[GROOVY-10052] STC sometimes requires explicit closure to SAM cast inside of another closure

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.0.8
• Fix Version/s: 4.0.0-beta-1, 3.0.9, 2.5.16
• Component/s: Static compilation, Static Type Checker
• Assignee: Eric Milles
• Reporter: Damir Murat
• Created: 23/Apr/21 08:51
• Updated: 01/Mar/22 20:06
• Resolved: 26/Apr/21 12:44

Description

After upgrading to the 3.0.8 version, I observed several improvements in STC, but it looks like some cases are still uncovered. Say I have the following script (tested with Groovy console):

import groovy.transform.CompileStatic

import java.util.function.Predicate
import java.util.function.Supplier
import java.util.stream.Collectors

@CompileStatic
class Test {
  void testMe() {
    String maybeSomeString = Optional.of("Some Name")
        .orElseThrow({ new NoSuchElementException("Nothing found") }) // line 1 - no explicit cast

    println maybeSomeString
    
    List stringList = ["1ab", "2cd"]
        .stream()
        .flatMap({ it.toList().stream() })
        .filter({ Character.isLetterOrDigit(it.chars[0]) }) // line 2 - no explicit cast
        .collect(Collectors.toList())
    
    println stringList

    [1, 2, 3]
        .stream()
        .map({
          String maybeSomeOtherString = Optional
              .of("Some Other Name")
              .orElseThrow({ new NoSuchElementException("Nothing found") } as Supplier) // line 3 - explicit cast needed

          return "$it: $maybeSomeOtherString".toString()
        })
        .peek({
          List list = it.toList()
              .stream()
              .filter({ Character.isLetterOrDigit(it.chars[0]) }) // line 4 - no explicit cast and no explicit parameter type
              .collect(Collectors.toList())

          println list
        })
        .forEach({println it })
  }
}

new Test().testMe()

In line 3, an explicit cast to Supplier is needed. It wasn't required for 3.0.7. When removed, the checker throws

[Static type checking] - Cannot call <X extends java.lang.Throwable> java.util.Optional <java.lang.String>#orElseThrow(java.util.function.Supplier <? extends X>) with arguments [groovy.lang.Closure <java.util.NoSuchElementException>] 
 at line: 26, column: 41

As can be seen from the sample, coercion works if the closure is not nested inside of another closure.

For comparison, with the 3.0.7 version, line 1 and 2 behaves the same as for 3.0.8.
In 3.0.7, line 3 did not require an explicit cast. In line 4, 3.0.7 requires explicit cast and explicit parameter declaration, while 3.0.8 does not require either.

Tnx