From df471c03cc67b86f12ecb772d8dc7f9e53dfa6a1 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Fri, 8 Dec 2023 12:17:36 +0000
Subject: [PATCH 1/8] Improve binding element type inference

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 src/compiler/checker.ts | 28 ++++++++++++++++++----------
 1 file changed, 18 insertions(+), 10 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 56c4d8a2114c9..3ab8bd8fb6af0 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11416,7 +11416,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             // contextual type or, if the element itself is a binding pattern, with the type implied by that binding
             // pattern.
             const contextualType = isBindingPattern(element.name) ? getTypeFromBindingPattern(element.name, /*includePatternInType*/ true, /*reportErrors*/ false) : unknownType;
-            return addOptionality(widenTypeInferredFromInitializer(element, checkDeclarationInitializer(element, CheckMode.Normal, contextualType)));
+            return addOptionality(widenTypeInferredFromInitializer(element, checkDeclarationInitializer(element, reportErrors ? CheckMode.Normal : CheckMode.Contextual, contextualType)));
         }
         if (isBindingPattern(element.name)) {
             return getTypeFromBindingPattern(element.name, includePatternInType, reportErrors);
@@ -11574,16 +11574,16 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return false;
     }
 
-    function getTypeOfVariableOrParameterOrProperty(symbol: Symbol): Type {
+    function getTypeOfVariableOrParameterOrProperty(symbol: Symbol, checkMode?: CheckMode): Type {
         const links = getSymbolLinks(symbol);
         if (!links.type) {
-            const type = getTypeOfVariableOrParameterOrPropertyWorker(symbol);
+            const type = getTypeOfVariableOrParameterOrPropertyWorker(symbol, checkMode);
             // For a contextually typed parameter it is possible that a type has already
             // been assigned (in assignTypeToParameterAndFixTypeParameters), and we want
             // to preserve this type. In fact, we need to _prefer_ that type, but it won't
             // be assigned until contextual typing is complete, so we need to defer in
             // cases where contextual typing may take place.
-            if (!links.type && !isParameterOfContextSensitiveSignature(symbol)) {
+            if (!links.type && !isParameterOfContextSensitiveSignature(symbol) && !(checkMode !== CheckMode.Normal && checkMode)) {
                 links.type = type;
             }
             return type;
@@ -11591,7 +11591,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return links.type;
     }
 
-    function getTypeOfVariableOrParameterOrPropertyWorker(symbol: Symbol): Type {
+    function getTypeOfVariableOrParameterOrPropertyWorker(symbol: Symbol, checkMode?: CheckMode): Type {
         // Handle prototype property
         if (symbol.flags & SymbolFlags.Prototype) {
             return getTypeOfPrototypeProperty(symbol);
@@ -11634,6 +11634,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (symbol.flags & SymbolFlags.ValueModule && !(symbol.flags & SymbolFlags.Assignment)) {
                 return getTypeOfFuncClassEnumModule(symbol);
             }
+
+            if (checkMode !== CheckMode.Normal && checkMode) {
+                return anyType;
+            }
             return reportCircularityError(symbol);
         }
         let type: Type;
@@ -11706,6 +11710,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             if (symbol.flags & SymbolFlags.ValueModule && !(symbol.flags & SymbolFlags.Assignment)) {
                 return getTypeOfFuncClassEnumModule(symbol);
             }
+
+            if (checkMode !== CheckMode.Normal && checkMode) {
+                return type;
+            }
             return reportCircularityError(symbol);
         }
         return type;
@@ -11988,7 +11996,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         return getTypeOfSymbol(symbol);
     }
 
-    function getTypeOfSymbol(symbol: Symbol): Type {
+    function getTypeOfSymbol(symbol: Symbol, checkMode?: CheckMode): Type {
         const checkFlags = getCheckFlags(symbol);
         if (checkFlags & CheckFlags.DeferredType) {
             return getTypeOfSymbolWithDeferredType(symbol);
@@ -12003,7 +12011,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             return getTypeOfReverseMappedSymbol(symbol as ReverseMappedSymbol);
         }
         if (symbol.flags & (SymbolFlags.Variable | SymbolFlags.Property)) {
-            return getTypeOfVariableOrParameterOrProperty(symbol);
+            return getTypeOfVariableOrParameterOrProperty(symbol, checkMode);
         }
         if (symbol.flags & (SymbolFlags.Function | SymbolFlags.Method | SymbolFlags.Class | SymbolFlags.Enum | SymbolFlags.ValueModule)) {
             return getTypeOfFuncClassEnumModule(symbol);
@@ -28823,8 +28831,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
         }
     }
 
