[GROOVY-8946] @CompileStatic ignores declared type and forces a cast when inside a closure

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.4.16, 3.0.0-alpha-3, 2.5.5
• Fix Version/s: 4.0.0-beta-1, 3.0.20
• Component/s: Static compilation
• Environment: Linux wmtz00088 4.4.0-141-generic #167-Ubuntu SMP Wed Dec 5 10:40:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
• Assignee: Eric Milles
• Reporter: Frederico Costa Galvão
• Created: 05/Jan/19 00:53
• Updated: 29/Nov/23 21:34
• Resolved: 17/May/21 17:59

Description

Using a typed method (which happens to be JSON.parse from grails.converters) but handling stuff as `Object` still makes the compiler generate a cast to the infered value type, ignoring the declared type.

I wanted to reproduce this as close as possible, so my snippet grabs stuff from grails. However, this should be easily reproducible with a simple typed method instead of using the grails stuff.

For readability's sake, the original code (greatly simplified bellow) tries to navigate through a json object with a deep path like 'a.b.c', in which the values at each step could be multidimensional or not (and implicit `collect`s happens when it is).

@GrabResolver(name = 'r1', root = 'https://repo.grails.org/grails/core')
@Grapes([
    @Grab("org.grails.plugins:converters:3.3.+"),
    @Grab(group='org.grails', module='grails-encoder', version='3.3.+'),
    @Grab(group='org.grails', module='grails-web', version='3.3.+'),
    @Grab(group='commons-io', module='commons-io', version='2.3'),
])
@GrabExclude('org.codehaus.groovy:groovy-xml')

import grails.converters.JSON
import groovy.transform.CompileStatic

@CompileStatic
class What {

    static void _do() {
        Object json = JSON.parse('[{"a":1},{"a":2}]')

        Object json2 = json['a']

        final boolean res = 'a'.tokenize('.').every { final String token ->
            json = json[token]//MARK, ERROR
            return json
        }

        assert json == [1, 2]
    }

}

What._do()

When I try to run that, I get:

org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object '[1, 2]' with class 'java.util.ArrayList' to class 'org.grails.web.json.JSONElement' due to: groovy.lang.GroovyRuntimeException: Could not find matching constructor for: org.grails.web.json.JSONElement(Integer, Integer)

And the bytecode for the relevant code is (Luyten is the decompiler of choice, but I've seen the same with groovyConsole's inner AST browser):

• outside `json` reference

  final Reference json = new Reference((Object)JSON.parse("[{\"a\":1},{\"a\":2}]"));
  

• closure inner `json` reference

  private /* synthetic */ Reference json;
  

• marked line

  json.set((Object)ScriptBytecodeAdapter.castToType(DefaultGroovyMethods.getAt(json.get(), token), (Class)JSONElement.class));
  

I have tried that with:
groovy: 2.4.[11..16], 2.5.5, 3.0.0-alpha-3

Things that make it run/compile properly:

• casting `JSON.parse(...)` to `(Object)`
• removing `@CompileStatic`
• using `JsonSlurper` (which already returns `Object`)

I understand that type inference from flow is useful and important, but shouldn't it respect the declared type whenever it differs from def? It's not inside a type guard, nor did I use a `JSONElement` specific member to rightfully trigger it. I may be wrong and this could be by design, but I couldn't find something to justify this out-of-sight cast.

Also, the other/last argument I have in favor of it being a bug is that the same operation, if done outside the closure, compiles just fine:

final Object json2 = DefaultGroovyMethods.getAt((Object)json.get(), "a");