Instantiate more type variables to hard unions

Fixes scala#14770

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

@@ -12,6 +12,7 @@ import config.Printers.typr
 import typer.ProtoTypes.{newTypeVar, representedParamRef}
 import UnificationDirection.*
 import NameKinds.AvoidNameKind
+import NullOpsDecorator.stripNull
 
 /** Methods for adding constraints and solving them.
  *
@@ -525,10 +526,12 @@ trait ConstraintHandling {
    * At this point we also drop the @Repeated annotation to avoid inferring type arguments with it,
    * as those could leak the annotation to users (see run/inferred-repeated-result).
    */
-  def widenInferred(inst: Type, bound: Type)(using Context): Type =
+  def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
     def widenOr(tp: Type) =
-      val tpw = tp.widenUnion
-      if (tpw ne tp) && (tpw <:< bound) then tpw else tp
+      if widenUnions then
+        val tpw = tp.widenUnion
+        if (tpw ne tp) && (tpw <:< bound) then tpw else tp
+      else tp.hardenUnions
 
     def widenSingle(tp: Type) =
       val tpw = tp.widenSingletons
@@ -548,24 +551,40 @@ trait ConstraintHandling {
         wideInst.dropRepeatedAnnot
   end widenInferred
 
+  extension (tp: Type) private def hardenUnions(using Context): Type = tp.widen match
+    case tp: AndType =>
+      tp.derivedAndType(tp.tp1.hardenUnions, tp.tp2.hardenUnions)
+    case tp: RefinedType =>
+      tp.derivedRefinedType(tp.parent.hardenUnions, tp.refinedName, tp.refinedInfo)
+    case tp: RecType =>
+      tp.rebind(tp.parent.hardenUnions)
+    case tp: HKTypeLambda =>
+      tp.derivedLambdaType(resType = tp.resType.hardenUnions)
+    case tp: OrType =>
+      val tp1 = tp.stripNull
+      if tp1 ne tp then tp.derivedOrType(tp1.hardenUnions, defn.NullType)
+      else tp.derivedOrType(tp.tp1.hardenUnions, tp.tp2.hardenUnions, soft = false)
+    case _ =>
+      tp
+
   /** The instance type of `param` in the current constraint (which contains `param`).
    *  If `fromBelow` is true, the instance type is the lub of the parameter's
    *  lower bounds; otherwise it is the glb of its upper bounds. However,
    *  a lower bound instantiation can be a singleton type only if the upper bound
    *  is also a singleton type.
    */
-  def instanceType(param: TypeParamRef, fromBelow: Boolean)(using Context): Type = {
+  def instanceType(param: TypeParamRef, fromBelow: Boolean, widenUnions: Boolean)(using Context): Type = {
     val approx = approximation(param, fromBelow).simplified
     if fromBelow then
-      val widened = widenInferred(approx, param)
+      val widened = widenInferred(approx, param, widenUnions)
       // Widening can add extra constraints, in particular the widened type might
       // be a type variable which is now instantiated to `param`, and therefore
       // cannot be used as an instantiation of `param` without creating a loop.
       // If that happens, we run `instanceType` again to find a new instantation.
       // (we do not check for non-toplevel occurences: those should never occur
       // since `addOneBound` disallows recursive lower bounds).
       if constraint.occursAtToplevel(param, widened) then
-        instanceType(param, fromBelow)
+        instanceType(param, fromBelow, widenUnions)
       else
         widened
     else

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

@@ -492,23 +492,35 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
           case AndType(tp21, tp22) => constrainRHSVars(tp21) && constrainRHSVars(tp22)
           case _ => true
 
-        widenOK
-        || joinOK
-        || (tp1.isSoft || constrainRHSVars(tp2)) && recur(tp11, tp2) && recur(tp12, tp2)
-        || containsAnd(tp1)
-            && !joined
-            && {
-              joined = true
-              try inFrozenGadt(recur(tp1.join, tp2))
-              finally joined = false
-            }
-            // An & on the left side loses information. We compensate by also trying the join.
-            // This is less ad-hoc than it looks since we produce joins in type inference,
-            // and then need to check that they are indeed supertypes of the original types
-            // under -Ycheck. Test case is i7965.scala.
-            // On the other hand, we could get a combinatorial explosion by applying such joins
-            // recursively, so we do it only once. See i14870.scala as a test case, which would
-            // loop for a very long time without the recursion brake.
+        def hardenTypeVars(tp2: Type): Unit = tp2.dealiasKeepRefiningAnnots match
+          case tvar: TypeVar if constraint.contains(tvar.origin) =>
+            tvar.widenUnions = false
+          case tp2: TypeParamRef if constraint.contains(tp2) =>
+            hardenTypeVars(constraint.typeVarOfParam(tp2))
+          case tp2: AndOrType =>
+            hardenTypeVars(tp2.tp1)
+            hardenTypeVars(tp2.tp2)
+          case _ =>
+
+        val res = widenOK
+          || joinOK
+          || (tp1.isSoft || constrainRHSVars(tp2)) && recur(tp11, tp2) && recur(tp12, tp2)
+          || containsAnd(tp1)
+              && !joined
+              && {
+                joined = true
+                try inFrozenGadt(recur(tp1.join, tp2))
+                finally joined = false
+              }
+              // An & on the left side loses information. We compensate by also trying the join.
+              // This is less ad-hoc than it looks since we produce joins in type inference,
+              // and then need to check that they are indeed supertypes of the original types
+              // under -Ycheck. Test case is i7965.scala.
+              // On the other hand, we could get a combinatorial explosion by applying such joins
+              // recursively, so we do it only once. See i14870.scala as a test case, which would
+              // loop for a very long time without the recursion brake.
+        if res then hardenTypeVars(tp2)
+        res
 
      case tp1: MatchType =>
         val reduced = tp1.reduced
@@ -2851,8 +2863,8 @@ object TypeComparer {
   def subtypeCheckInProgress(using Context): Boolean =
     comparing(_.subtypeCheckInProgress)
 
