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, 27 insertions, 0 deletions

diff --git a/test/files/neg/t8430.check b/test/files/neg/t8430.check
new file mode 100644
index 0000000000..7c6a73ce53
--- /dev/null
+++ b/test/files/neg/t8430.check
@@ -0,0 +1,27 @@
+t8430.scala:15: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+t8430.scala:16: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+t8430.scala:17: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+t8430.scala:18: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+t8430.scala:19: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+t8430.scala:20: warning: match may not be exhaustive.
+It would fail on the following inputs: ??, LetC, LetF, LetL(IntLit), LetP
+  (tree: Tree) => tree match {case LetL(CharLit) => ??? }
+                  ^
+error: No warnings can be incurred under -Xfatal-warnings.
+6 warnings found
+one error found