Filter the type of a binding pattern to not include undefined is the pattern parent has an initializer (#37309)

Author: weswigham

src/compiler/checker.ts

@@ -7164,6 +7164,10 @@ namespace ts {
             if (strictNullChecks && declaration.flags & NodeFlags.Ambient && isParameterDeclaration(declaration)) {
                 parentType = getNonNullableType(parentType);
             }
+            // Filter `undefined` from the type we check against if the parent has an initializer (which handles the `undefined` case implicitly)
+            else if (strictNullChecks && pattern.parent.initializer) {
+                parentType = getTypeWithFacts(parentType, TypeFacts.NEUndefined);
+            }
 
             let type: Type | undefined;
             if (pattern.kind === SyntaxKind.ObjectBindingPattern) {

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.js

@@ -0,0 +1,17 @@
+//// [contextualTypeForInitalizedVariablesFiltersUndefined.ts]
+const fInferred = ({ a = 0 } = {}) => a;
+// const fInferred: ({ a }?: { a?: number; }) => number
+
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+
+//// [contextualTypeForInitalizedVariablesFiltersUndefined.js]
+"use strict";
+var fInferred = function (_a) {
+    var _b = (_a === void 0 ? {} : _a).a, a = _b === void 0 ? 0 : _b;
+    return a;
+};
+// const fInferred: ({ a }?: { a?: number; }) => number
+var fAnnotated = function (_a) {
+    var _b = (_a === void 0 ? {} : _a).a, a = _b === void 0 ? 0 : _b;
+    return a;
+};

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.symbols

@@ -0,0 +1,14 @@
+=== tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts ===
+const fInferred = ({ a = 0 } = {}) => a;
+>fInferred : Symbol(fInferred, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 0, 5))
+>a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 0, 20))
+>a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 0, 20))
+
+// const fInferred: ({ a }?: { a?: number; }) => number
+
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+>fAnnotated : Symbol(fAnnotated, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 3, 5))
+>fInferred : Symbol(fInferred, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 0, 5))
+>a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 3, 39))
+>a : Symbol(a, Decl(contextualTypeForInitalizedVariablesFiltersUndefined.ts, 3, 39))
+

tests/baselines/reference/contextualTypeForInitalizedVariablesFiltersUndefined.types

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts ===
+const fInferred = ({ a = 0 } = {}) => a;
+>fInferred : ({ a }?: { a?: number | undefined; }) => number
+>({ a = 0 } = {}) => a : ({ a }?: { a?: number | undefined; }) => number
+>a : number
+>0 : 0
+>{} : { a?: number | undefined; }
+>a : number
+
+// const fInferred: ({ a }?: { a?: number; }) => number
+
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;
+>fAnnotated : ({ a }?: { a?: number | undefined; }) => number
+>fInferred : ({ a }?: { a?: number | undefined; }) => number
+>({ a = 0 } = {}) => a : ({ a }?: { a?: number | undefined; } | undefined) => number
+>a : number
+>0 : 0
+>{} : {}
+>a : number
+

tests/cases/compiler/contextualTypeForInitalizedVariablesFiltersUndefined.ts

@@ -0,0 +1,5 @@
+// @strict: true
+const fInferred = ({ a = 0 } = {}) => a;
+// const fInferred: ({ a }?: { a?: number; }) => number
+
+const fAnnotated: typeof fInferred = ({ a = 0 } = {}) => a;