SI-6434 Pretty print function types with by name arg as (=> A) => B

We were pretty printing a function type with one by name arg as
=> A => B, but because => is right associative that's formally
equivalent to => (A => B) and that's entirely a different thing. This
commit changes the pretty printer in Typers.scala to check for a
byname argument on a function type and wrap it in parens. A REPL test
is included.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/files/run/t6434.scala b/test/files/run/t6434.scala
new file mode 100644
index 0000000000..e4a4579613
--- /dev/null
+++ b/test/files/run/t6434.scala
@@ -0,0 +1,8 @@
+import scala.tools.partest.ReplTest
+
+object Test extends ReplTest {
+  def code =
+"""def f(x: => Int): Int = x
+f _
+"""
+}