PY-21161 Check that index fragment of .format() string doesn't contain backslash

python/src/com/jetbrains/python/codeInsight/intentions/PyConvertToFStringIntention.java

@@ -360,6 +360,9 @@ public class PyConvertToFStringIntention extends PyBaseIntentionAction {
         escaped.add(part);
       }
       else if (part.startsWith("[")) {
+        if (part.contains("\\")) {
+          return null;
+        }
         final String indexText = part.substring(1, part.length() - 1);
         if (indexText.matches("\\d+")) {
           escaped.add(part);

python/testData/intentions/PyConvertToFStringIntentionTest/formatMethodIndexContainsBackslash.py

@@ -0,0 +1 @@
+'{0[\\]}'.format(undefined)

python/testData/intentions/PyConvertToFStringIntentionTest/formatMethodIndexContainsBackslash_after.py

@@ -0,0 +1 @@
+'{0[\\]}'.format(undefined)

python/testSrc/com/jetbrains/python/intentions/PyConvertToFStringIntentionTest.java

@@ -99,6 +99,10 @@ public class PyConvertToFStringIntentionTest extends PyIntentionTestCase {
   public void testFormatMethodIndexContainsHostAlternativeQuote() {
     doTest();
   }
+  
+  public void testFormatMethodIndexContainsBackslash() {
+    doTest();
+  }
 
   public void testPercentOperatorWidthAndPrecision() {
     doTest();