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diff --git a/‎src/compiler/binder.ts
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+9-7Lines changed: 9 additions & 7 deletions b/‎src/compiler/binder.ts
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+9-7Lines changed: 9 additions & 7 deletions
diff --git a/‎src/compiler/checker.ts
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+84-44Lines changed: 84 additions & 44 deletions b/‎src/compiler/checker.ts
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+84-44Lines changed: 84 additions & 44 deletions
diff --git a/‎src/compiler/commandLineParser.ts
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+7Lines changed: 7 additions & 0 deletions b/‎src/compiler/commandLineParser.ts
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+7Lines changed: 7 additions & 0 deletions
diff --git a/‎src/compiler/core.ts
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+1-1Lines changed: 1 addition & 1 deletion b/‎src/compiler/core.ts
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+1-1Lines changed: 1 addition & 1 deletion
diff --git a/‎src/compiler/diagnosticMessages.json
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+12Lines changed: 12 additions & 0 deletions b/‎src/compiler/diagnosticMessages.json
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diff --git a/‎src/compiler/types.ts
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+2Lines changed: 2 additions & 0 deletions b/‎src/compiler/types.ts
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+2Lines changed: 2 additions & 0 deletions
diff --git a/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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+1Lines changed: 1 addition & 0 deletions b/‎tests/baselines/reference/api/tsserverlibrary.d.ts
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diff --git a/‎tests/baselines/reference/api/typescript.d.ts
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+1Lines changed: 1 addition & 0 deletions b/‎tests/baselines/reference/api/typescript.d.ts
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diff --git a/‎tests/baselines/reference/indexedAccessTypeConstraints.js
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+3-2Lines changed: 3 additions & 2 deletions b/‎tests/baselines/reference/indexedAccessTypeConstraints.js
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+3-2Lines changed: 3 additions & 2 deletions
diff --git a/‎tests/baselines/reference/indexedAccessTypeConstraints.symbols
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+9-9Lines changed: 9 additions & 9 deletions b/‎tests/baselines/reference/indexedAccessTypeConstraints.symbols
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+9-9Lines changed: 9 additions & 9 deletions
@@ -517,16 +517,15 @@ namespace ts {
                 const isIIFE = containerFlags & ContainerFlags.IsFunctionExpression && !hasModifier(node, ModifierFlags.Async) && !!getImmediatelyInvokedFunctionExpression(node);
                 // A non-async IIFE is considered part of the containing control flow. Return statements behave
                 // similarly to break statements that exit to a label just past the statement body.
-                if (isIIFE) {
-                    currentReturnTarget = createBranchLabel();
-                }
-                else {
+                if (!isIIFE) {
                     currentFlow = { flags: FlowFlags.Start };
                     if (containerFlags & (ContainerFlags.IsFunctionExpression | ContainerFlags.IsObjectLiteralOrClassExpressionMethod)) {
                         (<FlowStart>currentFlow).container = <FunctionExpression | ArrowFunction | MethodDeclaration>node;
                     }
-                    currentReturnTarget = undefined;
                 }
+                // We create a return control flow graph for IIFEs and constructors. For constructors
+                // we use the return control flow graph in strict property intialization checks.
+                currentReturnTarget = isIIFE || node.kind === SyntaxKind.Constructor ? createBranchLabel() : undefined;
                 currentBreakTarget = undefined;
                 currentContinueTarget = undefined;
                 activeLabels = undefined;
@@ -541,11 +540,14 @@ namespace ts {
                 if (node.kind === SyntaxKind.SourceFile) {
                     node.flags |= emitFlags;
                 }
-                if (isIIFE) {
+                if (currentReturnTarget) {
                     addAntecedent(currentReturnTarget, currentFlow);
                     currentFlow = finishFlowLabel(currentReturnTarget);
+                    if (node.kind === SyntaxKind.Constructor) {
+                        (<ConstructorDeclaration>node).returnFlowNode = currentFlow;
+                    }
                 }
-                else {
+                if (!isIIFE) {
                     currentFlow = saveCurrentFlow;
                 }
                 currentBreakTarget = saveBreakTarget;
 