-    function getNarrowedTypeOfSymbol(symbol: Symbol, location: Identifier) {
-        const type = getTypeOfSymbol(symbol);
+    function getNarrowedTypeOfSymbol(symbol: Symbol, location: Identifier, checkMode?: CheckMode) {
+        const type = getTypeOfSymbol(symbol, checkMode);
         const declaration = symbol.valueDeclaration;
         if (declaration) {
             // If we have a non-rest binding element with no initializer declared as a const variable or a const-like
@@ -28987,7 +28995,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
 
         checkNestedBlockScopedBinding(node, symbol);
 
-        let type = getNarrowedTypeOfSymbol(localOrExportSymbol, node);
+        let type = getNarrowedTypeOfSymbol(localOrExportSymbol, node, checkMode);
         const assignmentKind = getAssignmentTargetKind(node);
 
         if (assignmentKind) {

From 58db89a97b6eed3b95da204880530cd8d2f989f6 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Fri, 8 Dec 2023 12:21:12 +0000
Subject: [PATCH 2/8] Modify existing test case's '.types' file

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 ...tructuringArrayBindingPatternAndAssignment3.types | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment3.types b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment3.types
index 72aaa6378e5af..791ba83c243dc 100644
--- a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment3.types
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment3.types
@@ -2,16 +2,16 @@
 
 === destructuringArrayBindingPatternAndAssignment3.ts ===
 const [a, b = a] = [1]; // ok
->a : any
->b : any
->a : any
+>a : number
+>b : number
+>a : number
 >[1] : [number]
 >1 : 1
 
 const [c, d = c, e = e] = [1]; // error for e = e
->c : any
->d : any
->c : any
+>c : number
+>d : number
+>c : number
 >e : any
 >e : any
 >[1] : [number]

From 354ebb07c2c809cba0e3583b9d9af11506c1e608 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Mon, 11 Dec 2023 13:42:43 +0000
Subject: [PATCH 3/8] Add tests for array binding pattern assignment

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 ...PatternAndAssignment5SiblingInitializer.js | 38 +++++++++++
 ...rnAndAssignment5SiblingInitializer.symbols | 48 +++++++++++++
 ...ternAndAssignment5SiblingInitializer.types | 68 +++++++++++++++++++
 ...PatternAndAssignment5SiblingInitializer.ts | 19 ++++++
 4 files changed, 173 insertions(+)
 create mode 100644 tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
 create mode 100644 tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
 create mode 100644 tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
 create mode 100644 tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts

diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
new file mode 100644
index 0000000000000..58d662d96f1b8
--- /dev/null
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
@@ -0,0 +1,38 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts] ////
+
+//// [destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts]
+// To be inferred as `number`
+function f1() {
+    const [a1, b1 = a1] = [1];
+    const [a2, b2 = 1 + a2] = [1];
+}
+
+// To be inferred as `string`
+function f2() {
+    const [a1, b1 = a1] = ['hi'];
+    const [a2, b2 = a2 + '!'] = ['hi'];
+}
+
+// To be inferred as `string | number`
+function f3() {
+    const [a1, b1 = a1] = ['hi', 1];
+    const [a2, b2 = a2 + '!'] = ['hi', 1];
+}
+
+
+//// [destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js]
+// To be inferred as `number`
+function f1() {
+    var _a = [1], a1 = _a[0], _b = _a[1], b1 = _b === void 0 ? a1 : _b;
+    var _c = [1], a2 = _c[0], _d = _c[1], b2 = _d === void 0 ? 1 + a2 : _d;
+}
+// To be inferred as `string`
+function f2() {
+    var _a = ['hi'], a1 = _a[0], _b = _a[1], b1 = _b === void 0 ? a1 : _b;
+    var _c = ['hi'], a2 = _c[0], _d = _c[1], b2 = _d === void 0 ? a2 + '!' : _d;
+}
+// To be inferred as `string | number`
+function f3() {
+    var _a = ['hi', 1], a1 = _a[0], _b = _a[1], b1 = _b === void 0 ? a1 : _b;
+    var _c = ['hi', 1], a2 = _c[0], _d = _c[1], b2 = _d === void 0 ? a2 + '!' : _d;
+}
diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
new file mode 100644
index 0000000000000..115f9f7f0a6f8
--- /dev/null
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
@@ -0,0 +1,48 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts] ////
+
+=== destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts ===
+// To be inferred as `number`
+function f1() {
+>f1 : Symbol(f1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 0, 0))
+
+    const [a1, b1 = a1] = [1];
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 2, 11))
+>b1 : Symbol(b1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 2, 14))
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 2, 11))
+
+    const [a2, b2 = 1 + a2] = [1];
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 3, 11))
+>b2 : Symbol(b2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 3, 14))
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 3, 11))
+}
+
+// To be inferred as `string`
+function f2() {
+>f2 : Symbol(f2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 4, 1))
+
+    const [a1, b1 = a1] = ['hi'];
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 8, 11))
+>b1 : Symbol(b1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 8, 14))
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 8, 11))
+
+    const [a2, b2 = a2 + '!'] = ['hi'];
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 9, 11))
+>b2 : Symbol(b2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 9, 14))
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 9, 11))
+}
+
+// To be inferred as `string | number`
+function f3() {
+>f3 : Symbol(f3, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 10, 1))
+
+    const [a1, b1 = a1] = ['hi', 1];
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 14, 11))
+>b1 : Symbol(b1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 14, 14))
+>a1 : Symbol(a1, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 14, 11))
+
+    const [a2, b2 = a2 + '!'] = ['hi', 1];
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 15, 11))
+>b2 : Symbol(b2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 15, 14))
+>a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 15, 11))
+}
+
diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
new file mode 100644
index 0000000000000..df11e27070dfb
--- /dev/null
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
@@ -0,0 +1,68 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts] ////
+
+=== destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts ===
+// To be inferred as `number`
+function f1() {
+>f1 : () => void
+
+    const [a1, b1 = a1] = [1];
+>a1 : number
+>b1 : number
+>a1 : number
+>[1] : [number]
+>1 : 1
+
+    const [a2, b2 = 1 + a2] = [1];
+>a2 : number
+>b2 : number
+>1 + a2 : number
+>1 : 1
+>a2 : number
+>[1] : [number]
+>1 : 1
+}
+
+// To be inferred as `string`
+function f2() {
+>f2 : () => void
+
+    const [a1, b1 = a1] = ['hi'];
+>a1 : string
+>b1 : string
+>a1 : string
+>['hi'] : [string]
+>'hi' : "hi"
+
+    const [a2, b2 = a2 + '!'] = ['hi'];
+>a2 : string
+>b2 : string
+>a2 + '!' : string
+>a2 : string
+>'!' : "!"
+>['hi'] : [string]
+>'hi' : "hi"
+}
+
+// To be inferred as `string | number`
+function f3() {
+>f3 : () => void
+
+    const [a1, b1 = a1] = ['hi', 1];
+>a1 : string
+>b1 : string | number
+>a1 : string
+>['hi', 1] : [string, number]
+>'hi' : "hi"
+>1 : 1
+
+    const [a2, b2 = a2 + '!'] = ['hi', 1];
+>a2 : string
+>b2 : string | number
+>a2 + '!' : string
+>a2 : string
+>'!' : "!"
+>['hi', 1] : [string, number]
+>'hi' : "hi"
+>1 : 1
+}
+
diff --git a/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts b/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts
new file mode 100644
index 0000000000000..fecc40937fcd3
--- /dev/null
+++ b/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts
@@ -0,0 +1,19 @@
+// @noImplicitAny: true
+
+// To be inferred as `number`
+function f1() {
+    const [a1, b1 = a1] = [1];
+    const [a2, b2 = 1 + a2] = [1];
+}
+
+// To be inferred as `string`
+function f2() {
+    const [a1, b1 = a1] = ['hi'];
+    const [a2, b2 = a2 + '!'] = ['hi'];
+}
+
+// To be inferred as `string | number`
+function f3() {
+    const [a1, b1 = a1] = ['hi', 1];
+    const [a2, b2 = a2 + '!'] = ['hi', 1];
+}

