From b7af3a22840e28e6fa92af20c5ebe1f5e35d0228 Mon Sep 17 00:00:00 2001
From: Jake Bailey <5341706+jakebailey@users.noreply.github.com>
Date: Sat, 28 Mar 2026 21:12:51 -0700
Subject: [PATCH] Port tsgo PR 3276 for testing

---
 src/compiler/checker.ts                       | 14 +++++---
 ...dingPatternOptionalParameterCached.symbols | 27 +++++++++++++++
 ...indingPatternOptionalParameterCached.types | 34 +++++++++++++++++++
 .../bindingPatternOptionalParameterCached.ts  | 15 ++++++++
 4 files changed, 86 insertions(+), 4 deletions(-)
 create mode 100644 tests/baselines/reference/bindingPatternOptionalParameterCached.symbols
 create mode 100644 tests/baselines/reference/bindingPatternOptionalParameterCached.types
 create mode 100644 tests/cases/compiler/bindingPatternOptionalParameterCached.ts

diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 0567712f11da3..f21d227649663 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -11601,11 +11601,17 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
     // Return the type of a binding element parent. We check SymbolLinks first to see if a type has been
     // assigned by contextual typing.
     function getTypeForBindingElementParent(node: BindingElementGrandparent, checkMode: CheckMode) {
-        if (checkMode !== CheckMode.Normal) {
-            return getTypeForVariableLikeDeclaration(node, /*includeOptionality*/ false, checkMode);
+        if (checkMode === CheckMode.Normal) {
+            // We can use a cached resolved type if no optionality was included in that type.
+            const symbol = getSymbolOfDeclaration(node);
+            if (symbol) {
+                const type = getSymbolLinks(symbol).type;
+                if (type && !(strictNullChecks && isOptionalDeclaration(node))) {
+                    return type;
+                }
+            }
         }
-        const symbol = getSymbolOfDeclaration(node);
-        return symbol && getSymbolLinks(symbol).type || getTypeForVariableLikeDeclaration(node, /*includeOptionality*/ false, checkMode);
+        return getTypeForVariableLikeDeclaration(node, /*includeOptionality*/ false, checkMode);
     }
 
     function getRestType(source: Type, properties: PropertyName[], symbol: Symbol | undefined): Type {
diff --git a/tests/baselines/reference/bindingPatternOptionalParameterCached.symbols b/tests/baselines/reference/bindingPatternOptionalParameterCached.symbols
new file mode 100644
index 0000000000000..eeccb919821e1
--- /dev/null
+++ b/tests/baselines/reference/bindingPatternOptionalParameterCached.symbols
@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/bindingPatternOptionalParameterCached.ts] ////
+
+=== bindingPatternOptionalParameterCached.ts ===
+// https://github.com/microsoft/typescript-go/issues/3276
+
+// There should be no error in this test, but previously there was because of the
+// declaration of mock. Commenting it out would make the error disappear.
+
+export const mock: I = {
+>mock : Symbol(mock, Decl(bindingPatternOptionalParameterCached.ts, 5, 12))
+>I : Symbol(I, Decl(bindingPatternOptionalParameterCached.ts, 7, 2))
+
+    m: (_) => {},
+>m : Symbol(m, Decl(bindingPatternOptionalParameterCached.ts, 5, 24))
+>_ : Symbol(_, Decl(bindingPatternOptionalParameterCached.ts, 6, 8))
+
+};
+
+export interface I {
+>I : Symbol(I, Decl(bindingPatternOptionalParameterCached.ts, 7, 2))
+
+    m({ x }?: { x: boolean }): void
+>m : Symbol(I.m, Decl(bindingPatternOptionalParameterCached.ts, 9, 20))
+>x : Symbol(x, Decl(bindingPatternOptionalParameterCached.ts, 10, 7))
+>x : Symbol(x, Decl(bindingPatternOptionalParameterCached.ts, 10, 15))
+}
+
diff --git a/tests/baselines/reference/bindingPatternOptionalParameterCached.types b/tests/baselines/reference/bindingPatternOptionalParameterCached.types
new file mode 100644
index 0000000000000..a1205ddc8fdcd
--- /dev/null
+++ b/tests/baselines/reference/bindingPatternOptionalParameterCached.types
@@ -0,0 +1,34 @@
+//// [tests/cases/compiler/bindingPatternOptionalParameterCached.ts] ////
+
+=== bindingPatternOptionalParameterCached.ts ===
+// https://github.com/microsoft/typescript-go/issues/3276
+
+// There should be no error in this test, but previously there was because of the
+// declaration of mock. Commenting it out would make the error disappear.
+
+export const mock: I = {
+>mock : I
+>     : ^
+>{    m: (_) => {},} : { m: (_: { x: boolean; } | undefined) => void; }
+>                    : ^^^^^^ ^^^^^^^       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+    m: (_) => {},
+>m : (_: { x: boolean; } | undefined) => void
+>  : ^ ^^^^^^^       ^^^^^^^^^^^^^^^^^^^^^^^^
+>(_) => {} : (_: { x: boolean; } | undefined) => void
+>          : ^ ^^^^^^^       ^^^^^^^^^^^^^^^^^^^^^^^^
+>_ : { x: boolean; } | undefined
+>  : ^^^^^       ^^^^^^^^^^^^^^^
+
+};
+
+export interface I {
+    m({ x }?: { x: boolean }): void
+>m : ({ x }?: { x: boolean; }) => void
+>  : ^     ^^^               ^^^^^    
+>x : boolean
+>  : ^^^^^^^
+>x : boolean
+>  : ^^^^^^^
+}
+
diff --git a/tests/cases/compiler/bindingPatternOptionalParameterCached.ts b/tests/cases/compiler/bindingPatternOptionalParameterCached.ts
new file mode 100644
index 0000000000000..54e0da5444564
--- /dev/null
+++ b/tests/cases/compiler/bindingPatternOptionalParameterCached.ts
@@ -0,0 +1,15 @@
+// @strict: true
+// @noEmit: true
+
+// https://github.com/microsoft/typescript-go/issues/3276
+
+// There should be no error in this test, but previously there was because of the
+// declaration of mock. Commenting it out would make the error disappear.
+
+export const mock: I = {
+    m: (_) => {},
+};
+
+export interface I {
+    m({ x }?: { x: boolean }): void
+}