@@ -69,6 +69,7 @@ namespace ts {
         const allowSyntheticDefaultImports = getAllowSyntheticDefaultImports(compilerOptions);
         const strictNullChecks = getStrictOptionValue(compilerOptions, "strictNullChecks");
         const strictFunctionTypes = getStrictOptionValue(compilerOptions, "strictFunctionTypes");
+        const strictPropertyInitialization = getStrictOptionValue(compilerOptions, "strictPropertyInitialization");
         const noImplicitAny = getStrictOptionValue(compilerOptions, "noImplicitAny");
         const noImplicitThis = getStrictOptionValue(compilerOptions, "noImplicitThis");
 
@@ -12283,7 +12284,7 @@ namespace ts {
             // on empty arrays are possible without implicit any errors and new element types can be inferred without
             // type mismatch errors.
             const resultType = getObjectFlags(evolvedType) & ObjectFlags.EvolvingArray && isEvolvingArrayOperationTarget(reference) ? anyArrayType : finalizeEvolvingArrayType(evolvedType);
-            if (reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(resultType, TypeFacts.NEUndefinedOrNull).flags & TypeFlags.Never) {
+            if (reference.parent && reference.parent.kind === SyntaxKind.NonNullExpression && getTypeWithFacts(resultType, TypeFacts.NEUndefinedOrNull).flags & TypeFlags.Never) {
                 return declaredType;
             }
             return resultType;
@@ -15561,63 +15562,68 @@ namespace ts {
         }
 