From 3f1b5244e41fa488cbef4180285961ffb4d41699 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Mon, 11 Dec 2023 13:43:21 +0000
Subject: [PATCH 4/8] Add tests for object binding pattern assignment

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 ...PatternAndAssignment9SiblingInitializer.js | 37 +++++++++
 ...rnAndAssignment9SiblingInitializer.symbols | 55 ++++++++++++++
 ...ternAndAssignment9SiblingInitializer.types | 75 +++++++++++++++++++
 ...PatternAndAssignment9SiblingInitializer.ts | 19 +++++
 4 files changed, 186 insertions(+)
 create mode 100644 tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
 create mode 100644 tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
 create mode 100644 tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
 create mode 100644 tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts

diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
new file mode 100644
index 0000000000000..7ce61e4b8a1ac
--- /dev/null
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
@@ -0,0 +1,37 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts] ////
+
+//// [destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts]
+// To be inferred as `number`
+function f1() {
+    const { a1, b1 = a1 } = { a1: 1 };
+    const { a2, b2 = 1 + a2 } = { a2: 1 };
+}
+
+// To be inferred as `string`
+function f2() {
+    const { a1, b1 = a1 } = { a1: 'hi' };
+    const { a2, b2 = a2 + '!' } = { a2: 'hi' };
+}
+
+// To be inferred as `string | number`
+function f3() {
+    const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
+    const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
+}
+
+//// [destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js]
+// To be inferred as `number`
+function f1() {
+    var _a = { a1: 1 }, a1 = _a.a1, _b = _a.b1, b1 = _b === void 0 ? a1 : _b;
+    var _c = { a2: 1 }, a2 = _c.a2, _d = _c.b2, b2 = _d === void 0 ? 1 + a2 : _d;
+}
+// To be inferred as `string`
+function f2() {
+    var _a = { a1: 'hi' }, a1 = _a.a1, _b = _a.b1, b1 = _b === void 0 ? a1 : _b;
+    var _c = { a2: 'hi' }, a2 = _c.a2, _d = _c.b2, b2 = _d === void 0 ? a2 + '!' : _d;
+}
+// To be inferred as `string | number`
+function f3() {
+    var _a = { a1: 'hi', b1: 1 }, a1 = _a.a1, _b = _a.b1, b1 = _b === void 0 ? a1 : _b;
+    var _c = { a2: 'hi', b2: 1 }, a2 = _c.a2, _d = _c.b2, b2 = _d === void 0 ? a2 + '!' : _d;
+}
diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
new file mode 100644
index 0000000000000..57eb6eb863156
--- /dev/null
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
@@ -0,0 +1,55 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts] ////
+
+=== destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts ===
+// To be inferred as `number`
+function f1() {
+>f1 : Symbol(f1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 0, 0))
+
+    const { a1, b1 = a1 } = { a1: 1 };
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 2, 11))
+>b1 : Symbol(b1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 2, 15))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 2, 11))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 2, 29))
+
+    const { a2, b2 = 1 + a2 } = { a2: 1 };
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 3, 11))
+>b2 : Symbol(b2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 3, 15))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 3, 11))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 3, 33))
+}
+
+// To be inferred as `string`
+function f2() {
+>f2 : Symbol(f2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 4, 1))
+
+    const { a1, b1 = a1 } = { a1: 'hi' };
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 8, 11))
+>b1 : Symbol(b1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 8, 15))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 8, 11))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 8, 29))
+
+    const { a2, b2 = a2 + '!' } = { a2: 'hi' };
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 9, 11))
+>b2 : Symbol(b2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 9, 15))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 9, 11))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 9, 35))
+}
+
+// To be inferred as `string | number`
+function f3() {
+>f3 : Symbol(f3, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 10, 1))
+
+    const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 14, 11))
+>b1 : Symbol(b1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 14, 15))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 14, 11))
+>a1 : Symbol(a1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 14, 29))
+>b1 : Symbol(b1, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 14, 39))
+
+    const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 11))
+>b2 : Symbol(b2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 15))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 11))
+>a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 35))
+>b2 : Symbol(b2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 45))
+}
diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
new file mode 100644
index 0000000000000..2b7bc7d2aa1b4
--- /dev/null
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
@@ -0,0 +1,75 @@
+//// [tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts] ////
+
+=== destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts ===
+// To be inferred as `number`
+function f1() {
+>f1 : () => void
+
+    const { a1, b1 = a1 } = { a1: 1 };
+>a1 : number
+>b1 : number
+>a1 : number
+>{ a1: 1 } : { a1: number; b1?: number; }
+>a1 : number
+>1 : 1
+
+    const { a2, b2 = 1 + a2 } = { a2: 1 };
+>a2 : number
+>b2 : number
+>1 + a2 : number
+>1 : 1
+>a2 : number
+>{ a2: 1 } : { a2: number; b2?: number; }
+>a2 : number
+>1 : 1
+}
+
+// To be inferred as `string`
+function f2() {
+>f2 : () => void
+
+    const { a1, b1 = a1 } = { a1: 'hi' };
+>a1 : string
+>b1 : string
+>a1 : string
+>{ a1: 'hi' } : { a1: string; b1?: string; }
+>a1 : string
+>'hi' : "hi"
+
+    const { a2, b2 = a2 + '!' } = { a2: 'hi' };
+>a2 : string
+>b2 : string
+>a2 + '!' : string
+>a2 : string
+>'!' : "!"
+>{ a2: 'hi' } : { a2: string; b2?: string; }
+>a2 : string
+>'hi' : "hi"
+}
+
+// To be inferred as `string | number`
+function f3() {
+>f3 : () => void
+
+    const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
+>a1 : string
+>b1 : string | number
+>a1 : string
+>{ a1: 'hi', b1: 1 } : { a1: string; b1?: number; }
+>a1 : string
+>'hi' : "hi"
+>b1 : number
+>1 : 1
+
+    const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
+>a2 : string
+>b2 : string | number
+>a2 + '!' : string
+>a2 : string
+>'!' : "!"
+>{ a2: 'hi', b2: 1 } : { a2: string; b2?: number; }
+>a2 : string
+>'hi' : "hi"
+>b2 : number
+>1 : 1
+}
diff --git a/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts b/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts
new file mode 100644
index 0000000000000..200e75e336389
--- /dev/null
+++ b/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts
@@ -0,0 +1,19 @@
+// @noImplicitAny: true
+
+// To be inferred as `number`
+function f1() {
+    const { a1, b1 = a1 } = { a1: 1 };
+    const { a2, b2 = 1 + a2 } = { a2: 1 };
+}
+
+// To be inferred as `string`
+function f2() {
+    const { a1, b1 = a1 } = { a1: 'hi' };
+    const { a2, b2 = a2 + '!' } = { a2: 'hi' };
+}
+
+// To be inferred as `string | number`
+function f3() {
+    const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
+    const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
+}
\ No newline at end of file

