Fix SI-5284.

The problem was the false assumption that methods specialized
on their type parameter, such as this one:

    class Foo[@spec(Int) T](val x: T) {
      def bar[@spec(Int) S >: T](f: S => S) = f(x)
    }

have their normalized versions (`bar$mIc$sp`) never called
from the base specialization class `Foo`.
This meant that the implementation of `bar$mIc$sp` in `Foo`
simply threw an exception.

This assumption is not true, however. See this:

    object Baz {
      def apply[T]() = new Foo[T]
    }

Calling `Baz.apply[Int]()` will create an instance of the
base specialization class `Foo` at `Int`.
Calling `bar` on this instance will be rewritten by
specialization to calling `bar$mIc$sp`, hence the error.

So, we have to emit a valid implementation for `bar`,
obviously.
Problem is, such an implementation would have conflicting
type bounds in the base specialization class `Foo`, since
we don't know if `T` is a subtype of `S = Int`.
In other words, we cannot emit:

    def bar$mIc$sp(f: Int => Int) = f(x) // x: T

without typechecking errors.

However, notice that the bounds are valid if and only if
`T = Int`. In the same time, invocations of `bar$mIc$sp` will only
be emitted in callsites where the type bounds hold.
This means we can cast the expressions in method applications
to the required specialized type bound.

The following changes have been made:
1) The decision of whether or not to create a normalized
version of the specialized method is not done on the
`conflicting` relation anymore.
Instead, it's done based on the `satisfiable` relation,
which is true if there is possibly an instantiation of
the type parameters where the bounds hold.
2) The `satisfiable` method has a new variant called
`satisfiableConstraints`, which does unification to
figure out how the type parameters should be instantiated
in order to satisfy the bounds.
3) The `Duplicators` are changed to transform a tree
using the `castType` method which just returns the tree
by default.
In specialization, the `castType` in `Duplicators` is overridden,
and uses a map from type parameters to types.
This map is obtained by `satisfiableConstraints` from 2).
If the type of the expression is not equal to the expected type,
and this map contains a mapping to the expected type, then
the tree is cast, as discussed above.

Additional tests added.

Review by @dragos
Review by @VladUreche

Conflicts:

	src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
	src/compiler/scala/tools/nsc/typechecker/Duplicators.scala

1 files changed, 72 insertions, 13 deletions

diff --git a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
index c4c769d7cf..1d820afe11 100644
--- a/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
+++ b/src/compiler/scala/tools/nsc/transform/SpecializeTypes.scala
@@ -450,7 +450,12 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
 
   /** Type parameters that survive when specializing in the specified environment. */
   def survivingParams(params: List[Symbol], env: TypeEnv) =
-    params.filter(p => !p.isSpecialized || !isPrimitiveValueType(env(p)))
+    params filter {
+      p =>
+      !p.isSpecialized || 
+      !env.contains(p) ||
+      !isPrimitiveValueType(env(p))
+    }
 
   /** Produces the symbols from type parameters `syms` of the original owner,
    *  in the given type environment `env`. The new owner is `nowner`.
@@ -1176,7 +1181,7 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
         || specializedTypeVars(t1).nonEmpty
         || specializedTypeVars(t2).nonEmpty)
      }
-
+    
     env forall { case (tvar, tpe) =>
       matches(tvar.info.bounds.lo, tpe) && matches(tpe, tvar.info.bounds.hi) || {
         if (warnings)
@@ -1192,10 +1197,58 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
       }
     }
   }
+  
+  def satisfiabilityConstraints(env: TypeEnv): Option[TypeEnv] = {
+    val noconstraints = Some(emptyEnv)
+    def matches(tpe1: Type, tpe2: Type): Option[TypeEnv] = {
+      val t1 = subst(env, tpe1)
+      val t2 = subst(env, tpe2)
+      // log("---------> " + tpe1 + " matches " + tpe2)
+      // log(t1 + ", " + specializedTypeVars(t1))
+      // log(t2 + ", " + specializedTypeVars(t2))
+      // log("unify: " + unify(t1, t2, env, false, false) + " in " + env)
+      if (t1 <:< t2) noconstraints
+      else if (specializedTypeVars(t1).nonEmpty) Some(unify(t1, t2, env, false, false) -- env.keys)
+      else if (specializedTypeVars(t2).nonEmpty) Some(unify(t2, t1, env, false, false) -- env.keys)
+      else None
+    }
+
+    env.foldLeft[Option[TypeEnv]](noconstraints) {
+      case (constraints, (tvar, tpe)) =>
+        val loconstraints = matches(tvar.info.bounds.lo, tpe)
+        val hiconstraints = matches(tpe, tvar.info.bounds.hi)
+        val allconstraints = for (c <- constraints; l <- loconstraints; h <- hiconstraints) yield c ++ l ++ h
+        allconstraints
+    }
+  }
 
