Corrects behavior of finalResultType.

The implementation had come to depend on finalResultType
accidentally doing things beyond its charter - in particular,
widening types. After hunting down and fixing the call sites
depending on the bugs, I was able to rewrite the method to do
only what it's supposed to do.

I threw in a different way of writing it entirely to suggest how
some correctness might be obtained in the future. It's a lot
harder for a method written like this to break.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
index 263b5ad784..787c7ebf63 100644
--- a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
@@ -413,7 +413,7 @@ trait MethodSynthesis {
         // starts compiling (instead of failing like it's supposed to) because the typer
         // expects to be able to identify escaping locals in typedDefDef, and fails to
         // spot that brand of them. In other words it's an artifact of the implementation.
-        val tpt = atPos(derivedSym.pos.focus)(derivedSym.tpe.finalResultType match {
+        val tpt = atPos(derivedSym.pos.focus)(derivedSym.tpe_*.finalResultType.widen match {
           case ExistentialType(_, _)  => TypeTree()
           case _ if mods.isDeferred   => TypeTree()
           case tp                     => TypeTree(tp)