From d00a5b7e7c0ba16b41acd361f7d8f37736366d91 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Sat, 16 Dec 2023 21:03:46 +0000
Subject: [PATCH 5/8] Improve test cases

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 ...PatternAndAssignment5SiblingInitializer.js | 10 ++++++++++
 ...rnAndAssignment5SiblingInitializer.symbols | 15 ++++++++++++++
 ...ternAndAssignment5SiblingInitializer.types | 17 ++++++++++++++++
 ...PatternAndAssignment9SiblingInitializer.js | 13 +++++++++++-
 ...rnAndAssignment9SiblingInitializer.symbols | 18 +++++++++++++++++
 ...ternAndAssignment9SiblingInitializer.types | 20 +++++++++++++++++++
 ...PatternAndAssignment5SiblingInitializer.ts |  7 +++++++
 ...PatternAndAssignment9SiblingInitializer.ts |  9 ++++++++-
 8 files changed, 107 insertions(+), 2 deletions(-)

diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
index 58d662d96f1b8..47eaab26cd17b 100644
--- a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js
@@ -18,6 +18,13 @@ function f3() {
     const [a1, b1 = a1] = ['hi', 1];
     const [a2, b2 = a2 + '!'] = ['hi', 1];
 }
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: [number, number] | undefined
+function f4() {
+    const [ a, b = a ] = yadda ?? [];
+}
 
