Issue #17431 has been updated by ibylich (Ilya Bylich).


The following patch seems to be the fix:

```
diff --git a/parse.y b/parse.y
index c65a469a..a4f6bcc8 100644
--- a/parse.y
+++ b/parse.y
@@ -9668,6 +9669,7 @@ parser_yylex(struct parser_params *p)
        p->lex.paren_nest++;
        if (IS_AFTER_OPERATOR()) {
            if ((c = nextc(p)) == ']') {
+                p->lex.paren_nest--;
                SET_LEX_STATE(EXPR_ARG);
                if ((c = nextc(p)) == '=') {
                    return tASET;
```

----------------------------------------
Bug #17431: paren_nest is not reset in parse.y after "foo.[]= value"
https://bugs.ruby-lang.org/issues/17431#change-89447

* Author: ibylich (Ilya Bylich)
* Status: Assigned
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* ruby -v: ruby 3.0.0dev (2020-12-23T14:40:04Z master 94015200b6) [x86_64-darwin19]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
----------------------------------------
`p->lex.paren_nest` is equal to 1 after the following code:

```ruby
self.[]= foo
```

`paren_nest` is used to differentiate `...` for ranges/forwarded arguments, and so `./miniruby -we 'foo.[]= bar, ...'` gives no `... at EOL, should be parenthesized?` warning (`./miniruby -we 'foo.x= bar, ...'` does).

Also it's used to differentiate `kDO` vs `kDO_LAMBDA`, so I believe there are code samples with lambdas and `.[]=` calls that can't be handled by Ruby but are technically valid.



-- 
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