         function checkPropertyAccessExpressionOrQualifiedName(node: PropertyAccessExpression | QualifiedName, left: Expression | QualifiedName, right: Identifier) {
-            const type = checkNonNullExpression(left);
-            if (isTypeAny(type) || type === silentNeverType) {
-                return type;
-            }
-
-            const apparentType = getApparentType(getWidenedType(type));
-            if (apparentType === unknownType || (type.flags & TypeFlags.TypeParameter && isTypeAny(apparentType))) {
-                // handle cases when type is Type parameter with invalid or any constraint
+            let propType: Type;
+            const leftType = checkNonNullExpression(left);
+            const apparentType = getApparentType(getWidenedType(leftType));
+            if (isTypeAny(apparentType) || apparentType === silentNeverType) {
                 return apparentType;
             }
+            const assignmentKind = getAssignmentTargetKind(node);
             const prop = getPropertyOfType(apparentType, right.escapedText);
             if (!prop) {
                 const indexInfo = getIndexInfoOfType(apparentType, IndexKind.String);
-                if (indexInfo && indexInfo.type) {
-                    if (indexInfo.isReadonly && (isAssignmentTarget(node) || isDeleteTarget(node))) {
-                        error(node, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(apparentType));
+                if (!(indexInfo && indexInfo.type)) {
+                    if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
+                        reportNonexistentProperty(right, leftType.flags & TypeFlags.TypeParameter && (leftType as TypeParameter).isThisType ? apparentType : leftType);
                     }
-                    return getFlowTypeOfPropertyAccess(node, /*prop*/ undefined, indexInfo.type, getAssignmentTargetKind(node));
+                    return unknownType;
                 }
-                if (right.escapedText && !checkAndReportErrorForExtendingInterface(node)) {
-                    reportNonexistentProperty(right, type.flags & TypeFlags.TypeParameter && (type as TypeParameter).isThisType ? apparentType : type);
+                if (indexInfo.isReadonly && (isAssignmentTarget(node) || isDeleteTarget(node))) {
+                    error(node, Diagnostics.Index_signature_in_type_0_only_permits_reading, typeToString(apparentType));
                 }
-                return unknownType;
+                propType = indexInfo.type;
             }
-
-            checkPropertyNotUsedBeforeDeclaration(prop, node, right);
-
-            markPropertyAsReferenced(prop, node, left.kind === SyntaxKind.ThisKeyword);
-
-            getNodeLinks(node).resolvedSymbol = prop;
-
-            checkPropertyAccessibility(node, left, apparentType, prop);
-
-            const propType = getDeclaredOrApparentType(prop, node);
-            const assignmentKind = getAssignmentTargetKind(node);
-
-            if (assignmentKind) {
-                if (isReferenceToReadonlyEntity(<Expression>node, prop) || isReferenceThroughNamespaceImport(<Expression>node)) {
-                    error(right, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, idText(right));
-                    return unknownType;
+            else {
+                checkPropertyNotUsedBeforeDeclaration(prop, node, right);
+                markPropertyAsReferenced(prop, node, left.kind === SyntaxKind.ThisKeyword);
+                getNodeLinks(node).resolvedSymbol = prop;
+                checkPropertyAccessibility(node, left, apparentType, prop);
+                if (assignmentKind) {
+                    if (isReferenceToReadonlyEntity(<Expression>node, prop) || isReferenceThroughNamespaceImport(<Expression>node)) {
+                        error(right, Diagnostics.Cannot_assign_to_0_because_it_is_a_constant_or_a_read_only_property, idText(right));
+                        return unknownType;
+                    }
                 }
+                propType = getDeclaredOrApparentType(prop, node);
             }
-            return getFlowTypeOfPropertyAccess(node, prop, propType, assignmentKind);
-        }
-
-        /**
-         * Only compute control flow type if this is a property access expression that isn't an
-         * assignment target, and the referenced property was declared as a variable, property,
-         * accessor, or optional method.
-         */
-        function getFlowTypeOfPropertyAccess(node: PropertyAccessExpression | QualifiedName, prop: Symbol | undefined, type: Type, assignmentKind: AssignmentKind) {
+            // Only compute control flow type if this is a property access expression that isn't an
+            // assignment target, and the referenced property was declared as a variable, property,
+            // accessor, or optional method.
             if (node.kind !== SyntaxKind.PropertyAccessExpression ||
                 assignmentKind === AssignmentKind.Definite ||
-                prop && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) && !(prop.flags & SymbolFlags.Method && type.flags & TypeFlags.Union)) {
-                return type;
+                prop && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) && !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
+                return propType;
+            }
+            // If strict null checks and strict property initialization checks are enabled, if we have
+            // a this.xxx property access, if the property is an instance property without an initializer,
+            // and if we are in a constructor of the same class as the property declaration, assume that
+            // the property is uninitialized at the top of the control flow.
+            let assumeUninitialized = false;
+            if (strictNullChecks && strictPropertyInitialization && left.kind === SyntaxKind.ThisKeyword) {
+                const declaration = prop && prop.valueDeclaration;
+                if (declaration && isInstancePropertyWithoutInitializer(declaration)) {
+                    const flowContainer = getControlFlowContainer(node);
+                    if (flowContainer.kind === SyntaxKind.Constructor && flowContainer.parent === declaration.parent) {
+                        assumeUninitialized = true;
+                    }
+                }
+            }
+            const flowType = getFlowTypeOfReference(node, propType, assumeUninitialized ? getOptionalType(propType) : propType);
+            if (assumeUninitialized && !(getFalsyFlags(propType) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
+                error(right, Diagnostics.Property_0_is_used_before_being_assigned, symbolToString(prop));
+                // Return the declared type to reduce follow-on errors
+                return propType;
             }
-            const flowType = getFlowTypeOfReference(node, type);
             return assignmentKind ? getBaseTypeOfLiteralType(flowType) : flowType;
         }
 
@@ -22483,6 +22489,7 @@ namespace ts {
             if (produceDiagnostics) {
                 checkIndexConstraints(type);
                 checkTypeForDuplicateIndexSignatures(node);
+                checkPropertyInitialization(node);
             }
         }
 