 
 //// [destructuringArrayBindingPatternAndAssignment5SiblingInitializer.js]
@@ -36,3 +43,6 @@ function f3() {
     var _a = ['hi', 1], a1 = _a[0], _b = _a[1], b1 = _b === void 0 ? a1 : _b;
     var _c = ['hi', 1], a2 = _c[0], _d = _c[1], b2 = _d === void 0 ? a2 + '!' : _d;
 }
+function f4() {
+    var _a = yadda !== null && yadda !== void 0 ? yadda : [], a = _a[0], _b = _a[1], b = _b === void 0 ? a : _b;
+}
diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
index 115f9f7f0a6f8..2ff79c20083f2 100644
--- a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.symbols
@@ -46,3 +46,18 @@ function f3() {
 >a2 : Symbol(a2, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 15, 11))
 }
 
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: [number, number] | undefined
+>yadda : Symbol(yadda, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 20, 13))
+
+function f4() {
+>f4 : Symbol(f4, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 20, 49))
+
+    const [ a, b = a ] = yadda ?? [];
+>a : Symbol(a, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 22, 11))
+>b : Symbol(b, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 22, 14))
+>a : Symbol(a, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 22, 11))
+>yadda : Symbol(yadda, Decl(destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts, 20, 13))
+}
+
diff --git a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
index df11e27070dfb..009a62ee2d987 100644
--- a/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
+++ b/tests/baselines/reference/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.types
@@ -66,3 +66,20 @@ function f3() {
 >1 : 1
 }
 
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: [number, number] | undefined
+>yadda : [number, number]
+
+function f4() {
+>f4 : () => void
+
+    const [ a, b = a ] = yadda ?? [];
+>a : number
+>b : number
+>a : number
+>yadda ?? [] : [number, number] | []
+>yadda : [number, number]
+>[] : []
+}
+
diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
index 7ce61e4b8a1ac..92a7a56842bfc 100644
--- a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js
@@ -17,7 +17,15 @@ function f2() {
 function f3() {
     const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
     const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
-}
+}
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: { a?: number, b?: number } | undefined
+function f4() {
+    const { a, b = a } = yadda ?? {};
+}
+
 
 //// [destructuringObjectBindingPatternAndAssignment9SiblingInitializer.js]
 // To be inferred as `number`
@@ -35,3 +43,6 @@ function f3() {
     var _a = { a1: 'hi', b1: 1 }, a1 = _a.a1, _b = _a.b1, b1 = _b === void 0 ? a1 : _b;
     var _c = { a2: 'hi', b2: 1 }, a2 = _c.a2, _d = _c.b2, b2 = _d === void 0 ? a2 + '!' : _d;
 }
+function f4() {
+    var _a = yadda !== null && yadda !== void 0 ? yadda : {}, a = _a.a, _b = _a.b, b = _b === void 0 ? a : _b;
+}
diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
index 57eb6eb863156..3c76a5d2b045d 100644
--- a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.symbols
@@ -53,3 +53,21 @@ function f3() {
 >a2 : Symbol(a2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 35))
 >b2 : Symbol(b2, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 15, 45))
 }
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: { a?: number, b?: number } | undefined
+>yadda : Symbol(yadda, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 20, 13))
+>a : Symbol(a, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 20, 22))
+>b : Symbol(b, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 20, 34))
+
+function f4() {
+>f4 : Symbol(f4, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 20, 59))
+
+    const { a, b = a } = yadda ?? {};
+>a : Symbol(a, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 22, 11))
+>b : Symbol(b, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 22, 14))
+>a : Symbol(a, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 22, 11))
+>yadda : Symbol(yadda, Decl(destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts, 20, 13))
+}
+
diff --git a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
index 2b7bc7d2aa1b4..c5ebaabe82453 100644
--- a/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
+++ b/tests/baselines/reference/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.types
@@ -73,3 +73,23 @@ function f3() {
 >b2 : number
 >1 : 1
 }
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: { a?: number, b?: number } | undefined
+>yadda : { a?: number; b?: number; }
+>a : number
+>b : number
+
+function f4() {
+>f4 : () => void
+
+    const { a, b = a } = yadda ?? {};
+>a : number
+>b : number
+>a : number
+>yadda ?? {} : { a?: number; b?: number; }
+>yadda : { a?: number; b?: number; }
+>{} : {}
+}
+
diff --git a/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts b/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts
index fecc40937fcd3..6183e22b61fbf 100644
--- a/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts
+++ b/tests/cases/conformance/es6/destructuring/destructuringArrayBindingPatternAndAssignment5SiblingInitializer.ts
@@ -17,3 +17,10 @@ function f3() {
     const [a1, b1 = a1] = ['hi', 1];
     const [a2, b2 = a2 + '!'] = ['hi', 1];
 }
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: [number, number] | undefined
+function f4() {
+    const [ a, b = a ] = yadda ?? [];
+}
diff --git a/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts b/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts
index 200e75e336389..d4d5cd7c6cd4d 100644
--- a/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts
+++ b/tests/cases/conformance/es6/destructuring/destructuringObjectBindingPatternAndAssignment9SiblingInitializer.ts
@@ -16,4 +16,11 @@ function f2() {
 function f3() {
     const { a1, b1 = a1 } = { a1: 'hi', b1: 1 };
     const { a2, b2 = a2 + '!' } = { a2: 'hi', b2: 1 };
-}
\ No newline at end of file
+}
+
+// Based on comment:
+//   - https://github.com/microsoft/TypeScript/issues/49989#issuecomment-1852694486
+declare const yadda: { a?: number, b?: number } | undefined
+function f4() {
+    const { a, b = a } = yadda ?? {};
+}

