Add space before return type of method that ends with special sign

Fixes #1002267

Author: kiritsuku

org.scala-refactoring.library/src/main/scala/scala/tools/refactoring/sourcegen/ReusingPrinter.scala

@@ -1005,8 +1005,13 @@ trait ReusingPrinter extends TreePrintingTraversals with AbstractPrinter {
 
       val noEqualNeeded = resultType == EmptyFragment || isAbstract
 
+      val resultType2 = {
+        def addLeadingSpace = name.isOperatorName || name.endsWith('_')
+        if (resultType != EmptyFragment && addLeadingSpace) Layout(" ") ++ resultType else resultType
+      }
+
       if (noEqualNeeded && !hasEqualInSource) {
-        l ++ modsAndName ++ typeParameters ++ parameters ++ resultType ++ body ++ r
+        l ++ modsAndName ++ typeParameters ++ parameters ++ resultType2 ++ body ++ r
       } else {
         val openingBrace = keepOpeningBrace(tree, tpt, rhs)
         // In case a Unit return type is added to a method like `def f {}`, we
@@ -1018,7 +1023,7 @@ trait ReusingPrinter extends TreePrintingTraversals with AbstractPrinter {
           else
             modsAndName
 
-        l ++ modsAndName2 ++ typeParameters ++ parameters ++ resultType ++ Requisite.anywhere("=", " = ") ++ openingBrace ++ body ++ r
+        l ++ modsAndName2 ++ typeParameters ++ parameters ++ resultType2 ++ Requisite.anywhere("=", " = ") ++ openingBrace ++ body ++ r
       }
     }
 

org.scala-refactoring.library/src/test/scala/scala/tools/refactoring/tests/sourcegen/ReusingPrinterTest.scala

@@ -204,6 +204,25 @@ class ReusingPrinterTest extends TestHelper with SilentTracing {
         d.copy(tpt = newTpt) replaces d
     }}}
 
+  @Test
+  def add_space_before_return_type_of_def_when_it_ends_with_special_sign() = """
+    package add_space_before_return_type_of_def_when_it_ends_with_special_sign
+    object X {
+      def foo_ = 0
+      def ++ = 0
+    }
+    """ becomes """
+    package add_space_before_return_type_of_def_when_it_ends_with_special_sign
+    object X {
+      def foo_ : Int = 0
+      def ++ : Int = 0
+    }
+    """ after topdown { matchingChildren { transform {
+      case d @ DefDef(_, _, _, _, tpt: TypeTree, _) =>
+        val newTpt = tpt setOriginal mkReturn(List(tpt.tpe.typeSymbol))
+        d.copy(tpt = newTpt) replaces d
+    }}}
+
   @Test
   def add_override_flag() = """
     package add_override_flag