microsoft · DukeDeSouth · Jun 17, 2026
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
@@ -29475,7 +29475,10 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 if (isIdentifier(expr)) {
                     const symbol = getResolvedSymbol(expr);
                     const declaration = getExportSymbolOfValueSymbolIfExported(symbol).valueDeclaration;
-                    if (declaration && (isBindingElement(declaration) || isParameter(declaration)) && reference === declaration.parent && !declaration.initializer && !declaration.dotDotDotToken) {
+                    if (
+                        declaration && (isBindingElement(declaration) || isParameter(declaration)) && reference === declaration.parent && !declaration.dotDotDotToken &&
+                        (!declaration.initializer || isBindingElement(declaration) && bindingElementDefaultNeverApplies(declaration))
+                    ) {
                         return declaration;
                     }
                 }
@@ -29509,6 +29512,21 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             function getCandidateVariableDeclarationInitializer(node: Node) {
                 return isVariableDeclaration(node) && !node.type && node.initializer ? skipParentheses(node.initializer) : undefined;
             }
+
+            function bindingElementDefaultNeverApplies(declaration: BindingElement) {
+                // A binding element with a default initializer (e.g. `{ kind = "a" }`) only faithfully mirrors its
+                // source property for discriminant narrowing when the default can never substitute for a real value,
+                // i.e. the property is present and non-undefined in every constituent of the declared union. For an
+                // optional or possibly-undefined property the default stands in for `undefined`, so narrowing the
+                // parent by this binding would be unsound.
+                const name = getAccessedPropertyName(declaration);
+                if (name === undefined) {
+                    return false;
+                }
+                const prop = getPropertyOfType(declaredType, name);
+                return !!prop && !(prop.flags & SymbolFlags.Optional) && !(getCheckFlags(prop) & CheckFlags.Partial) &&
+                    !maybeTypeOfKind(getTypeOfSymbol(prop), TypeFlags.Undefined);
+            }
         }
 
         function getDiscriminantPropertyAccess(expr: Expression, computedType: Type) {

diff --git a/tests/baselines/reference/dependentDestructuredVariablesWithDefaults.errors.txt b/tests/baselines/reference/dependentDestructuredVariablesWithDefaults.errors.txt
@@ -0,0 +1,117 @@
+dependentDestructuredVariablesWithDefaults.ts(74,15): error TS2322: Type 'string | number' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+dependentDestructuredVariablesWithDefaults.ts(85,15): error TS2322: Type 'string | number' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+dependentDestructuredVariablesWithDefaults.ts(96,15): error TS2322: Type 'string | number' is not assignable to type 'string'.
+  Type 'number' is not assignable to type 'string'.
+
+
+==== dependentDestructuredVariablesWithDefaults.ts (3 errors) ====
+    // https://github.com/microsoft/TypeScript/issues/50139
+    // A default on a *required* discriminant must not disable discriminant narrowing of siblings.
+
+    type Props =
+        | { isText: true, children: string }
+        | { isText: false, children: number };
+
+    // Baseline: no default — narrows (already worked).
+    function noDefault({ isText, children }: Props) {
+        if (isText === true) {
+            const s: string = children;
+        } else {
+            const n: number = children;
+        }
+    }
+
+    // The bug: default on a required discriminant should still narrow siblings.
+    function withDefault({ isText = false, children }: Props) {
+        if (isText === true) {
+            const s: string = children;
+        } else {
+            const n: number = children;
+        }
+    }
+
+    // switch form.
+    function withDefaultSwitch({ isText = false, children }: Props) {
+        switch (isText) {
+            case true: { const s: string = children; break; }
+            case false: { const n: number = children; break; }
+        }
+    }
+
+    // Renamed binding `{ isText: t = false }`.
+    function renamed({ isText: t = false, children }: Props) {
+        if (t === true) {
+            const s: string = children;
+        } else {
+            const n: number = children;
+        }
+    }
+
+    // Three-way union with a string discriminant.
+    type Three =
+        | { t: "i", v: number }
+        | { t: "s", v: string }
+        | { t: "b", v: boolean };
+
+    function threeWay({ t = "i", v }: Three) {
+        if (t === "s") {
+            const s: string = v;
+        }
+    }
+
+    // const destructuring (not a parameter).
+    declare const props: Props;
+    function constDestructure() {
+        const { isText = false, children } = props;
+        if (isText === true) {
+            const s: string = children;
+        }
+    }
+
+    // --- Soundness boundary: these must still error ---
+
+    // Optional discriminant + default: calling with the property omitted yields the default,
+    // so the sibling cannot be safely narrowed.
+    type OptDisc =
+        | { kind?: "a", x: string }
+        | { kind?: "b", x: number };
+
+    function optionalDiscriminant({ kind = "a", x }: OptDisc) {
+        if (kind === "a") {
+            const s: string = x; // error
+                  ~
+!!! error TS2322: Type 'string | number' is not assignable to type 'string'.