@@ -22639,6 +22646,39 @@ namespace ts {
             return ok;
         }
 
+        function checkPropertyInitialization(node: ClassLikeDeclaration) {
+            if (!strictNullChecks || !strictPropertyInitialization || node.flags & NodeFlags.Ambient) {
+                return;
+            }
+            const constructor = findConstructorDeclaration(node);
+            for (const member of node.members) {
+                if (isInstancePropertyWithoutInitializer(member)) {
+                    const propName = (<PropertyDeclaration>member).name;
+                    if (isIdentifier(propName)) {
+                        const type = getTypeOfSymbol(getSymbolOfNode(member));
+                        if (!(type.flags & TypeFlags.Any || getFalsyFlags(type) & TypeFlags.Undefined)) {
+                            if (!constructor || !isPropertyInitializedInConstructor(propName, type, constructor)) {
+                                error(member.name, Diagnostics.Property_0_has_no_initializer_and_is_not_definitely_assigned_in_the_constructor, declarationNameToString(propName));
+                            }
+                        }
+                    }
+                }
+            }
+        }
+
+        function isInstancePropertyWithoutInitializer(node: Node) {
+            return node.kind === SyntaxKind.PropertyDeclaration &&
+                !hasModifier(node, ModifierFlags.Static | ModifierFlags.Abstract) &&
+                !(<PropertyDeclaration>node).initializer;
+        }
+
+        function isPropertyInitializedInConstructor(propName: Identifier, propType: Type, constructor: ConstructorDeclaration) {
+            const reference = createPropertyAccess(createThis(), propName);
+            reference.flowNode = constructor.returnFlowNode;
+            const flowType = getFlowTypeOfReference(reference, propType, getOptionalType(propType));
+            return !(getFalsyFlags(flowType) & TypeFlags.Undefined);
+        }
+
         function checkInterfaceDeclaration(node: InterfaceDeclaration) {
             // Grammar checking
             if (!checkGrammarDecoratorsAndModifiers(node)) checkGrammarInterfaceDeclaration(node);
 
@@ -277,6 +277,13 @@ namespace ts {
             category: Diagnostics.Strict_Type_Checking_Options,
             description: Diagnostics.Enable_strict_checking_of_function_types
         },
+        {
+            name: "strictPropertyInitialization",
+            type: "boolean",
+            showInSimplifiedHelpView: true,
+            category: Diagnostics.Strict_Type_Checking_Options,
+            description: Diagnostics.Enable_strict_checking_of_property_initialization_in_classes
+        },
         {
             name: "noImplicitThis",
             type: "boolean",
 
@@ -1967,7 +1967,7 @@ namespace ts {
             : moduleKind === ModuleKind.System;
     }
 
-    export type StrictOptionName = "noImplicitAny" | "noImplicitThis" | "strictNullChecks" | "strictFunctionTypes" | "alwaysStrict";
+    export type StrictOptionName = "noImplicitAny" | "noImplicitThis" | "strictNullChecks" | "strictFunctionTypes" | "strictPropertyInitialization" | "alwaysStrict";
 
     export function getStrictOptionValue(compilerOptions: CompilerOptions, flag: StrictOptionName): boolean {
         return compilerOptions[flag] === undefined ? compilerOptions.strict : compilerOptions[flag];
 
@@ -1952,6 +1952,14 @@
         "category": "Error",
         "code": 2563
     },
+    "Property '{0}' has no initializer and is not definitely assigned in the constructor.": {
+        "category": "Error",
+        "code": 2564
+    },
+    "Property '{0}' is used before being assigned.": {
+        "category": "Error",
+        "code": 2565
+    },
     "JSX element attributes type '{0}' may not be a union type.": {
         "category": "Error",
         "code": 2600
@@ -3391,6 +3399,10 @@
         "category": "Message",
         "code": 6186
     },
+    "Enable strict checking of property initialization in classes.": {
+        "category": "Message",
+        "code": 6187
+    },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
         "code": 7005
 
@@ -947,6 +947,7 @@ namespace ts {
         kind: SyntaxKind.Constructor;
         parent?: ClassDeclaration | ClassExpression;
         body?: FunctionBody;
+        /* @internal */ returnFlowNode?: FlowNode;
     }
 
     /** For when we encounter a semicolon in a class declaration. ES6 allows these as class elements. */
@@ -3854,6 +3855,7 @@ namespace ts {
         strict?: boolean;
         strictFunctionTypes?: boolean;  // Always combine with strict property
         strictNullChecks?: boolean;  // Always combine with strict property
+        strictPropertyInitialization?: boolean;  // Always combine with strict property
         /* @internal */ stripInternal?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;
 
@@ -2287,6 +2287,7 @@ declare namespace ts {
         strict?: boolean;
         strictFunctionTypes?: boolean;
         strictNullChecks?: boolean;
+        strictPropertyInitialization?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;
         target?: ScriptTarget;
 