From 86a543b9ad33dff0b4ae7abac3b6c8195b0fd5cb Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Sat, 16 Dec 2023 22:02:30 +0000
Subject: [PATCH 6/8] Restrict ignoring circular error to binding element
 contextual type checking

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 src/compiler/checker.ts | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3ab8bd8fb6af0..7be367dc6dd76 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11635,7 +11635,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 return getTypeOfFuncClassEnumModule(symbol);
             }
 
-            if (checkMode !== CheckMode.Normal && checkMode) {
+            // When trying to get the *contextual* type of a binding element, it's possible to fall in a loop and therefore
+            // end up in a circularity-like situation. This is not a true circularity so we should not report such an error.
+            // For example, here the looping could happen when trying to get the type of `a` (binding element):
+            //
+            //   const { a, b = a } = { a: 0 }
+            //
+            if (isBindingElement(declaration) && checkMode === CheckMode.Contextual) {
                 return anyType;
             }
             return reportCircularityError(symbol);
@@ -11711,7 +11717,13 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 return getTypeOfFuncClassEnumModule(symbol);
             }
 
-            if (checkMode !== CheckMode.Normal && checkMode) {
+            // When trying to get the *contextual* type of a binding element, it's possible to fall in a loop and therefore
+            // end up in a circularity-like situation. This is not a true circularity so we should not report such an error.
+            // For example, here the looping could happen when trying to get the type of `a` (binding element):
+            //
+            //   const { a, b = a } = { a: 0 }
+            //
+            if (isBindingElement(declaration) && checkMode === CheckMode.Contextual) {
                 return type;
             }
             return reportCircularityError(symbol);

From e872af56a8c9139cc17fdad641f6ef07b3e5a290 Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Sat, 16 Dec 2023 22:05:49 +0000
Subject: [PATCH 7/8] Return `errorType` when resolving binding element
 contextual type falls in a loop

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 7be367dc6dd76..3dc2dd68f2f1d 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11642,7 +11642,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             //   const { a, b = a } = { a: 0 }
             //
             if (isBindingElement(declaration) && checkMode === CheckMode.Contextual) {
-                return anyType;
+                return errorType;
             }
             return reportCircularityError(symbol);
         }

From 6b7d58033d9292ed41fb37bf47869d11416ecd9b Mon Sep 17 00:00:00 2001
From: "Babak K. Shandiz" <babak.k.shandiz@gmail.com>
Date: Wed, 10 Jan 2024 21:12:42 +0000
Subject: [PATCH 8/8] Simplify condition on `checkMode`

Signed-off-by: Babak K. Shandiz <babak.k.shandiz@gmail.com>
---
 src/compiler/checker.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 3dc2dd68f2f1d..c98e5743478ef 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11583,7 +11583,7 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             // to preserve this type. In fact, we need to _prefer_ that type, but it won't
             // be assigned until contextual typing is complete, so we need to defer in
             // cases where contextual typing may take place.
-            if (!links.type && !isParameterOfContextSensitiveSignature(symbol) && !(checkMode !== CheckMode.Normal && checkMode)) {
+            if (!links.type && !isParameterOfContextSensitiveSignature(symbol) && !checkMode) {
                 links.type = type;
             }
             return type;