SI-7226 Fix inference regression caused by TypeVar equality.

TypeVars, being mutable creatures, mustn't have structural
equality/hashing, otherwise TypeRefs that differ only by
having distinct TypeVars as components get wrongly uniqued
together.

The reported bug showed the disaterous consequences: constraints
from the `C?[Int]` in the return type applied to the `?C[?A]` in
the parameter list.

This commit overrides `equals` and `hashCode` in `TypeVar`
to use reference equality. An alternative fix would be to drop
the `case`-ness of the class, as was the case before 0cde930b
when this regressed.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/reflect/scala/reflect/internal/Types.scala b/src/reflect/scala/reflect/internal/Types.scala
index ea193465ad..a27b37dae5 100644
--- a/src/reflect/scala/reflect/internal/Types.scala
+++ b/src/reflect/scala/reflect/internal/Types.scala
@@ -3052,6 +3052,12 @@ trait Types extends api.Types { self: SymbolTable =>
     val origin: Type,
     var constr: TypeConstraint
   ) extends Type {
+
+    // We don't want case class equality/hashing as TypeVar-s are mutable,
+    // and TypeRefs based on them get wrongly `uniqued` otherwise. See SI-7226.
+    override def hashCode(): Int = System.identityHashCode(this)
+    override def equals(other: Any): Boolean = this eq other.asInstanceOf[AnyRef]
+
     def untouchable = false   // by other typevars
     override def params: List[Symbol] = Nil
     override def typeArgs: List[Type] = Nil