Unify in-scope name types

Enable construction of names with a more concrete type than the one in
scope.

tlair/src/main/scala/at/forsyte/apalache/tla/typecomp/subbuilder/LiteralAndNameBuilder.scala

@@ -3,6 +3,7 @@ package at.forsyte.apalache.tla.typecomp.subbuilder
 import at.forsyte.apalache.tla.lir._
 import at.forsyte.apalache.tla.typecomp._
 import at.forsyte.apalache.tla.typecomp.unsafe.UnsafeLiteralAndNameBuilder
+import at.forsyte.apalache.tla.types.{Substitution, TypeUnifier, TypeVarPool}
 import scalaz.Scalaz._
 import scalaz._
 
@@ -58,16 +59,22 @@ trait LiteralAndNameBuilder {
   def name(exprName: String, t: TlaType1): TBuilderInstruction = State[TBuilderContext, TlaEx] { mi =>
     val scope = mi.freeNameScope
 
-    // If already in scope, type must be the same
-    scope.get(exprName).foreach { tt =>
-      if (tt != t)
-        throw new TBuilderScopeException(
+    // If already in scope, type must be unifiable
+    val unifT = scope.get(exprName).map { tt =>
+      val unifOpt = new TypeUnifier(new TypeVarPool()).unify(Substitution.empty, tt, t)
+      unifOpt match {
+        case Some((_, unifiedOperT)) => unifiedOperT
+        case None =>
+          throw new TBuilderScopeException(
             s"Name $exprName with type $t constructed in scope where expected type is $tt."
-        )
+          )
+      }
     }
+    // If not in scope, just use `t`
+    val finalT = unifT.getOrElse(t)
 
-    val ret = unsafeBuilder.name(exprName, t)
-    (mi.copy(freeNameScope = scope + (exprName -> t), usedNames = mi.usedNames + exprName), ret)
+    val ret = unsafeBuilder.name(exprName, finalT)
+    (mi.copy(freeNameScope = scope + (exprName -> finalT), usedNames = mi.usedNames + exprName), ret)
   }
 
   /** Attempt to infer the type from the scope. Fails if exprName is not in scope. */