-  def instanceType(param: TypeParamRef, fromBelow: Boolean)(using Context): Type =
-    comparing(_.instanceType(param, fromBelow))
+  def instanceType(param: TypeParamRef, fromBelow: Boolean, widenUnions: Boolean)(using Context): Type =
+    comparing(_.instanceType(param, fromBelow, widenUnions))
 
   def approximation(param: TypeParamRef, fromBelow: Boolean)(using Context): Type =
     comparing(_.approximation(param, fromBelow))
@@ -2872,8 +2884,8 @@ object TypeComparer {
   def addToConstraint(tl: TypeLambda, tvars: List[TypeVar])(using Context): Boolean =
     comparing(_.addToConstraint(tl, tvars))
 
-  def widenInferred(inst: Type, bound: Type)(using Context): Type =
-    comparing(_.widenInferred(inst, bound))
+  def widenInferred(inst: Type, bound: Type, widenUnions: Boolean)(using Context): Type =
+    comparing(_.widenInferred(inst, bound, widenUnions))
 
   def dropTransparentTraits(tp: Type, bound: Type)(using Context): Type =
     comparing(_.dropTransparentTraits(tp, bound))

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

@@ -517,7 +517,9 @@ object TypeOps:
       override def apply(tp: Type): Type = tp match
         case tp: TypeVar if mapCtx.typerState.constraint.contains(tp) =>
           val lo = TypeComparer.instanceType(
-            tp.origin, fromBelow = variance > 0 || variance == 0 && tp.hasLowerBound)(using mapCtx)
+            tp.origin,
+            fromBelow = variance > 0 || variance == 0 && tp.hasLowerBound,
+            widenUnions = tp.widenUnions)(using mapCtx)
           val lo1 = apply(lo)
           if (lo1 ne lo) lo1 else tp
         case _ =>

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

@@ -4507,6 +4507,8 @@ object Types {
   final class TypeVar private(initOrigin: TypeParamRef, creatorState: TyperState | Null, val nestingLevel: Int) extends CachedProxyType with ValueType {
     private var currentOrigin = initOrigin
 
+    var widenUnions = true
+
     def origin: TypeParamRef = currentOrigin
 
     /** Set origin to new parameter. Called if we merge two conflicting constraints.
@@ -4569,7 +4571,7 @@ object Types {
      *  is also a singleton type.
      */
     def instantiate(fromBelow: Boolean)(using Context): Type =
-      val tp = TypeComparer.instanceType(origin, fromBelow)
+      val tp = TypeComparer.instanceType(origin, fromBelow, widenUnions)
       if myInst.exists then // The line above might have triggered instantiation of the current type variable
         myInst
       else

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1884,7 +1884,7 @@ class Namer { typer: Typer =>
         TypeOps.simplify(tp.widenTermRefExpr,
             if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null) match
           case ctp: ConstantType if sym.isInlineVal => ctp
-          case tp => TypeComparer.widenInferred(tp, pt)
+          case tp => TypeComparer.widenInferred(tp, pt, widenUnions = true)
 
     // Replace aliases to Unit by Unit itself. If we leave the alias in
     // it would be erased to BoxedUnit.

compiler/src/dotty/tools/dotc/typer/Synthesizer.scala

@@ -489,7 +489,7 @@ class Synthesizer(typer: Typer)(using @constructorOnly c: Context):
                   val tparams = poly.paramRefs
                   val variances = childClass.typeParams.map(_.paramVarianceSign)
                   val instanceTypes = tparams.lazyZip(variances).map((tparam, variance) =>
-                    TypeComparer.instanceType(tparam, fromBelow = variance < 0))
+                    TypeComparer.instanceType(tparam, fromBelow = variance < 0, widenUnions = true))
                   resType.substParams(poly, instanceTypes)
                 instantiate(using ctx.fresh.setExploreTyperState().setOwner(childClass))
               case _ =>

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2808,7 +2808,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         if (ctx.mode.is(Mode.Pattern)) app1
         else {
           val elemTpes = elems.lazyZip(pts).map((elem, pt) =>
-            TypeComparer.widenInferred(elem.tpe, pt))
+            TypeComparer.widenInferred(elem.tpe, pt, widenUnions = true))
           val resTpe = TypeOps.nestedPairs(elemTpes)
           app1.cast(resTpe)
         }

tests/pos/i14770.scala

@@ -0,0 +1,25 @@
+type UndefOr[A] = A | Unit
+
+extension [A](maybe: UndefOr[A])
+  def foreach(f: A => Unit): Unit =
+    maybe match
+      case () => ()
+      case a: A => f(a)
+
+trait Foo
+trait Bar
+
+object Baz:
+  var booBap: Foo | Bar = _
+
+def z: UndefOr[Foo | Bar] = ???
+
+@main
+def main =
+  z.foreach(x => Baz.booBap = x)
+
+def test[A](v: A | Unit): A | Unit =  v
+val x1 = test(5: Int | Unit)
+val x2 = test(5: String | Int | Unit)
+val _: Int | Unit = x1
+val _: String | Int | Unit = x2