-  class Duplicator extends {
+  /** This duplicator additionally performs casts of expressions if that is allowed by the `casts` map. */
+  class Duplicator(casts: Map[Symbol, Type]) extends {
     val global: SpecializeTypes.this.global.type = SpecializeTypes.this.global
-  } with typechecker.Duplicators
+  } with typechecker.Duplicators {
+    private val (castfrom, castto) = casts.unzip
+    private object CastMap extends SubstTypeMap(castfrom.toList, castto.toList)
+    
+    class BodyDuplicator(_context: Context) extends super.BodyDuplicator(_context) {
+      override def castType(tree: Tree, pt: Type): Tree = {
+        // log(" expected type: " + pt)
+        // log(" tree type: " + tree.tpe)
+        tree.tpe = if (tree.tpe != null) fixType(tree.tpe) else null
+        // log(" tree type: " + tree.tpe)
+        val ntree = if (tree.tpe != null && !(tree.tpe <:< pt)) {
+          val casttpe = CastMap(tree.tpe)
+          if (casttpe <:< pt) gen.mkCast(tree, casttpe)
+          else if (casttpe <:< CastMap(pt)) gen.mkCast(tree, pt)
+          else tree
+        } else tree
+        ntree.tpe = null
+        ntree
+      }
+    }
+    
+    protected override def newBodyDuplicator(context: Context) = new BodyDuplicator(context)
+    
+  }
 
   /** A tree symbol substituter that substitutes on type skolems.
    *  If a type parameter is a skolem, it looks for the original
@@ -1475,14 +1528,14 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
               deriveDefDef(tree1)(transform)
 
             case NormalizedMember(target) =>
-              debuglog("Normalized member: " + symbol + ", target: " + target)
-              if (target.isDeferred || conflicting(typeEnv(symbol))) {
+              val constraints = satisfiabilityConstraints(typeEnv(symbol))
+              log("constraints: " + constraints)
+              if (target.isDeferred || constraints == None) {
                 deriveDefDef(tree)(_ => localTyper typed gen.mkSysErrorCall("Fatal error in code generation: this should never be called."))
-              }
-              else {
+              } else {
                 // we have an rhs, specialize it
                 val tree1 = reportTypeError {
-                  duplicateBody(ddef, target)
+                  duplicateBody(ddef, target, constraints.get)
                 }
                 debuglog("implementation: " + tree1)
                 deriveDefDef(tree1)(transform)
@@ -1546,7 +1599,7 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
           val tree1 = deriveValDef(tree)(_ => body(symbol.alias).duplicate)
           debuglog("now typing: " + tree1 + " in " + tree.symbol.owner.fullName)
 
-          val d = new Duplicator
+          val d = new Duplicator(emptyEnv)
           val newValDef = d.retyped(
             localTyper.context1.asInstanceOf[d.Context],
             tree1,
@@ -1571,12 +1624,18 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
           super.transform(tree)
       }
     }
-
-    private def duplicateBody(tree: DefDef, source: Symbol) = {
+    
+    /** Duplicate the body of the given method `tree` to the new symbol `source`.
+     *  
+     *  Knowing that the method can be invoked only in the `castmap` type environment,
+     *  this method will insert casts for all the expressions of types mappend in the
+     *  `castmap`.
+     */
+    private def duplicateBody(tree: DefDef, source: Symbol, castmap: TypeEnv = emptyEnv) = {
       val symbol = tree.symbol
       val meth   = addBody(tree, source)
 
-      val d = new Duplicator
+      val d = new Duplicator(castmap)
       debuglog("-->d DUPLICATING: " + meth)
       d.retyped(
         localTyper.context1.asInstanceOf[d.Context],