Expose tpd problem, temporary .toString hack

The situation is a follows:

Without this commit `testOnly dotty.tools.dotc.CompilationTests -- *testPickling` fail with more precise types after pickling.

Reverting tpd.scala to master breaks the "tree referencial equiality" condition used in Simplify to detect a fix point. For example `vulpix t7336.scala` (this one also requires the Skolem#toString change).

This hack is sufficent for Simplify to reach its fix points without breaking pickling...

[SQUASH] [TODO] fix tpd .toString hack

Author: OlivierBlanvillain

compiler/src/dotty/tools/dotc/ast/tpd.scala

@@ -473,23 +473,30 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     }
 
     override def Apply(tree: Tree)(fun: Tree, args: List[Tree])(implicit ctx: Context): Apply = {
+      // @Dmitry: current implementation in master:
+      // ta.assignType(untpd.cpy.Apply(tree)(fun, args), fun, args)
       val untyped = untpd.cpy.Apply(tree)(fun, args)
-      if (untyped ne tree)
-        ta.assignType(untyped, fun, args)
-      else tree.asInstanceOf[Apply]
+      val typed   = ta.assignType(untyped, fun, args)
+      if (untyped.ne(tree) || tree.tpe.toString != typed.tpe.toString)
+        typed
+      else
+        tree.asInstanceOf[Apply]
     }
       // Note: Reassigning the original type if `fun` and `args` have the same types as before
       // does not work here: The computed type depends on the widened function type, not
       // the function type itself. A treetransform may keep the function type the
       // same but its widened type might change.
 
     override def TypeApply(tree: Tree)(fun: Tree, args: List[Tree])(implicit ctx: Context): TypeApply = {
+      // @Dmitry: current implementation in master:
+      // ta.assignType(untpd.cpy.TypeApply(tree)(fun, args), fun, args)
       val untyped = untpd.cpy.TypeApply(tree)(fun, args)
-      if (untyped ne tree)
-        ta.assignType(untyped, fun, args)
-      else tree.asInstanceOf[TypeApply]
+      val typed   = ta.assignType(untyped, fun, args)
+      if (untyped.ne(tree) || tree.tpe.toString != typed.tpe.toString)
+        typed
+      else
+        tree.asInstanceOf[TypeApply]
     }
-    // FIXME
       // Same remark as for Apply
 
     override def Literal(tree: Tree)(const: Constant)(implicit ctx: Context): Literal =
@@ -524,11 +531,14 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
     }
 
     override def Closure(tree: Tree)(env: List[Tree], meth: Tree, tpt: Tree)(implicit ctx: Context): Closure = {
+      // @Dmitry: current implementation in master:
+      // ta.assignType(untpd.cpy.Closure(tree)(env, meth, tpt), meth, tpt)
       val untyped = untpd.cpy.Closure(tree)(env, meth, tpt)
-      if (untyped ne tree)
-        ta.assignType(untyped, meth, tpt)
-      else tree.asInstanceOf[Closure]
-
+      val typed   = ta.assignType(untyped, meth, tpt)
+      if (untyped.ne(tree) || tree.tpe.toString != typed.tpe.toString)
+        typed
+      else
+        tree.asInstanceOf[Closure]
     }
       // Same remark as for Apply