SI-9658 Fix crosstalk between partial fun. and GADT match

When typechecking the synthetic default case of a pattern matching
anonymous partial function, we failed to create a new `Context`.
This led to crosstalk with the management of the saved type bounds
of an enclosing GADT pattern match.

This commit avoids the direct call to `typeCase` and instead
indirects through `typedCases`, which spawns a new nested typer
context, and hence avoids the crosstalk when `restoreSavedTypeBounds`
runs.

1 files changed, 10 insertions, 0 deletions

diff --git a/test/files/pos/t9658.scala b/test/files/pos/t9658.scala
new file mode 100644
index 0000000000..a2c695a8ae
--- /dev/null
+++ b/test/files/pos/t9658.scala
@@ -0,0 +1,10 @@
+sealed trait G[T]
+case object GI extends G[Int]
+
+class C {
+  def typerFail[T](rt: G[T]): T = rt match {
+    case GI =>
+      { case x => x } : PartialFunction[Any, Any] // comment this line, compiles.
+      0 // found Int, required T
+  }
+}