SI-5330, SI-6014 deal with existential self-type

This has been broken since https://github.com/scala/scala/commit/b7b81ca2#L0L567.
The existential rash is treated in a similar manner as in fc24db4c.

Conceptually, the fix would be `def selfTypeSkolemized =
widen.skolemizeExistential.narrow`, but simply widening before
narrowing achieves the same thing. Since we're in existential voodoo
territory, let's go for the minimal fix: replacing `this.narrow` by
`widen.narrow`.

--
Original patch by @retronym in #1074, refined by @paulp to
only perform widen.narrow incantation if there are
existentials present in the widened type, as
narrowing is expensive when the type is not a singleton.

The result is that compiling the entirety of quick, that
code path is hit only 143 times.  All the other calls hit
.narrow directly as before.  It looks like the definition
of negligible in the diff of -Ystatistics when compiling
src/library/scala/collection:

    < #symbols                      : 306315
    ---
    > #symbols                      : 306320
    12c13
    < #unique types                 : 293859
    ---
    > #unique types                 : 293865

I'm assuming based on the 2/1000ths of a percent increase
in symbol and type creation that wall clock is manageable,
but I didn't measure it.

1 files changed, 15 insertions, 3 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index 3104c1e74e..02cdac4046 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -321,6 +321,18 @@ trait Types extends api.Types { self: SymbolTable =>
     }
   }
 
+  /** Same as a call to narrow unless existentials are visible
+   *  after widening the type. In that case, narrow from the widened
+   *  type instead of the proxy. This gives buried existentials a
+   *  chance to make peace with the other types. See SI-5330.
+   */
+  private def narrowForFindMember(tp: Type): Type = {
+    val w = tp.widen
+    // Only narrow on widened type when we have to -- narrow is expensive unless the target is a singleton type.
+    if ((tp ne w) && containsExistential(w)) w.narrow
+    else tp.narrow
+  }
+
   /** The base class for all types */
   abstract class Type extends TypeApiImpl with Annotatable[Type] {
     /** Types for which asSeenFrom always is the identity, no matter what
@@ -1070,7 +1082,7 @@ trait Types extends api.Types { self: SymbolTable =>
                            (other ne sym) &&
                            ((other.owner eq sym.owner) ||
                             (flags & PRIVATE) != 0 || {
-                               if (self eq null) self = this.narrow
+                               if (self eq null) self = narrowForFindMember(this)
                                if (symtpe eq null) symtpe = self.memberType(sym)
                                !(self.memberType(other) matches symtpe)
                             })}) {
@@ -1148,7 +1160,7 @@ trait Types extends api.Types { self: SymbolTable =>
                     if ((member ne sym) &&
                       ((member.owner eq sym.owner) ||
                         (flags & PRIVATE) != 0 || {
-                          if (self eq null) self = this.narrow
+                          if (self eq null) self = narrowForFindMember(this)
                           if (membertpe eq null) membertpe = self.memberType(member)
                           !(membertpe matches self.memberType(sym))
                         })) {
@@ -1163,7 +1175,7 @@ trait Types extends api.Types { self: SymbolTable =>
                       (other ne sym) &&
                         ((other.owner eq sym.owner) ||
                           (flags & PRIVATE) != 0 || {
-                            if (self eq null) self = this.narrow
+                            if (self eq null) self = narrowForFindMember(this)
                             if (symtpe eq null) symtpe = self.memberType(sym)
                             !(self.memberType(other) matches symtpe)
                                })}) {