Workaround flags of interpolated regexp string parts causing issues

Author: eregon

src/main/java/org/truffleruby/parser/YARPBaseTranslator.java

@@ -215,6 +215,10 @@ protected final void copyNewlineFlag(Nodes.Node yarpNode, RubyNode rubyNode) {
         }
     }
 
+    protected void copyNewLineFlag(Nodes.Node source, Nodes.Node target) {
+        target.setNewLineFlag(source.hasNewLineFlag());
+    }
+
     protected static RubyNode sequence(Nodes.Node yarpNode, RubyNode... sequence) {
         assert !yarpNode.hasNewLineFlag() : "Expected node passed to sequence() to not have a newline flag";
 

src/main/java/org/truffleruby/parser/YARPTranslator.java

@@ -2387,7 +2387,7 @@ public RubyNode visitInterpolatedMatchLastLineNode(Nodes.InterpolatedMatchLastLi
     public RubyNode visitInterpolatedRegularExpressionNode(Nodes.InterpolatedRegularExpressionNode node) {
         var encodingAndOptions = getRegexpEncodingAndOptions(new Nodes.RegularExpressionFlags(node.flags));
         var options = encodingAndOptions.options;
-        final ToSNode[] children = translateInterpolatedParts(node.parts);
+        final ToSNode[] children = translateInterpolatedPartsIgnoreForceEncodingFlags(node.parts);
 
         final RubyEncoding encoding;
         if (!options.isKcodeDefault()) { // explicit encoding
@@ -2526,6 +2526,31 @@ private ToSNode[] translateInterpolatedParts(Nodes.Node[] parts) {
         return children;
     }
 
+    /** Regexp encoding negotiation does not work correctly if such flags are kept, e.g. for /#{ }\xc2\xa1/e in
+     * test_m17n.rb. Not clear what is a good solution yet. */
+    private ToSNode[] translateInterpolatedPartsIgnoreForceEncodingFlags(Nodes.Node[] parts) {
+        final ToSNode[] children = new ToSNode[parts.length];
+
+        for (int i = 0; i < parts.length; i++) {
+            RubyNode expression;
+            if (parts[i] instanceof Nodes.StringNode stringNode) {
+                short flags = stringNode.isFrozen() ? Nodes.StringFlags.FROZEN : NO_FLAGS;
+                Nodes.StringNode stringNodeNoForceEncoding = new Nodes.StringNode(flags, stringNode.unescaped,
+                        stringNode.startOffset, stringNode.length);
+
+                // Prism might assign new line flag not to the outer regexp node but to its first part instead
+                copyNewLineFlag(stringNode, stringNodeNoForceEncoding);
+
+                expression = stringNodeNoForceEncoding.accept(this);
+            } else {
+                expression = parts[i].accept(this);
+            }
+            children[i] = ToSNodeGen.create(expression);
+        }
+
+        return children;
+    }
+
     @Override
     public RubyNode visitItParametersNode(Nodes.ItParametersNode node) {
         throw CompilerDirectives.shouldNotReachHere("ItParametersNode is only from Ruby 3.4");