Add use-before-def error for uninitialized property

src/compiler/checker.ts

@@ -1387,8 +1387,10 @@ namespace ts {
                     return !findAncestor(usage, n => isComputedPropertyName(n) && n.parent.parent === declaration);
                 }
                 else if (isPropertyDeclaration(declaration)) {
+                    // still might be illegal if the property has no initializer.
                     // still might be illegal if a self-referencing property initializer (eg private x = this.x)
-                    return !isPropertyImmediatelyReferencedWithinDeclaration(declaration, usage, /*stopAtAnyPropertyDeclaration*/ false);
+                    return !isUninitializedPropertyReferencedWithinDeclaration(declaration, usage)
+                        && !isPropertyImmediatelyReferencedWithinDeclaration(declaration, usage, /*stopAtAnyPropertyDeclaration*/ false);
                 }
                 else if (isParameterPropertyDeclaration(declaration, declaration.parent)) {
                     // foo = this.bar is illegal in esnext+useDefineForClassFields when bar is a parameter property
@@ -1486,6 +1488,36 @@ namespace ts {
                 });
             }
 
+            function isUninitializedPropertyReferencedWithinDeclaration(declaration: PropertyDeclaration, usage: Node) {
+                if (!strictNullChecks || !strictPropertyInitialization || declaration.flags & NodeFlags.Ambient) {
+                    return;
+                }
+                const ancestor = findAncestor(usage, (node: Node) => {
+                    switch (node.kind) {
+                        case SyntaxKind.ClassDeclaration:
+                        case SyntaxKind.ClassExpression:
+                        case SyntaxKind.ArrowFunction:
+                        case SyntaxKind.Block:
+                            return "quit";
+                        case SyntaxKind.PropertyDeclaration:
+                            return true;
+                        default:
+                            return isStatement(node) ? "quit" : false;
+                    }
+                });
+
+                if (!ancestor || ancestor.parent !== declaration.parent) {
+                    return false;
+                }
+
+                if (!isInstancePropertyWithoutInitializer(declaration)) {
+                    return false;
+                }
+
+                const type = getTypeOfSymbol(getSymbolOfNode(declaration));
+                return !(type.flags & TypeFlags.AnyOrUnknown || getFalsyFlags(type) & TypeFlags.Undefined);
+            }
+
             /** stopAtAnyPropertyDeclaration is used for detecting ES-standard class field use-before-def errors */
             function isPropertyImmediatelyReferencedWithinDeclaration(declaration: PropertyDeclaration | ParameterPropertyDeclaration, usage: Node, stopAtAnyPropertyDeclaration: boolean) {
                 // always legal if usage is after declaration

tests/baselines/reference/initializerWithThisPropertyAccess.errors.txt

@@ -1,8 +1,9 @@
 tests/cases/compiler/initializerWithThisPropertyAccess.ts(3,14): error TS2729: Property 'a' is used before its initialization.
+tests/cases/compiler/initializerWithThisPropertyAccess.ts(5,19): error TS2729: Property 'a' is used before its initialization.
 tests/cases/compiler/initializerWithThisPropertyAccess.ts(24,29): error TS2729: Property 'bar' is used before its initialization.
 
 
-==== tests/cases/compiler/initializerWithThisPropertyAccess.ts (2 errors) ====
+==== tests/cases/compiler/initializerWithThisPropertyAccess.ts (3 errors) ====
     class A {
         a: number;
         b = this.a;  // Error
@@ -11,6 +12,9 @@ tests/cases/compiler/initializerWithThisPropertyAccess.ts(24,29): error TS2729:
 !!! related TS2728 tests/cases/compiler/initializerWithThisPropertyAccess.ts:2:5: 'a' is declared here.
         c = () => this.a;
         d = (new A()).a;
+                      ~
+!!! error TS2729: Property 'a' is used before its initialization.
+!!! related TS2728 tests/cases/compiler/initializerWithThisPropertyAccess.ts:2:5: 'a' is declared here.
         constructor() {
             this.a = 1;
         }

tests/baselines/reference/useBeforeDeclaration_propertyDeclaration.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts(7,21): error TS2729: Property 'num' is used before its initialization.
+
+
+==== tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts (1 errors) ====
+    class A {
+        num: number;
+        constructor(num:number) {
+            this.num = num;
+        }
+
+        computed = this.num * 10;
+                        ~~~
+!!! error TS2729: Property 'num' is used before its initialization.
+!!! related TS2728 tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts:2:5: 'num' is declared here.
+    }

tests/baselines/reference/useBeforeDeclaration_propertyDeclaration.js

@@ -0,0 +1,19 @@
+//// [useBeforeDeclaration_propertyDeclaration.ts]
+class A {
+    num: number;
+    constructor(num:number) {
+        this.num = num;
+    }
+
+    computed = this.num * 10;
+}
+
+//// [useBeforeDeclaration_propertyDeclaration.js]
+"use strict";
+var A = /** @class */ (function () {
+    function A(num) {
+        this.computed = this.num * 10;
+        this.num = num;
+    }
+    return A;
+}());

tests/baselines/reference/useBeforeDeclaration_propertyDeclaration.symbols

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts ===
+class A {
+>A : Symbol(A, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 0))
+
+    num: number;
+>num : Symbol(A.num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 9))
+
+    constructor(num:number) {
+>num : Symbol(num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 2, 16))
+
+        this.num = num;
+>this.num : Symbol(A.num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 9))
+>this : Symbol(A, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 0))
+>num : Symbol(A.num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 9))
+>num : Symbol(num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 2, 16))
+    }
+
+    computed = this.num * 10;
+>computed : Symbol(A.computed, Decl(useBeforeDeclaration_propertyDeclaration.ts, 4, 5))
+>this.num : Symbol(A.num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 9))
+>this : Symbol(A, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 0))
+>num : Symbol(A.num, Decl(useBeforeDeclaration_propertyDeclaration.ts, 0, 9))
+}

tests/baselines/reference/useBeforeDeclaration_propertyDeclaration.types

@@ -0,0 +1,26 @@
+=== tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts ===
+class A {
+>A : A
+
+    num: number;
+>num : number
+
+    constructor(num:number) {
+>num : number
+
+        this.num = num;
+>this.num = num : number
+>this.num : number
+>this : this
+>num : number
+>num : number
+    }
+
+    computed = this.num * 10;
+>computed : number
+>this.num * 10 : number
+>this.num : number
+>this : this
+>num : number
+>10 : 10
+}

tests/cases/compiler/useBeforeDeclaration_propertyDeclaration.ts

@@ -0,0 +1,9 @@
+// @strict: true
+class A {
+    num: number;
+    constructor(num:number) {
+        this.num = num;
+    }
+
+    computed = this.num * 10;
+}