Check that we don't leak uninstantiated type variables

Type variable instantiation should only occur in Typer, TreeChecker and
during exhaustiveness checking, this means that no uninstantiated type
variable should exist outside of these phases. This was not enforced
until now, and was not always true before #4080.

Author: smarter

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -94,4 +94,6 @@ object Mode {
   /** We are in the IDE */
   val Interactive = newMode(20, "Interactive")
 
+  /** Allow instantiating type variables after Typer */
+  val AllowTypevarsInstantiation = newMode(21, "AllowTypevarsInstantiation")
 }

compiler/src/dotty/tools/dotc/core/Phases.scala

@@ -310,6 +310,9 @@ object Phases {
      */
     def isTyper = false
 
+    /** Is this phase an instance of TreeChecker ? */
+    def isTreeChecker = false
+
     /** Can this transform create or delete non-private members? */
     def changesMembers: Boolean = false
 

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -3384,7 +3384,11 @@ object Types {
      *  uninstantiated
      */
     def instanceOpt(implicit ctx: Context): Type =
-      if (inst.exists) inst else ctx.typerState.instType(this)
+      if (inst.exists) inst else {
+        assert(ctx.phase.isTyper || ctx.phase.isTreeChecker || ctx.mode.is(Mode.AllowTypevarsInstantiation),
+          s"Detected uninstantiated type variable $this in phase ${ctx.phase}")
+        ctx.typerState.instType(this)
+      }
 
     /** Is the variable already instantiated? */
     def isInstantiated(implicit ctx: Context) = instanceOpt.exists

compiler/src/dotty/tools/dotc/transform/PatternMatcher.scala

@@ -32,7 +32,7 @@ class PatternMatcher extends MiniPhase {
     val translated = new Translator(tree.tpe, this).translateMatch(tree)
 
     // check exhaustivity and unreachability
-    val engine = new patmat.SpaceEngine
+    val engine = new patmat.SpaceEngine(ctx)
 
     if (engine.checkable(tree)) {
       engine.checkExhaustivity(tree)

compiler/src/dotty/tools/dotc/transform/PatternMatcherOld.scala

@@ -51,7 +51,7 @@ class PatternMatcherOld extends MiniPhase with DenotTransformer {
     val translated = new Translator()(ctx).translator.translateMatch(tree)
 
     // check exhaustivity and unreachability
-    val engine = new SpaceEngine
+    val engine = new SpaceEngine(ctx)
     if (engine.checkable(tree)) {
       engine.checkExhaustivity(tree)
       engine.checkRedundancy(tree)

compiler/src/dotty/tools/dotc/transform/TreeChecker.scala

@@ -49,6 +49,8 @@ class TreeChecker extends Phase with SymTransformer {
   import ast.tpd._
   import TreeChecker._
 
+  override def isTreeChecker = true
+
   private val seenClasses = collection.mutable.HashMap[String, Symbol]()
   private val seenModuleVals = collection.mutable.HashMap[String, Symbol]()
 

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -279,9 +279,11 @@ trait SpaceLogic {
 }
 
 /** Scala implementation of space logic */
-class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
+class SpaceEngine(ictx: Context) extends SpaceLogic {
   import tpd._
 
+  implicit val ctx: Context = ictx.addMode(Mode.AllowTypevarsInstantiation)
+
   private val scalaSomeClass       = ctx.requiredClass("scala.Some")
   private val scalaSeqFactoryClass = ctx.requiredClass("scala.collection.generic.SeqFactory")
   private val scalaListType        = ctx.requiredClassRef("scala.collection.immutable.List")