SI-8366 make partial function and match trees disjoint

Previously one could match a partial function with match quasiquote:

    scala> val q"$scrutinee match { case ..$cases }" = q"{ case Foo => Bar
    }"
    scrutinee: universe.Tree = <empty>
    cases: List[universe.CaseDef] = List(case Foo => Bar)

This was quite annoying as it leaked encoding of partial functions as
Match trees with empty tree in place of scrutinee.

This commit make sure that matches and partial functions are disjoint
and don't match one another (while preserving original encoding under
the hood out of sight of the end user.)

2 files changed, 14 insertions, 6 deletions

diff --git a/test/files/scalacheck/quasiquotes/TermConstructionProps.scala b/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
index 74d0d54ea8..7447037399 100644
--- a/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
+++ b/test/files/scalacheck/quasiquotes/TermConstructionProps.scala
@@ -95,12 +95,6 @@ object TermConstructionProps extends QuasiquoteProperties("term construction") {
     body1 ≈ body && cond1 ≈ cond
   }
 
-  property("unquote trees into alternative") = forAll { (c: Tree, A: Tree, B: Tree) =>
-    q"$c match { case $A | $B => }" ≈
-      Match(c, List(
-        CaseDef(Alternative(List(A, B)), EmptyTree, Literal(Constant(())))))
-  }
-
   def blockInvariant(quote: Tree, trees: List[Tree]) =
     quote ≈ (trees match {
       case Nil => q"{}"
@@ -303,4 +297,12 @@ object TermConstructionProps extends QuasiquoteProperties("term construction") {
   property("SI-8385 b") = test {
     assertEqAst(q"(() => ())()", "(() => ())()")
   }
+
+  property("match scrutinee may not be empty") = test {
+    assertThrows[IllegalArgumentException] {
+      val scrutinee = q""
+      val cases = List(cq"_ =>")
+      q"$scrutinee match { case ..$cases }"
+    }
+  }
 }
diff --git a/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala b/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
index f558a2f078..993ceea8e9 100644
--- a/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
+++ b/test/files/scalacheck/quasiquotes/TermDeconstructionProps.scala
@@ -211,4 +211,10 @@ object TermDeconstructionProps extends QuasiquoteProperties("term deconstruction
       val q"$f[..$targs]" = tq"foo[bar]"
     }
   }
+
+  property("match doesn't match partial function") = test {
+    assertThrows[MatchError] {
+      val q"$_ match { case ..$_ }" = q"{ case _ => }"
+    }
+  }
 }