BinPack.ParentCtors: add new ForceBreak option

docs/configuration.md

@@ -5138,6 +5138,21 @@ object A {
 }
 ```
 
+#### `binPack.parentConstructors=ForceBreak`
+
+This option will enforce a break before each parent. As usual, the break is only
+actually introduced if indented position on the next line is less than the current.
+
+```scala mdoc:scalafmt
+binPack.parentConstructors = ForceBreak
+maxColumn = 45
+---
+object A {
+  class Foo(a: Int) extends Bar with Baz
+  class Foo(a: Int, b: Int, c: String, d: Double) extends Bar with Baz
+}
+```
+
 ### `binPack.xxxSite`
 
 Controls binpacking around method/type definition sites (`binPack.defnSite`) or

scalafmt-core/shared/src/main/scala/org/scalafmt/config/BinPack.scala

@@ -103,6 +103,7 @@ object BinPack {
     case object keep extends ParentCtors
     case object Always extends ParentCtors
     case object Never extends ParentCtors
+    case object ForceBreak extends ParentCtors
     case object Oneline extends ParentCtors
     case object OnelineIfPrimaryOneline extends ParentCtors
 
@@ -112,6 +113,7 @@ object BinPack {
         keep,
         Always,
         Never,
+        ForceBreak,
         Oneline,
         OnelineIfPrimaryOneline,
       ) {

scalafmt-tests/shared/src/test/resources/binPack/ParentConstructors.stat

@@ -275,3 +275,65 @@ object foo extends bar(baz) with foo with baz
 object foo
     extends bar(baz)
     with foo with baz
+<<< #4542 ForceBreak narrow single-line
+maxColumn = 40
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String) extends SomeBaseClass with SomeTrait
+>>>
+class SomeClass(
+    a: Int,
+    b: Double,
+    c: String
+) extends SomeBaseClass
+    with SomeTrait
+<<< #4542 ForceBreak medium single-line
+maxColumn = 50
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String) extends SomeBaseClass with SomeTrait
+>>>
+class SomeClass(a: Int, b: Double, c: String)
+    extends SomeBaseClass
+    with SomeTrait
+<<< #4542 ForceBreak wide single-line
+maxColumn = 100
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String) extends SomeBaseClass with SomeTrait
+>>>
+class SomeClass(a: Int, b: Double, c: String) extends SomeBaseClass with SomeTrait
+<<< #4542 ForceBreak narrow multi-line
+maxColumn = 40
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String)
+extends SomeBaseClass
+with SomeTrait
+>>>
+class SomeClass(
+    a: Int,
+    b: Double,
+    c: String
+) extends SomeBaseClass
+    with SomeTrait
+<<< #4542 ForceBreak medium multi-line
+maxColumn = 50
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String)
+extends SomeBaseClass
+with SomeTrait
+>>>
+class SomeClass(a: Int, b: Double, c: String)
+    extends SomeBaseClass
+    with SomeTrait
+<<< #4542 ForceBreak wide multi-line
+maxColumn = 100
+binPack.parentConstructors = ForceBreak
+===
+class SomeClass(a: Int, b: Double, c: String)
+extends SomeBaseClass
+with SomeTrait
+>>>
+class SomeClass(a: Int, b: Double, c: String) extends SomeBaseClass with SomeTrait

scalafmt-tests/shared/src/test/scala/org/scalafmt/FormatTests.scala

@@ -137,7 +137,7 @@ class FormatTests extends FunSuite with CanRunTests with FormatAssertions {
     val explored = Debug.explored.get()
     logger.debug(s"Total explored: $explored")
     if (!onlyUnit && !onlyManual)
-      assertEquals(explored, 1117062, "total explored")
+      assertEquals(explored, 1117354, "total explored")
     val results = debugResults.result()
     // TODO(olafur) don't block printing out test results.
     // I don't want to deal with scalaz's Tasks :'(