@@ -2287,6 +2287,7 @@ declare namespace ts {
         strict?: boolean;
         strictFunctionTypes?: boolean;
         strictNullChecks?: boolean;
+        strictPropertyInitialization?: boolean;
         suppressExcessPropertyErrors?: boolean;
         suppressImplicitAnyIndexErrors?: boolean;
         target?: ScriptTarget;
 
@@ -10,7 +10,7 @@ type Data<T> = {
 };
 
 class Parent<M> {
-    private data: Data<M>;
+    constructor(private data: Data<M>) {}
     getData(): Data<M> {
         return this.data;
     }
@@ -50,7 +50,8 @@ var __extends = (this && this.__extends) || (function () {
 })();
 exports.__esModule = true;
 var Parent = /** @class */ (function () {
-    function Parent() {
+    function Parent(data) {
+        this.data = data;
     }
     Parent.prototype.getData = function () {
         return this.data;
 
@@ -29,20 +29,20 @@ class Parent<M> {
 >Parent : Symbol(Parent, Decl(indexedAccessTypeConstraints.ts, 8, 2))
 >M : Symbol(M, Decl(indexedAccessTypeConstraints.ts, 10, 13))
 
-    private data: Data<M>;
->data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 10, 17))
+    constructor(private data: Data<M>) {}
+>data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 11, 16))
 >Data : Symbol(Data, Decl(indexedAccessTypeConstraints.ts, 4, 1))
 >M : Symbol(M, Decl(indexedAccessTypeConstraints.ts, 10, 13))
 
     getData(): Data<M> {
->getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 26))
+>getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 41))
 >Data : Symbol(Data, Decl(indexedAccessTypeConstraints.ts, 4, 1))
 >M : Symbol(M, Decl(indexedAccessTypeConstraints.ts, 10, 13))
 
         return this.data;
->this.data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 10, 17))
+>this.data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 11, 16))
 >this : Symbol(Parent, Decl(indexedAccessTypeConstraints.ts, 8, 2))
->data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 10, 17))
+>data : Symbol(Parent.data, Decl(indexedAccessTypeConstraints.ts, 11, 16))
     }
 }
 
@@ -59,9 +59,9 @@ export class Foo<C> extends Parent<IData<C>> {
 
         return this.getData().get('content');
 >this.getData().get : Symbol(get, Decl(indexedAccessTypeConstraints.ts, 6, 16))
->this.getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 26))
+>this.getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 41))
 >this : Symbol(Foo, Decl(indexedAccessTypeConstraints.ts, 15, 1))
->getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 26))
+>getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 41))
 >get : Symbol(get, Decl(indexedAccessTypeConstraints.ts, 6, 16))
     }
 }
@@ -81,9 +81,9 @@ export class Bar<C, T extends IData<C>> extends Parent<T> {
 
         return this.getData().get('content');
 >this.getData().get : Symbol(get, Decl(indexedAccessTypeConstraints.ts, 6, 16))
->this.getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 26))
+>this.getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 41))
 >this : Symbol(Bar, Decl(indexedAccessTypeConstraints.ts, 21, 1))
->getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 26))
+>getData : Symbol(Parent.getData, Decl(indexedAccessTypeConstraints.ts, 11, 41))
 >get : Symbol(get, Decl(indexedAccessTypeConstraints.ts, 6, 16))
     }
 }
Original file line number	Diff line number	Diff line change
`@@ -1967,7 +1967,7 @@ namespace ts {`
`1967`	`1967`	`: moduleKind === ModuleKind.System;`
`1968`	`1968`	`}`
`1969`	`1969`
`1970`		`- export type StrictOptionName = "noImplicitAny" \| "noImplicitThis" \| "strictNullChecks" \| "strictFunctionTypes" \| "alwaysStrict";`
	`1970`	`+ export type StrictOptionName = "noImplicitAny" \| "noImplicitThis" \| "strictNullChecks" \| "strictFunctionTypes" \| "strictPropertyInitialization" \| "alwaysStrict";`
`1971`	`1971`
`1972`	`1972`	`export function getStrictOptionValue(compilerOptions: CompilerOptions, flag: StrictOptionName): boolean {`
`1973`	`1973`	`return compilerOptions[flag] === undefined ? compilerOptions.strict : compilerOptions[flag];`