Print parentheses around identifier let where necessary

packages/babel-generator/src/node/parentheses.ts

@@ -74,7 +74,10 @@ export function ObjectExpression(
   parent: any,
   printStack: Array<any>,
 ): boolean {
-  return isFirstInStatement(printStack, { considerArrow: true });
+  return isFirstInContext(printStack, {
+    expressionStatement: true,
+    arrowBody: true,
+  });
 }
 
 export function DoExpression(
@@ -83,7 +86,9 @@ export function DoExpression(
   printStack: Array<any>,
 ): boolean {
   // `async do` can start an expression statement
-  return !node.async && isFirstInStatement(printStack);
+  return (
+    !node.async && isFirstInContext(printStack, { expressionStatement: true })
+  );
 }
 
 export function Binary(node: any, parent: any): boolean {
@@ -214,7 +219,10 @@ export function ClassExpression(
   parent: any,
   printStack: Array<any>,
 ): boolean {
-  return isFirstInStatement(printStack, { considerDefaultExports: true });
+  return isFirstInContext(printStack, {
+    expressionStatement: true,
+    exportDefault: true,
+  });
 }
 
 export function UnaryLike(node: any, parent: any): boolean {
@@ -230,7 +238,10 @@ export function FunctionExpression(
   parent: any,
   printStack: Array<any>,
 ): boolean {
-  return isFirstInStatement(printStack, { considerDefaultExports: true });
+  return isFirstInContext(printStack, {
+    expressionStatement: true,
+    exportDefault: true,
+  });
 }
 
 export function ArrowFunctionExpression(node: any, parent: any): boolean {
@@ -281,7 +292,28 @@ export function LogicalExpression(node: any, parent: any): boolean {
   }
 }
 
-export function Identifier(node: t.Identifier, parent: t.Node): boolean {
+export function Identifier(
+  node: t.Identifier,
+  parent: t.Node,
+  printStack: Array<t.Node>,
+): boolean {
+  // Non-strict code allows the identifier `let`, but it cannot occur as-is in
+  // certain contexts to avoid ambiguity with contextual keyword `let`.
+  if (node.name === "let") {
+    // Some contexts only forbid `let [`, so check if the next token would
+    // be the left bracket of a computed member expression.
+    const isFollowedByBracket = t.isMemberExpression(parent, {
+      object: node,
+      computed: true,
+    });
+    return isFirstInContext(printStack, {
+      expressionStatement: isFollowedByBracket,
+      forHead: isFollowedByBracket,
+      forInHead: isFollowedByBracket,
+      forOfHead: true,
+    });
+  }
+
   // ECMAScript specifically forbids a for-of loop from starting with the
   // token sequence `for (async of`, because it would be ambiguous with
   // `for (async of => {};;)`, so we need to add extra parentheses.
@@ -296,21 +328,32 @@ export function Identifier(node: t.Identifier, parent: t.Node): boolean {
 }
 
 // Walk up the print stack to determine if our node can come first
-// in statement.
-function isFirstInStatement(
-  printStack: Array<any>,
-  { considerArrow = false, considerDefaultExports = false } = {},
+// in a particular context.
+function isFirstInContext(
+  printStack: Array<t.Node>,
+  {
+    expressionStatement = false,
+    arrowBody = false,
+    exportDefault = false,
+    forHead = false,
+    forInHead = false,
+    forOfHead = false,
+  },
 ): boolean {
   let i = printStack.length - 1;
   let node = printStack[i];
   i--;
   let parent = printStack[i];
   while (i >= 0) {
     if (
-      t.isExpressionStatement(parent, { expression: node }) ||
-      (considerDefaultExports &&
+      (expressionStatement &&
+        t.isExpressionStatement(parent, { expression: node })) ||
+      (exportDefault &&
         t.isExportDefaultDeclaration(parent, { declaration: node })) ||
-      (considerArrow && t.isArrowFunctionExpression(parent, { body: node }))
+      (arrowBody && t.isArrowFunctionExpression(parent, { body: node })) ||
+      (forHead && t.isForStatement(parent, { init: node })) ||
+      (forInHead && t.isForInStatement(parent, { left: node })) ||
+      (forOfHead && t.isForOfStatement(parent, { left: node }))
     ) {
       return true;
     }

packages/babel-generator/test/fixtures/edgecase/let-identifier/input.js

@@ -0,0 +1,31 @@
+/* ExpressionStatement */
+let;
+\u006cet[x];
+(let)[x];
+a[let[x]];
+
+/* ForStatement */
+for (let;;);
+for (\u006cet[x];;);
+for ((let)[x];;);
+for (a[let[x]];;);
+
+/* ForInStatement */
+for (let in {});
+for (\u006cet[x] in {});
+for ((let)[x] in {});
+for (a[let[x]] in {});
+
+/* ForOfStatement { await: false } */
+for ((let) of []);
+for (\u006cet of []);
+for ((let)[x] of []);
+for (a[let] of []);
+
+/* ForOfStatement { await: true } */
+async () => {
+  for await ((let) of []);
+  for await (\u006cet of []);
+  for await ((let)[x] of []);
+  for await (a[let] of []);
+}

packages/babel-generator/test/fixtures/edgecase/let-identifier/options.json

@@ -0,0 +1,4 @@
+{
+  "sourceType": "script",
+  "strictMode": false
+}

packages/babel-generator/test/fixtures/edgecase/let-identifier/output.js

@@ -0,0 +1,46 @@
+/* ExpressionStatement */
+let;
+(let)[x];
+(let)[x];
+a[let[x]];
+/* ForStatement */
+
+for (let;;);
+
+for ((let)[x];;);
+
+for ((let)[x];;);
+
+for (a[let[x]];;);
+/* ForInStatement */
+
+
+for (let in {});
+
+for ((let)[x] in {});
+
+for ((let)[x] in {});
+
+for (a[let[x]] in {});
+/* ForOfStatement { await: false } */
+
+
+for ((let) of []);
+
+for ((let) of []);
+
+for ((let)[x] of []);
+
+for (a[let] of []);
+/* ForOfStatement { await: true } */
+
+
+async () => {
+  for await ((let) of []);
+
+  for await ((let) of []);
+
+  for await ((let)[x] of []);
+
+  for await (a[let] of []);
+};