From fc368285681b6c7a07fe69d8ce0b73b82a771da9 Mon Sep 17 00:00:00 2001
From: Lily Lin <lily_lin2@apple.com>
Date: Mon, 15 Aug 2022 15:25:03 -0700
Subject: [PATCH] Don't assume quoted literals are non-empty during bytecodegen

---
 Sources/_StringProcessing/ByteCodeGen.swift | 2 +-
 Tests/RegexTests/MatchTests.swift           | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/Sources/_StringProcessing/ByteCodeGen.swift b/Sources/_StringProcessing/ByteCodeGen.swift
index 5111979b1..e0a6c7465 100644
--- a/Sources/_StringProcessing/ByteCodeGen.swift
+++ b/Sources/_StringProcessing/ByteCodeGen.swift
@@ -115,7 +115,7 @@ fileprivate extension Compiler.ByteCodeGen {
     }
 
     // Fast path for eliding boundary checks for an all ascii quoted literal
-    if optimizationsEnabled && s.allSatisfy(\.isASCII) {
+    if optimizationsEnabled && s.allSatisfy(\.isASCII) && !s.isEmpty {
       let lastIdx = s.unicodeScalars.indices.last!
       for idx in s.unicodeScalars.indices {
         let boundaryCheck = idx == lastIdx
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index 5025c8c47..2fa9b9381 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -2458,6 +2458,9 @@ extension RegexTests {
 
     // case insensitive tests
     firstMatchTest(#"(?i)abc\u{301}d"#, input: "AbC\u{301}d", match: "AbC\u{301}d", semanticLevel: .unicodeScalar)
+
+    // check that we don't crash on empty strings
+    firstMatchTest(#"\Q\E"#, input: "", match: "")
   }
   
   func testCase() {