Parse let declarations whose id starts with \ (#13325)

packages/babel-parser/src/parser/statement.js

@@ -188,7 +188,13 @@ export default class StatementParser extends ExpressionParser {
     // Statement) is allowed here. If context is not empty then only a Statement
     // is allowed. However, `let [` is an explicit negative lookahead for
     // ExpressionStatement, so special-case it first.
-    if (nextCh === charCodes.leftSquareBracket) return true;
+    // Also, `let \` is never valid as an expression so this must be a keyword.
+    if (
+      nextCh === charCodes.backslash ||
+      nextCh === charCodes.leftSquareBracket
+    ) {
+      return true;
+    }
     if (context) return false;
 
     if (nextCh === charCodes.leftCurlyBrace) return true;

packages/babel-parser/test/fixtures/es2015/let/let-declaration-escape-id/input.js

@@ -0,0 +1 @@
+let \u0275ResourceLoaderImpl_BaseFactory

packages/babel-parser/test/fixtures/es2015/let/let-declaration-escape-id/output.json

@@ -0,0 +1,30 @@
+{
+  "type": "File",
+  "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":40}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":40}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "VariableDeclaration",
+        "start":0,"end":40,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":40}},
+        "declarations": [
+          {
+            "type": "VariableDeclarator",
+            "start":4,"end":40,"loc":{"start":{"line":1,"column":4},"end":{"line":1,"column":40}},
+            "id": {
+              "type": "Identifier",
+              "start":4,"end":40,"loc":{"start":{"line":1,"column":4},"end":{"line":1,"column":40},"identifierName":"ɵResourceLoaderImpl_BaseFactory"},
+              "name": "ɵResourceLoaderImpl_BaseFactory"
+            },
+            "init": null
+          }
+        ],
+        "kind": "let"
+      }
+    ],
+    "directives": []
+  }
+}

packages/babel-parser/test/fixtures/es2015/let/let-expression-followed-by-escape/input.js

@@ -0,0 +1,2 @@
+1 + let
+\u0275ResourceLoaderImpl_BaseFactory

packages/babel-parser/test/fixtures/es2015/let/let-expression-followed-by-escape/output.json

@@ -0,0 +1,45 @@
+{
+  "type": "File",
+  "start":0,"end":44,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":36}},
+  "program": {
+    "type": "Program",
+    "start":0,"end":44,"loc":{"start":{"line":1,"column":0},"end":{"line":2,"column":36}},
+    "sourceType": "script",
+    "interpreter": null,
+    "body": [
+      {
+        "type": "ExpressionStatement",
+        "start":0,"end":7,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":7}},
+        "expression": {
+          "type": "BinaryExpression",
+          "start":0,"end":7,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":7}},
+          "left": {
+            "type": "NumericLiteral",
+            "start":0,"end":1,"loc":{"start":{"line":1,"column":0},"end":{"line":1,"column":1}},
+            "extra": {
+              "rawValue": 1,
+              "raw": "1"
+            },
+            "value": 1
+          },
+          "operator": "+",
+          "right": {
+            "type": "Identifier",
+            "start":4,"end":7,"loc":{"start":{"line":1,"column":4},"end":{"line":1,"column":7},"identifierName":"let"},
+            "name": "let"
+          }
+        }
+      },
+      {
+        "type": "ExpressionStatement",
+        "start":8,"end":44,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":36}},
+        "expression": {
+          "type": "Identifier",
+          "start":8,"end":44,"loc":{"start":{"line":2,"column":0},"end":{"line":2,"column":36},"identifierName":"ɵResourceLoaderImpl_BaseFactory"},
+          "name": "ɵResourceLoaderImpl_BaseFactory"
+        }
+      }
+    ],
+    "directives": []
+  }
+}