The parser in Groovy 3 fails early on multiple methods with the same name.
For example in a Spock specification with Groovy 2.5 you can have mutliple methods with the same name.
After the Spock AST transformations are done, there are only unique method names left and all is fine.
But with Groovy 3, the parser fails with error before the AST transformation had a chance to fix things up.
Setting the system property groovy.antlr4=false restores the previous behavior.