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https://issues.apache.org/jira/browse/GROOVY-9971?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17297530#comment-17297530
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mgroovy commented on GROOVY-9971:
---------------------------------

* For the case that the return type of a Closure is deduced to be 
String|GString I would expect the result of calling the Closure<String> to be a 
String, same as when I pass a GString to a method that expects a String
** i.e. Groovy should call toString() on the result (I assumed that was what 
was happening until now; evidently from what you are saying, the resulting type 
remained GString, and the toString call happened much later...)
* After all I can e.g. do:
{code}
final x = 123
final String s = { false ? 'abc' : "x=$x" }()
{code}
it just fails, when I try to do:
{code}
final Closure<String> strCls = { false ? 'abc' : "x=$x" }
final String s = strCls()
{code}

> @TypeChecked: Closure<String> no longer compatible with Closure<GString> 
> argument
> ---------------------------------------------------------------------------------
>
>                 Key: GROOVY-9971
>                 URL: https://issues.apache.org/jira/browse/GROOVY-9971
>             Project: Groovy
>          Issue Type: Bug
>          Components: Compiler
>    Affects Versions: 3.0.7
>         Environment: Windows 10
> jdk-11.0.10.9-hotspot
> IntelliJ 2020.3.2
>            Reporter: mgroovy
>            Priority: Major
>
> *Problem*
> A Closure<String> is in Groovy 3 no longer compatible with a closure that 
> returns a GString (or String|GString), even though GString is autoconverted
> to String in Groovy.
> *Expected*
> A Closure returning a GString or a String|GString should be compatible with 
> Closure<String>
> *Sample Code*
> {code:java}
> import groovy.transform.TypeChecked
> import org.junit.Ignore
> import org.junit.Test
> @TypeChecked
> class Groovy3_ClosureOfString_GString_Compatible {
>       @Test @Ignore
>       void 'Groovy 3-0-7 Closure with String return type not compatible with 
> GString argument'() {
>               final x = 123
>               // Cannot find matching method 
> execStringCls(Closure<groovy.lang.GString>)
>               assert execStringCls { "x=$x" } == 'x=123'
>               //assert execStringCls { "x=$x".toString() } == 'x=123'  // 
> works
>       }
>       @Test @Ignore
>       void 'Groovy 3-0-7 Closure with String return type not compatible with 
> GString argument 2'() {
>               final x = 123
>               // Cannot find matching method 
> execStringCls(Closure<java.io.Serializable>)
>               assert execStringCls { false ? 'abc' : "x=$x" } == 'x=123'
>               //assert execStringCls { true ? 'abc' : "x=$x".toString() } == 
> 'x=123' // works
>       }
>       String execStringCls(Closure<String> cls) {
>               cls()
>       }
> }
> {code}



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