SI-8871 Fix specialization under REPL / FSC

The transformation of applications to specialized methods
relies on the owner of said method having had the specialization
info transform run which stashes a bunch of related data into
per-run caches such as `SpecializeTypes#{typeEnv}`.

Recently, we found that per-run caches didn't quite live up
to there name, and in fact weren't being cleaned up before
a new run. This was remedied in 00e11ff.

However, no good deed goes unpunished, and this led to a
regression in specialization in the REPL and FSC.

This commit makes two changes:

  - change the specialization info tranformer to no longer
    directly enter specialized methods into the `info` of whatever
    the current phase happens to be. This stops them showing up
    `enteringTyper` of the following run.
  - change `adaptInfos` to simply discard all but the oldest
    entry in the type history when bringing a symbol from one
    run into the next. This generalizes the approach taken to
    fix SI-7801. The specialization info transformer will now
    execute in each run, and repopulate `typeEnv` and friends.

I see that we have a seemingly related bandaid for this sort
of problem since 08505bd. In a followup, I'll try to revert
that.

Author: retronym

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

@@ -861,11 +861,6 @@ abstract class SpecializeTypes extends InfoTransform with TypingTransformers {
           debuglog("%s expands to %s in %s".format(sym, specMember.name.decode, pp(env)))
           info(specMember) = NormalizedMember(sym)
           newOverload(sym, specMember, env)
-          // if this is a class, we insert the normalized member in scope,
-          // if this is a method, there's no attached scope for it (EmptyScope)
-          val decls = owner.info.decls
-          if (decls != EmptyScope)
-            decls.enter(specMember)
           specMember
         }
       }

src/reflect/scala/reflect/internal/Symbols.scala

@@ -173,7 +173,6 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
              with HasFlags
              with Annotatable[Symbol]
              with Attachable {
-
     // makes sure that all symbols that runtime reflection deals with are synchronized
     private def isSynchronized = this.isInstanceOf[scala.reflect.runtime.SynchronizedSymbols#SynchronizedSymbol]
     private def isAprioriThreadsafe = isThreadsafe(AllOps)
@@ -1592,13 +1591,11 @@ trait Symbols extends api.Symbols { self: SymbolTable =>
       assert(isCompilerUniverse)
       if (infos == null || runId(infos.validFrom) == currentRunId) {
         infos
-      } else if (isPackageClass) {
-        // SI-7801 early phase package scopes are mutated in new runs (Namers#enterPackage), so we have to
-        //         discard transformed infos, rather than just marking them as from this run.
-        val oldest = infos.oldest
-        oldest.validFrom = validTo
-        this.infos = oldest
-        oldest
+      } else if (infos ne infos.oldest) {
+        // SI-8871 Discard all but the first element of type history. Specialization only works in the resident
+        // compiler / REPL if re-run its info transformer in this run to correctly populate its
+        // per-run caches, e.g. typeEnv
+        adaptInfos(infos.oldest)
       } else {
         val prev1 = adaptInfos(infos.prev)
         if (prev1 ne infos.prev) prev1

test/files/res/t8871.check

@@ -0,0 +1,5 @@
+
+nsc> 
+nsc> 
+nsc> 
+nsc> 

test/files/res/t8871.res

@@ -0,0 +1,4 @@
+t8871/tag.scala
+t8871/usetag.scala
+t8871/usetag.scala
+

test/files/res/t8871/tag.scala

@@ -0,0 +1,3 @@
+class Tag {
+  @inline def apply[@specialized A, T](a: A): A = a
+}

test/files/res/t8871/usetag.scala

@@ -0,0 +1,6 @@
+trait Foo
+
+object Test {
+  val y = new Tag().apply[Double, Foo](3.3)
+  // under FSC, this gave t8871/usetag.scala:4: error: wrong number of type parameters for method apply$mDc$sp: [T](a: Double)Double
+}