[GR-38430] Fix String#[Regexp, Integer] when the capture group exists but is not matched

Author: eregon

CHANGELOG.md

@@ -10,6 +10,7 @@ Bug fixes:
 * Fix `MatchData#[]` exception when supplying a large negative index along with a length argument (@nirvdrum).
 * Fix capacity computation for huge `Hash` (#2635, @eregon).
 * Fix aliased methods to return the correct owner when method is from a superclass (@bjfish).
+* Fix `String#[Regexp, Integer]` when the capture group exists but is not matched (@eregon).
 
 Compatibility:
 

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -818,6 +818,8 @@ protected Object sliceCapture(VirtualFrame frame, Object string, RubyRegexp rege
                 @Cached ReadCallerVariablesNode readCallerStorageNode,
                 @Cached ConditionProfile unsetProfile,
                 @Cached ConditionProfile sameThreadProfile,
+                @Cached ConditionProfile notMatchedProfile,
+                @Cached ConditionProfile captureSetProfile,
                 @Cached StringDupAsStringInstanceNode dupNode) {
             final Object capture = RubyGuards.wasProvided(maybeCapture) ? maybeCapture : 0;
             final Object matchStrPair = callNode.call(
@@ -828,13 +830,19 @@ protected Object sliceCapture(VirtualFrame frame, Object string, RubyRegexp rege
                     capture);
 
             final SpecialVariableStorage variables = readCallerStorageNode.execute(frame);
-            if (matchStrPair == nil) {
+            if (notMatchedProfile.profile(matchStrPair == nil)) {
                 variables.setLastMatch(nil, getContext(), unsetProfile, sameThreadProfile);
                 return nil;
             } else {
                 final Object[] array = (Object[]) ((RubyArray) matchStrPair).store;
-                variables.setLastMatch(array[0], getContext(), unsetProfile, sameThreadProfile);
-                return dupNode.executeDupAsStringInstance(array[1]);
+                final Object matchData = array[0];
+                final Object captureStringOrNil = array[1];
+                variables.setLastMatch(matchData, getContext(), unsetProfile, sameThreadProfile);
+                if (captureSetProfile.profile(captureStringOrNil != nil)) {
+                    return dupNode.executeDupAsStringInstance(captureStringOrNil);
+                } else {
+                    return nil;
+                }
             }
         }