From 5560e8e4de1910333bed5e35227af78b6cea891c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Burzy=C5=84ski?= <mateuszburzynski@gmail.com>
Date: Thu, 21 Aug 2025 21:07:25 +0200
Subject: [PATCH] Narrow by constant element access callees

---
 src/compiler/binder.ts                        |  4 +-
 src/compiler/checker.ts                       |  4 +-
 ...dicateFromElementAccessExpression1.symbols | 47 ++++++++++++
 ...redicateFromElementAccessExpression1.types | 74 +++++++++++++++++++
 ...pePredicateFromElementAccessExpression1.ts | 25 +++++++
 5 files changed, 150 insertions(+), 4 deletions(-)
 create mode 100644 tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.symbols
 create mode 100644 tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.types
 create mode 100644 tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts

diff --git a/src/compiler/binder.ts b/src/compiler/binder.ts
index 27ec079f614ab..540799673102d 100644
--- a/src/compiler/binder.ts
+++ b/src/compiler/binder.ts
@@ -1295,8 +1295,8 @@ function createBinder(): (file: SourceFile, options: CompilerOptions) => void {
             }
         }
         if (
-            expr.expression.kind === SyntaxKind.PropertyAccessExpression &&
-            containsNarrowableReference((expr.expression as PropertyAccessExpression).expression)
+            (expr.expression.kind === SyntaxKind.PropertyAccessExpression || expr.expression.kind === SyntaxKind.ElementAccessExpression) &&
+            containsNarrowableReference((expr.expression as PropertyAccessExpression | ElementAccessExpression).expression)
         ) {
             return true;
         }
diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 75be36474222f..5f2fa22d5dd93 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -27970,8 +27970,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
             }
         }
         if (
-            expression.expression.kind === SyntaxKind.PropertyAccessExpression &&
-            isOrContainsMatchingReference(reference, (expression.expression as PropertyAccessExpression).expression)
+            (expression.expression.kind === SyntaxKind.PropertyAccessExpression || expression.expression.kind === SyntaxKind.ElementAccessExpression && isConstantReference((expression.expression as ElementAccessExpression).argumentExpression)) &&
+            isOrContainsMatchingReference(reference, (expression.expression as PropertyAccessExpression | ElementAccessExpression).expression)
         ) {
             return true;
         }
diff --git a/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.symbols b/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.symbols
new file mode 100644
index 0000000000000..0438824530cc0
--- /dev/null
+++ b/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.symbols
@@ -0,0 +1,47 @@
+//// [tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts] ////
+
+=== narrowByTypePredicateFromElementAccessExpression1.ts ===
+// https://github.com/microsoft/TypeScript/issues/62247
+
+const isBar = Symbol("isBar");
+>isBar : Symbol(isBar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 5))
+>Symbol : Symbol(Symbol, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.symbol.d.ts, --, --), Decl(lib.es2015.symbol.wellknown.d.ts, --, --), Decl(lib.es2019.symbol.d.ts, --, --))
+
+abstract class Foo {
+>Foo : Symbol(Foo, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 30))
+
+  abstract [isBar](): this is Bar;
+>[isBar] : Symbol(Foo[isBar], Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 4, 20))
+>isBar : Symbol(isBar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 5))
+>Bar : Symbol(Bar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 12, 1))
+
+  method(): void {
+>method : Symbol(Foo.method, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 5, 34))
+
+    if (this[isBar]()) {
+>this : Symbol(Foo, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 30))
+>isBar : Symbol(isBar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 5))
+
+      this.barMethod(); // ok
+>this.barMethod : Symbol(Bar.barMethod, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 17, 3))
+>barMethod : Symbol(Bar.barMethod, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 17, 3))
+    }
+  }
+}
+
+class Bar extends Foo {
+>Bar : Symbol(Bar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 12, 1))
+>Foo : Symbol(Foo, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 30))
+
+  override [isBar](): this is Bar {
+>[isBar] : Symbol(Bar[isBar], Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 14, 23))
+>isBar : Symbol(isBar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 2, 5))
+>Bar : Symbol(Bar, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 12, 1))
+
+    return true;
+  }
+
+  barMethod(): void {}
+>barMethod : Symbol(Bar.barMethod, Decl(narrowByTypePredicateFromElementAccessExpression1.ts, 17, 3))
+}
+
diff --git a/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.types b/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.types
new file mode 100644
index 0000000000000..8c67e1fcede27
--- /dev/null
+++ b/tests/baselines/reference/narrowByTypePredicateFromElementAccessExpression1.types
@@ -0,0 +1,74 @@
+//// [tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts] ////
+
+=== narrowByTypePredicateFromElementAccessExpression1.ts ===
+// https://github.com/microsoft/TypeScript/issues/62247
+
+const isBar = Symbol("isBar");
+>isBar : unique symbol
+>      : ^^^^^^^^^^^^^
+>Symbol("isBar") : unique symbol
+>                : ^^^^^^^^^^^^^
+>Symbol : SymbolConstructor
+>       : ^^^^^^^^^^^^^^^^^
+>"isBar" : "isBar"
+>        : ^^^^^^^
+
+abstract class Foo {
+>Foo : Foo
+>    : ^^^
+
+  abstract [isBar](): this is Bar;
+>[isBar] : () => this is Bar
+>        : ^^^^^^           
+>isBar : unique symbol
+>      : ^^^^^^^^^^^^^
+
+  method(): void {
+>method : () => void
+>       : ^^^^^^    
+
+    if (this[isBar]()) {
+>this[isBar]() : boolean
+>              : ^^^^^^^
+>this[isBar] : () => this is Bar
+>            : ^^^^^^           
+>this : this
+>     : ^^^^
+>isBar : unique symbol
+>      : ^^^^^^^^^^^^^
+
+      this.barMethod(); // ok
+>this.barMethod() : void
+>                 : ^^^^
+>this.barMethod : () => void
+>               : ^^^^^^    
+>this : this & Bar
+>     : ^^^^^^^^^^
+>barMethod : () => void
+>          : ^^^^^^    
+    }
+  }
+}
+
+class Bar extends Foo {
+>Bar : Bar
+>    : ^^^
+>Foo : Foo
+>    : ^^^
+
+  override [isBar](): this is Bar {
+>[isBar] : () => this is Bar
+>        : ^^^^^^           
+>isBar : unique symbol
+>      : ^^^^^^^^^^^^^
+
+    return true;
+>true : true
+>     : ^^^^
+  }
+
+  barMethod(): void {}
+>barMethod : () => void
+>          : ^^^^^^    
+}
+
diff --git a/tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts b/tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts
new file mode 100644
index 0000000000000..625b7e0572b6e
--- /dev/null
+++ b/tests/cases/compiler/narrowByTypePredicateFromElementAccessExpression1.ts
@@ -0,0 +1,25 @@
+// @strict: true
+// @target: esnext
+// @noEmit: true
+
+// https://github.com/microsoft/TypeScript/issues/62247
+
+const isBar = Symbol("isBar");
+
+abstract class Foo {
+  abstract [isBar](): this is Bar;
+
+  method(): void {
+    if (this[isBar]()) {
+      this.barMethod(); // ok
+    }
+  }
+}
+
+class Bar extends Foo {
+  override [isBar](): this is Bar {
+    return true;
+  }
+
+  barMethod(): void {}
+}