It seems like because of JetBrains/kotlin@a29762e#diff-22c3d54b52dd863ad546e639e0b3e7ed5b85d1486afa9e55c0f733b540b78645R44 outputFile now resolved to String property instead of File. This change introduced since Kotlin 1.5.20-M1-61

erokhins · mvicsokolova · commit e85739779b61 · 2021-07-06T17:16:17.000+03:00
diff --git a/atomicfu/build.gradle b/atomicfu/build.gradle
@@ -163,7 +163,7 @@ compileJsLegacy.configure {
     kotlinOptions {
         // NOTE: Module base-name must be equal to the package name declared in package.json
         def baseName = "kotlinx-atomicfu"
-        outputFile = new File(outputFile.parent, baseName + ".js")
+        outputFile = new File(new File(outputFile).parent, baseName + ".js")
     }
 }
 
@@ -188,7 +188,7 @@ compileTestJsLegacy.configure {
     kotlinOptions {
         // NOTE: Module base-name must be equal to the package name declared in package.json
         def baseName = "kotlinx-atomicfu"
-        outputFile = new File(outputFile.parent, baseName + ".js")
+        outputFile = new File(new File(outputFile).parent, baseName + ".js")
     }
 }
 def originalJsFile = compileTestJsLegacy.kotlinOptions.outputFile

Original file line number	Diff line number	Diff line change
`@@ -163,7 +163,7 @@ compileJsLegacy.configure {`
`163`	`163`	`kotlinOptions {`
`164`	`164`	`// NOTE: Module base-name must be equal to the package name declared in package.json`
`165`	`165`	`def baseName = "kotlinx-atomicfu"`
`166`		`- outputFile = new File(outputFile.parent, baseName + ".js")`
	`166`	`+ outputFile = new File(new File(outputFile).parent, baseName + ".js")`
`167`	`167`	`}`
`168`	`168`	`}`
`169`	`169`
`@@ -188,7 +188,7 @@ compileTestJsLegacy.configure {`
`188`	`188`	`kotlinOptions {`
`189`	`189`	`// NOTE: Module base-name must be equal to the package name declared in package.json`
`190`	`190`	`def baseName = "kotlinx-atomicfu"`
`191`		`- outputFile = new File(outputFile.parent, baseName + ".js")`
	`191`	`+ outputFile = new File(new File(outputFile).parent, baseName + ".js")`
`192`	`192`	`}`
`193`	`193`	`}`
`194`	`194`	`def originalJsFile = compileTestJsLegacy.kotlinOptions.outputFile`