added tests from @SirzBenjie

src/data/battler-tags.ts

@@ -1763,14 +1763,7 @@ export class HighestStatBoostTag extends AbilityBattlerTag {
     highestStat = highestStat!; // tell TS compiler it's defined!
     this.stat = highestStat;
 
-    switch (this.stat) {
+    this.multiplier = this.stat === Stat.SPD ? 1.5 : 1.3;
-      case Stat.SPD:
-        this.multiplier = 1.5;
-        break;
-      default:
-        this.multiplier = 1.3;
-        break;
-    }
     globalScene.queueMessage(i18next.t("battlerTags:highestStatBoostOnAdd", { pokemonNameWithAffix: getPokemonNameWithAffix(pokemon), statName: i18next.t(getStatKey(highestStat)) }), null, false, null, true);
   }
 

src/test/abilities/protosythesis.test.ts

@@ -0,0 +1,65 @@
+import { Abilities } from "#enums/abilities";
+import { Moves } from "#enums/moves";
+import { Nature } from "#enums/nature";
+import { Species } from "#enums/species";
+import { Stat } from "#enums/stat";
+import GameManager from "#test/utils/gameManager";
+import Phaser from "phaser";
+import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
+
+describe("Abilities - Protosynthesis", () => {
+  let phaserGame: Phaser.Game;
+  let game: GameManager;
+
+  beforeAll(() => {
+    phaserGame = new Phaser.Game({
+      type: Phaser.HEADLESS,
+    });
+  });
+
+  afterEach(() => {
+    game.phaseInterceptor.restoreOg();
+  });
+
+  beforeEach(() => {
+    game = new GameManager(phaserGame);
+    game.override
+      .moveset([ Moves.SPLASH ])
+      .ability(Abilities.PROTOSYNTHESIS)
+      .battleType("single")
+      .disableCrits()
+      .enemySpecies(Species.MAGIKARP)
+      .enemyAbility(Abilities.BALL_FETCH)
+      .enemyMoveset(Moves.SPLASH);
+  });
+
+  it("should not consider temporary items when determining which stat to boost", async() => {
+    // Mew has uniform base stats
+    game.override.startingModifier([{ name: "TEMP_STAT_STAGE_BOOSTER", type: Stat.DEF }])
+      .ability(Abilities.PROTOSYNTHESIS)
+      .enemyMoveset(Moves.SUNNY_DAY)
+      .startingLevel(100)
+      .enemyLevel(100)
+      .moveset([ Moves.TACKLE, Moves.SPLASH ]);
+    await game.classicMode.startBattle([ Species.MEW ]);
+    const mew = game.scene.getPlayerPokemon()!;
+    // Nature of starting mon is randomized. We need to fix it to a neutral nature for the automated test.
+    mew.setNature(Nature.HARDY);
+    const enemy = game.scene.getEnemyPokemon()!;
+    const def_before_boost = mew?.getEffectiveStat(Stat.DEF, undefined, undefined, false, undefined, false, false, true);
+    const atk_before_boost = mew?.getEffectiveStat(Stat.ATK, undefined, undefined, false, undefined, false, false, true);
+    const initialHp = enemy.hp;
+    game.move.select(Moves.TACKLE);
+    await game.toNextTurn();
+    const unboosted_dmg = initialHp - enemy.hp;
+    enemy.hp = initialHp;
+    const def_after_boost = mew?.getEffectiveStat(Stat.DEF, undefined, undefined, false, undefined, false, false, true);
+    const atk_after_boost = mew?.getEffectiveStat(Stat.ATK, undefined, undefined, false, undefined, false, false, true);
+    game.move.select(Moves.TACKLE);
+    await game.toNextTurn();
+    const boosted_dmg = initialHp - enemy.hp;
+    expect(boosted_dmg).toBeGreaterThan(unboosted_dmg);
+    expect(def_after_boost).toEqual(def_before_boost);
+    expect(atk_after_boost).toBeGreaterThan(atk_before_boost);
+  });
+});