SI-4859 Don't rewrite CC().CC2() to new CC2

Where CC and CC2 are case classes. Attempting to do so leads to
a "no legal prefix" error.

Now, we restrict this optimization (living in RefChecks ?!) to
case class applies with a "safe to inline" qualifier.

2 files changed, 8 insertions, 3 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
index e88447c46d..f0ced1a8d4 100644
--- a/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/RefChecks.scala
@@ -1389,9 +1389,12 @@ abstract class RefChecks extends InfoTransform with scala.reflect.internal.trans
         case TypeApply(fun, targs) =>
           isClassTypeAccessible(fun)
         case Select(module, apply) =>
-          // Fixes SI-5626. Classes in refinement types cannot be constructed with `new`. In this case,
-          // the companion class is actually not a ClassSymbol, but a reference to an abstract type.
-          module.symbol.companionClass.isClass
+          ( // SI-4859 `CaseClass1().InnerCaseClass2()` must not be rewritten to `new InnerCaseClass2()`
+            treeInfo.isExprSafeToInline(module) &&
+            // SI-5626 Classes in refinement types cannot be constructed with `new`. In this case,
+            // the companion class is actually not a ClassSymbol, but a reference to an abstract type.
+            module.symbol.companionClass.isClass
+          )
       }
 
       val doTransform =
diff --git a/test/pending/pos/t4859.scala b/test/files/pos/t4859.scala
index ec5abd966d..284a39b7ab 100644
--- a/test/pending/pos/t4859.scala
+++ b/test/files/pos/t4859.scala
@@ -1,5 +1,7 @@
 object O {
+  // error: C is not a legal prefix for a constructor
   C().CC()
+  // but this works.
   D().DD()
 }