SI-9343 Xlint less strict on pattern sequences

-Xlint:stars-align warns only if elementarity > 0, that is,
if an extracted sequence is not matched entirely by a pattern
sequence, that is, in SLS 8.1.9 on pattern sequences, n = 1
and that pattern is a pattern sequence.

This is still only triggered if productarity > 0, that is,
a non-pattern-sequence pattern is required for the match.

This is a sensitive area because it borders on exhaustiveness
checking: it would be preferable to verify just that the match
is exhaustive, and to emit this warning only if it is not.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/patmat/ScalacPatternExpanders.scala b/src/compiler/scala/tools/nsc/transform/patmat/ScalacPatternExpanders.scala
index 2753baa51d..b1783dc81f 100644
--- a/src/compiler/scala/tools/nsc/transform/patmat/ScalacPatternExpanders.scala
+++ b/src/compiler/scala/tools/nsc/transform/patmat/ScalacPatternExpanders.scala
@@ -112,8 +112,10 @@ trait ScalacPatternExpanders {
         arityError("not enough")
       else if (elementArity > 0 && !isSeq)
         arityError("too many")
-      else if (settings.warnStarsAlign && isSeq && productArity > 0 && (elementArity > 0 || !isStar))
-        warn("A repeated case parameter or extracted sequence should be matched only by a sequence wildcard (_*).")
+      else if (settings.warnStarsAlign && isSeq && productArity > 0 && elementArity > 0) warn {
+        if (isStar) "Sequence wildcard (_*) does not align with repeated case parameter or extracted sequence; the result may be unexpected."
+        else "A repeated case parameter or extracted sequence is not matched by a sequence wildcard (_*), and may fail at runtime."
+      }
 
       aligned
     }