SI-8430 Less non-determinism in patmat exhautiveness warnings

Another mole whacked on the head by using `LinkedHashMap`.

Caution: `LinkedHashMap` doesn't preserve its runtime type if
you map through the generic interface. I've noted this gotcha as
SI-8434. I've structured this patch to enforce that concrete
collection with types, which is a good idea anyway.

My method to track this down was to place breakpoints in
`Hash{Map,Set}`.{foreach,iterator}` to see where that was
used from within pattern match translation. This approach was
drastically faster than my previous rounds of whack-a-mole.

The counter-examples are still a bit off; I'm going to merge
that aspect of this ticket with SI-7746, in which we've
pinpointed the culpable part of the implementation, but haven't
had success in fixing the bug.

1 files changed, 32 insertions, 0 deletions

diff --git a/test/files/neg/t8430.scala b/test/files/neg/t8430.scala
new file mode 100644
index 0000000000..4166b08a0a
--- /dev/null
+++ b/test/files/neg/t8430.scala
@@ -0,0 +1,32 @@
+sealed trait CL3Literal
+case object IntLit extends CL3Literal
+case object CharLit extends CL3Literal
+case object BooleanLit extends CL3Literal
+case object UnitLit extends CL3Literal
+ 
+ 
+sealed trait Tree
+case class LetL(value: CL3Literal) extends Tree
+case object LetP extends Tree
+case object LetC extends Tree
+case object LetF extends Tree
+ 
+object Test {
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+  // After the first patch for SI-8430, we achieve stability: all of 
+  // these get the same warning:
+  //
+  // ??, LetC, LetF, LetL(IntLit), LetP
+  //
+  // Before, it was non-deterministic.
+  //
+  // However, we our list of counter examples is itself non-exhaustive.
+  // We need to rework counter example generation to fix that.
+  //
+  // That work is the subject of SI-7746
+}