+!!! error TS2322:   Type 'number' is not assignable to type 'string'.
+        }
+    }
+
+    // Discriminant whose type already includes undefined.
+    type UndefDisc =
+        | { kind: "a" | undefined, x: string }
+        | { kind: "b", x: number };
+
+    function undefinedDiscriminant({ kind = "a", x }: UndefDisc) {
+        if (kind === "a") {
+            const s: string = x; // error
+                  ~
+!!! error TS2322: Type 'string | number' is not assignable to type 'string'.
+!!! error TS2322:   Type 'number' is not assignable to type 'string'.
+        }
+    }
+
+    // Sibling has its OWN default: it must not be narrowed from the parent.
+    type PropsOpt =
+        | { isText: true, children?: string }
+        | { isText: false, children?: number };
+
+    function siblingDefault({ isText = false, children = 0 }: PropsOpt) {
+        if (isText === true) {
+            const s: string = children; // error
+                  ~
+!!! error TS2322: Type 'string | number' is not assignable to type 'string'.
+!!! error TS2322:   Type 'number' is not assignable to type 'string'.
+        }
+    }
+
diff --git a/tests/baselines/reference/dependentDestructuredVariablesWithDefaults.symbols b/tests/baselines/reference/dependentDestructuredVariablesWithDefaults.symbols
@@ -0,0 +1,238 @@
+//// [tests/cases/conformance/controlFlow/dependentDestructuredVariablesWithDefaults.ts] ////
+
+=== dependentDestructuredVariablesWithDefaults.ts ===
+// https://github.com/microsoft/TypeScript/issues/50139
+// A default on a *required* discriminant must not disable discriminant narrowing of siblings.
+
+type Props =
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+    | { isText: true, children: string }
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 4, 7))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 4, 21))
+
+    | { isText: false, children: number };
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 5, 7))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 5, 22))
+
+// Baseline: no default — narrows (already worked).
+function noDefault({ isText, children }: Props) {
+>noDefault : Symbol(noDefault, Decl(dependentDestructuredVariablesWithDefaults.ts, 5, 42))
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 8, 20))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 8, 28))
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+    if (isText === true) {
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 8, 20))
+
+        const s: string = children;
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 10, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 8, 28))
+
+    } else {
+        const n: number = children;
+>n : Symbol(n, Decl(dependentDestructuredVariablesWithDefaults.ts, 12, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 8, 28))
+    }
+}
+
+// The bug: default on a required discriminant should still narrow siblings.
+function withDefault({ isText = false, children }: Props) {
+>withDefault : Symbol(withDefault, Decl(dependentDestructuredVariablesWithDefaults.ts, 14, 1))
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 17, 22))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 17, 38))
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+    if (isText === true) {
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 17, 22))
+
+        const s: string = children;
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 19, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 17, 38))
+
+    } else {
+        const n: number = children;
+>n : Symbol(n, Decl(dependentDestructuredVariablesWithDefaults.ts, 21, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 17, 38))
+    }
+}
+
+// switch form.
+function withDefaultSwitch({ isText = false, children }: Props) {
+>withDefaultSwitch : Symbol(withDefaultSwitch, Decl(dependentDestructuredVariablesWithDefaults.ts, 23, 1))
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 26, 28))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 26, 44))
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+    switch (isText) {
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 26, 28))
+
+        case true: { const s: string = children; break; }
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 28, 26))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 26, 44))
+
+        case false: { const n: number = children; break; }
+>n : Symbol(n, Decl(dependentDestructuredVariablesWithDefaults.ts, 29, 27))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 26, 44))
+    }
+}
+
+// Renamed binding `{ isText: t = false }`.
+function renamed({ isText: t = false, children }: Props) {
+>renamed : Symbol(renamed, Decl(dependentDestructuredVariablesWithDefaults.ts, 31, 1))
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 4, 7), Decl(dependentDestructuredVariablesWithDefaults.ts, 5, 7))
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 34, 18))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 34, 37))
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+    if (t === true) {
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 34, 18))
+
+        const s: string = children;
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 36, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 34, 37))
+
+    } else {
+        const n: number = children;
+>n : Symbol(n, Decl(dependentDestructuredVariablesWithDefaults.ts, 38, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 34, 37))
+    }
+}
+
+// Three-way union with a string discriminant.
