SI-6968 Simple Tuple patterns aren't irrefutable

Reverts part of c82ecab. The parser can't assume that
a pattern `(a, b)` will match, as results of
`.isInstanceOf[Tuple2]` can't be statically known until
after the typer.

The reopens SI-1336, SI-5589 and SI-4574, in exchange for fixing
this regression SI-6968. Keeping all of those fixed will require
a better definition of irrefutability, and some acrobatics to
ensure safe passage to the ambiguous trees through typechecking.

1 files changed, 8 insertions, 10 deletions

diff --git a/src/reflect/scala/reflect/internal/TreeInfo.scala b/src/reflect/scala/reflect/internal/TreeInfo.scala
index e1a18570b2..98c1afb323 100644
--- a/src/reflect/scala/reflect/internal/TreeInfo.scala
+++ b/src/reflect/scala/reflect/internal/TreeInfo.scala
@@ -261,22 +261,24 @@ abstract class TreeInfo {
    * in the position `for { <tree> <- expr }` based only
    * on information at the `parser` phase? To qualify, there
    * may be no subtree that will be interpreted as a
-   * Stable Identifier Pattern.
+   * Stable Identifier Pattern, nor any type tests, even
+   * on TupleN. See SI-6968.
    *
    * For instance:
    *
    * {{{
-   * foo @ (bar, (baz, quux))
+   * (foo @ (bar @ _)) = 0
    * }}}
    *
-   * is a variable pattern; if the structure matches,
-   * then the remainder is inevitable.
+   * is a not a variable pattern; if only binds names.
    *
    * The following are not variable patterns.
    *
    * {{{
-   *   foo @ (bar, (`baz`, quux)) // back quoted ident, not at top level
-   *   foo @ (bar, Quux)          // UpperCase ident, not at top level
+   *   `bar`
+   *   Bar
+   *   (a, b)
+   *   _: T
    * }}}
    *
    * If the pattern is a simple identifier, it is always
@@ -305,10 +307,6 @@ abstract class TreeInfo {
       tree match {
         case Bind(name, pat)  => isVarPatternDeep0(pat)
         case Ident(name)      => isVarPattern(tree)
-        case Apply(sel, args) =>
-          (    isReferenceToScalaMember(sel, TupleClass(args.size).name.toTermName)
-            && (args forall isVarPatternDeep0)
-            )
         case _                => false
       }
     }