Fix await using double lookahead edge case

FIX: Don't recognize `await using` as contextual keywords when followed directly by a backslash.

Issue  #1382

Author: adams85

acorn/src/statement.js

@@ -84,17 +84,17 @@ pp.isUsingKeyword = function(isAwaitUsing, isFor) {
   if (lineBreak.test(this.input.slice(this.pos, next))) return false
 
   if (isAwaitUsing) {
-    let awaitEndPos = next + 5 /* await */, after
-    if (this.input.slice(next, awaitEndPos) !== "using" ||
-      awaitEndPos === this.input.length ||
-      isIdentifierChar(after = this.input.charCodeAt(awaitEndPos)) ||
-      (after > 0xd7ff && after < 0xdc00)
+    let usingEndPos = next + 5 /* using */, after
+    if (this.input.slice(next, usingEndPos) !== "using" ||
+      usingEndPos === this.input.length ||
+      isIdentifierChar(after = this.fullCharCodeAt(usingEndPos)) ||
+      after === 92 /* '\' */
     ) return false
 
-    skipWhiteSpace.lastIndex = awaitEndPos
+    skipWhiteSpace.lastIndex = usingEndPos
     let skipAfterUsing = skipWhiteSpace.exec(this.input)
-    next = awaitEndPos + skipAfterUsing[0].length
-    if (skipAfterUsing && lineBreak.test(this.input.slice(awaitEndPos, next))) return false
+    next = usingEndPos + skipAfterUsing[0].length
+    if (skipAfterUsing && lineBreak.test(this.input.slice(usingEndPos, next))) return false
   }
 
   let ch = this.fullCharCodeAt(next)

test/tests-using.js

@@ -1184,6 +1184,9 @@ testFail("for (await using a of x) {}", "Await using cannot appear outside of as
 testFail("switch (x) { case 1: using y = resource; }", "Using declaration cannot appear in the top level when source type is `script` or in the bare case statement (1:21)", {ecmaVersion: 17, sourceType: "module"});
 testFail("switch (x) { case 1: break; default: using y = resource; }", "Using declaration cannot appear in the top level when source type is `script` or in the bare case statement (1:37)", {ecmaVersion: 17, sourceType: "module"});
 
+// await using double lookahead should detect using keyword correctly
+testFail("async () => { await using\\u0061 b = c }", "Unexpected token (1:32)", {ecmaVersion: 17, sourceType: "module"});
+
 // =============================================================================
 // EDGE CASES - Unusual but valid scenarios and boundary conditions
 // =============================================================================