+type Three =
+>Three : Symbol(Three, Decl(dependentDestructuredVariablesWithDefaults.ts, 40, 1))
+
+    | { t: "i", v: number }
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 44, 7))
+>v : Symbol(v, Decl(dependentDestructuredVariablesWithDefaults.ts, 44, 15))
+
+    | { t: "s", v: string }
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 45, 7))
+>v : Symbol(v, Decl(dependentDestructuredVariablesWithDefaults.ts, 45, 15))
+
+    | { t: "b", v: boolean };
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 46, 7))
+>v : Symbol(v, Decl(dependentDestructuredVariablesWithDefaults.ts, 46, 15))
+
+function threeWay({ t = "i", v }: Three) {
+>threeWay : Symbol(threeWay, Decl(dependentDestructuredVariablesWithDefaults.ts, 46, 29))
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 48, 19))
+>v : Symbol(v, Decl(dependentDestructuredVariablesWithDefaults.ts, 48, 28))
+>Three : Symbol(Three, Decl(dependentDestructuredVariablesWithDefaults.ts, 40, 1))
+
+    if (t === "s") {
+>t : Symbol(t, Decl(dependentDestructuredVariablesWithDefaults.ts, 48, 19))
+
+        const s: string = v;
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 50, 13))
+>v : Symbol(v, Decl(dependentDestructuredVariablesWithDefaults.ts, 48, 28))
+    }
+}
+
+// const destructuring (not a parameter).
+declare const props: Props;
+>props : Symbol(props, Decl(dependentDestructuredVariablesWithDefaults.ts, 55, 13))
+>Props : Symbol(Props, Decl(dependentDestructuredVariablesWithDefaults.ts, 0, 0))
+
+function constDestructure() {
+>constDestructure : Symbol(constDestructure, Decl(dependentDestructuredVariablesWithDefaults.ts, 55, 27))
+
+    const { isText = false, children } = props;
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 57, 11))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 57, 27))
+>props : Symbol(props, Decl(dependentDestructuredVariablesWithDefaults.ts, 55, 13))
+
+    if (isText === true) {
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 57, 11))
+
+        const s: string = children;
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 59, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 57, 27))
+    }
+}
+
+// --- Soundness boundary: these must still error ---
+
+// Optional discriminant + default: calling with the property omitted yields the default,
+// so the sibling cannot be safely narrowed.
+type OptDisc =
+>OptDisc : Symbol(OptDisc, Decl(dependentDestructuredVariablesWithDefaults.ts, 61, 1))
+
+    | { kind?: "a", x: string }
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 68, 7))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 68, 19))
+
+    | { kind?: "b", x: number };
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 69, 7))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 69, 19))
+
+function optionalDiscriminant({ kind = "a", x }: OptDisc) {
+>optionalDiscriminant : Symbol(optionalDiscriminant, Decl(dependentDestructuredVariablesWithDefaults.ts, 69, 32))
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 71, 31))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 71, 43))
+>OptDisc : Symbol(OptDisc, Decl(dependentDestructuredVariablesWithDefaults.ts, 61, 1))
+
+    if (kind === "a") {
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 71, 31))
+
+        const s: string = x; // error
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 73, 13))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 71, 43))
+    }
+}
+
+// Discriminant whose type already includes undefined.
+type UndefDisc =
+>UndefDisc : Symbol(UndefDisc, Decl(dependentDestructuredVariablesWithDefaults.ts, 75, 1))
+
+    | { kind: "a" | undefined, x: string }
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 79, 7))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 79, 30))
+
+    | { kind: "b", x: number };
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 80, 7))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 80, 18))
+
+function undefinedDiscriminant({ kind = "a", x }: UndefDisc) {
+>undefinedDiscriminant : Symbol(undefinedDiscriminant, Decl(dependentDestructuredVariablesWithDefaults.ts, 80, 31))
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 82, 32))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 82, 44))
+>UndefDisc : Symbol(UndefDisc, Decl(dependentDestructuredVariablesWithDefaults.ts, 75, 1))
+
+    if (kind === "a") {
+>kind : Symbol(kind, Decl(dependentDestructuredVariablesWithDefaults.ts, 82, 32))
+
+        const s: string = x; // error
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 84, 13))
+>x : Symbol(x, Decl(dependentDestructuredVariablesWithDefaults.ts, 82, 44))
+    }
+}
+
+// Sibling has its OWN default: it must not be narrowed from the parent.
+type PropsOpt =
+>PropsOpt : Symbol(PropsOpt, Decl(dependentDestructuredVariablesWithDefaults.ts, 86, 1))
+
+    | { isText: true, children?: string }
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 90, 7))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 90, 21))
+
+    | { isText: false, children?: number };
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 91, 7))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 91, 22))
+
+function siblingDefault({ isText = false, children = 0 }: PropsOpt) {
+>siblingDefault : Symbol(siblingDefault, Decl(dependentDestructuredVariablesWithDefaults.ts, 91, 43))
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 93, 25))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 93, 41))
+>PropsOpt : Symbol(PropsOpt, Decl(dependentDestructuredVariablesWithDefaults.ts, 86, 1))
+
+    if (isText === true) {
+>isText : Symbol(isText, Decl(dependentDestructuredVariablesWithDefaults.ts, 93, 25))
+
+        const s: string = children; // error
+>s : Symbol(s, Decl(dependentDestructuredVariablesWithDefaults.ts, 95, 13))
+>children : Symbol(children, Decl(dependentDestructuredVariablesWithDefaults.ts, 93, 41))
+    }
+}
+