This is in response to this question which concerns the UA version of Improved Pact Weapon. The text for the Eldritch Invocation changed when it was published in Xanathar’s Guide to Everything.
The description for the Warlock’s Improved Pact Weapon Eldritch Invocation now states the following:
The weapon gains a +1 bonus to its attack and damage rolls, unless it is a magic weapon that already has a bonus to those rolls.
+1 weapons are normally considered magical for the purpose of spells like Elemental Weapon, which cannot be cast on a magic weapon.
Is a Warlock’s Improved Pact Weapon considered a magic weapon?