fix GADT adaption when typing if

fix type assigning for if

Author: Linyxus

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

@@ -1142,11 +1142,18 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         cpy.If(tree)(cond1, thenp1, elsep1).withType(defn.UnitType)
       else
         val thenp1 :: elsep1 :: Nil = harmonic(harmonize, pt) {
-          val thenp0 = typed(tree.thenp, branchPt)(using cond1.nullableContextIf(true))
-          val elsep0 = typed(tree.elsep, branchPt)(using cond1.nullableContextIf(false))
+          val thenp0 = typedUnadapted(tree.thenp, branchPt)(using cond1.nullableContextIf(true))
+          val elsep0 = typedUnadapted(tree.elsep, branchPt)(using cond1.nullableContextIf(false))
           thenp0 :: elsep0 :: Nil
         }
-        assignType(cpy.If(tree)(cond1, thenp1, elsep1), thenp1, elsep1)
+
+        val locked = ctx.typerState.ownedVars
+
+        val orTpe = thenp1.tpe | elsep1.tpe
+        val thenp2 = adapt(thenp1, orTpe, locked)
+        val elsep2 = adapt(elsep1, orTpe, locked)
+
+        assignType(cpy.If(tree)(cond1, thenp2, elsep2), thenp1, elsep1)
 
     def thenPathInfo = cond1.notNullInfoIf(true).seq(result.thenp.notNullInfo)
     def elsePathInfo = cond1.notNullInfoIf(false).seq(result.elsep.notNullInfo)