Count shorthand property reads of an initializer-less JavaScript let (#2252)

packages/compiler/src/frontend/lowering/lower-stmts.ts

@@ -5422,6 +5422,12 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
   const scope = decl.parent?.parent?.parent;
   if (symbol === undefined || (!ts.isBlock(scope) && !ts.isSourceFile(scope))) return null;
 
+  // A shorthand property's name (`{ width }`) resolves to the property
+  // symbol, but it reads the binding: the checker's shorthand-value symbol.
+  const refersToBinding = (node: ts.Identifier): boolean =>
+    (ts.isShorthandPropertyAssignment(node.parent) && node.parent.name === node
+      ? lowerer.checker.getShorthandAssignmentValueSymbol(node.parent)
+      : lowerer.checker.getSymbolAtLocation(node)) === symbol;
   const isPlainAssignmentTarget = (node: ts.Identifier): boolean => {
     let target: ts.Expression = node;
     while (ts.isParenthesizedExpression(target.parent) && target.parent.expression === target) {
@@ -5437,11 +5443,7 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
   const captures = (fn: ts.Node): boolean => {
     let captured = false;
     ts.walkPreorder(fn, (node) => {
-      if (
-        node !== fn &&
-        ts.isIdentifier(node) &&
-        lowerer.checker.getSymbolAtLocation(node) === symbol
-      ) {
+      if (node !== fn && ts.isIdentifier(node) && refersToBinding(node)) {
         captured = true;
         return "stop";
       }
@@ -5493,7 +5495,7 @@ function inferredEvolvingLetType(lowerer: Lowerer, decl: ts.VariableDeclaration)
     if (
       ts.isIdentifier(node) &&
       node !== decl.name &&
-      lowerer.checker.getSymbolAtLocation(node) === symbol &&
+      refersToBinding(node) &&
       !isPlainAssignmentTarget(node)
     ) {
       reads.push(node);

packages/compiler/test/ts7/baselines/order-parity/tests/corpus/evolving-let-shorthand-reads.js.json

@@ -0,0 +1,6 @@
+{
+ "order": [
+  "<repo>/tests/corpus/evolving-let-shorthand-reads.js"
+ ],
+ "diags": []
+}

tests/corpus/evolving-let-shorthand-reads.js

@@ -0,0 +1,41 @@
+// An initializer-less JavaScript `let` read through a shorthand property
+// (`{ width }`) still reads undefined on the paths that never assign it, as
+// npm packages do when they build an options or size object.
+function size(tty) {
+  let width;
+  if (tty) width = 80;
+  return { width };
+}
+
+/**
+ * @param {boolean} tty
+ * @returns {{ width: number | undefined }}
+ */
+function documented(tty) {
+  /** @type {number | undefined} */
+  let width;
+  if (tty) width = 80;
+  return { width };
+}
+
+function captured(tty) {
+  let width;
+  const read = () => ({ width });
+  const before = read().width;
+  if (tty) width = 80;
+  return [before, read().width];
+}
+
+function nested(flags) {
+  let columns;
+  let rows;
+  if (flags.columns) columns = 120;
+  if (flags.rows) rows = 40;
+  return { size: { columns, rows } };
+}
+
+console.log(size(true).width, size(false).width);
+console.log(documented(true).width, documented(false).width);
+console.log(JSON.stringify(size(false)), "width" in size(false));
+console.log(captured(true), captured(false));
+console.log(nested({ columns: true }), nested({ rows: true }), nested({}));

tests/harness/corpus-costs/evolving-let-shorthand-reads.js.json

@@ -0,0 +1 @@
+{ "plain": 3700, "san": 2800 }