SI-8329 Better hygiene for patmat partial functions

Don't enter synthetic parameters of `applyOrElse` et al into
scope when typechecking the user-code; instead reference those
symbolically.

There is an exception to this principle. Currently we allow:

    val x: PartialFunction[A, B] = x => x match { ... }

For this pattern of code, we use the given name `x` for the
corresponding method parameter of `applyOrElse` and `isDefinedAt`
and we actually need this to be in scope when we typecheck the
scrutinee. This construct is tested in `run/virtpatmat_partial.scala`.

A new parameter, `paramSynthetic`, differentiates this
case from the more typical `val x: PF[A, B] = { case ... => ... ; ... }
case, which uses a fresh name need not be in scope. (We could get
away with it, as it is fresh, but I thought it better to exclude it.)

1 files changed, 29 insertions, 0 deletions

diff --git a/test/files/pos/t8329.scala b/test/files/pos/t8329.scala
new file mode 100644
index 0000000000..fcd5e50b37
--- /dev/null
+++ b/test/files/pos/t8329.scala
@@ -0,0 +1,29 @@
+object Test {
+  def pf(pf: PartialFunction[Any, Unit]) = ()
+  def f1(pf: Function[Any, Unit]) = ()
+ 
+  class A1; class B1
+  def test1(x: String, x1: String, default: String) = pf {
+    case _ if (
+         x.isEmpty
+      && default.isEmpty // was binding to synthetic param
+      && x1.isEmpty      // was binding to synthetic param
+    ) =>
+      x.isEmpty
+      default.isEmpty // was binding to synthetic param
+      x1.isEmpty      // was binding to synthetic param
+      new A1; new B1
+  }
+
+  def test2(x: String, x1: String, default: String) = f1 {
+    case _ if (
+         x.isEmpty
+      && default.isEmpty
+      && x1.isEmpty
+    ) =>
+      x.isEmpty
+      default.isEmpty
+      x1.isEmpty
+      new A1; new B1
+  }
+}