                    return declaredType;
+                }
+
+                return undefined;
+            }
+
             function getTypeAtFlowAssignment(flow: FlowAssignment) {
                 const node = flow.node;
                 // Assignments only narrow the computed type if the declared type is a union type. Thus, we

src/compiler/types.ts

@@ -2042,12 +2042,13 @@ namespace ts {
         BranchLabel    = 1 << 2,  // Non-looping junction
         LoopLabel      = 1 << 3,  // Looping junction
         Assignment     = 1 << 4,  // Assignment
-        TrueCondition  = 1 << 5,  // Condition known to be true
-        FalseCondition = 1 << 6,  // Condition known to be false
-        SwitchClause   = 1 << 7,  // Switch statement clause
-        ArrayMutation  = 1 << 8,  // Potential array mutation
-        Referenced     = 1 << 9,  // Referenced as antecedent once
-        Shared         = 1 << 10, // Referenced as antecedent more than once
+        InitializedParameter = 1 << 5, // Parameter with initializer
+        TrueCondition  = 1 << 6,  // Condition known to be true
+        FalseCondition = 1 << 7,  // Condition known to be false
+        SwitchClause   = 1 << 8,  // Switch statement clause
+        ArrayMutation  = 1 << 9,  // Potential array mutation
+        Referenced     = 1 << 10,  // Referenced as antecedent once
+        Shared         = 1 << 11, // Referenced as antecedent more than once
         Label = BranchLabel | LoopLabel,
         Condition = TrueCondition | FalseCondition
     }
@@ -2097,6 +2098,11 @@ namespace ts {
         antecedent: FlowNode;
     }
 
+    export interface FlowInitializedParameter extends FlowNode {
+        node: ParameterDeclaration;
+        antecedent: FlowNode;
+    }
+
     export type FlowType = Type | IncompleteType;
 
     // Incomplete types occur during control flow analysis of loops. An IncompleteType