MapWtf.groovy

import groovy.transform.*

@CompileStatic
Map<String, ?> makeMap() {
  def map = [string: "string"]
  map.number = 1
  return map
}

def map = makeMap()
map.each { println it.value.getClass().name }

This surprised me – the method returns a Map<String, String> having seemingly inferred the value type from the initial assignment. When the integer value is added it gets coerced to String. This happens with any type (I stumbled on it while adding a List<Map<String, String>> to a map in a similar way).

Things to try:

1. Remove @CompileStatic
2. Replace def with Map<String, ?> on line 5
3. Replace def with Map<String, Object> on line 5
4. Replace the initial assignment with [:]
5. Reverse the order of adding stuff to the map
6. Add as Map<String, ?> at the end of line 5
7. Repeat 4 but follow with map.string = "string"

This is weird. I have also noticed it works for initialization like below:

def map = [string: 'string', numbers: [1, 2, 3, 4].collect { [n: it] } ]

println map.number.getClass().name

immediately after line 6 prints as String.