fix(promise): Check for a promise without indexing the value

src/promise/src/Shared/Promise.lua

@@ -41,7 +41,14 @@ export type Promise<T...> = typeof(setmetatable(
 	@return boolean
 ]=]
 function Promise.isPromise(value: any): boolean
-	return type(value) == "table" and value.ClassName == "Promise"
+	if type(value) ~= "table" then
+		return false
+	end
+
+	-- Read the metatable directly: indexing the value runs its __index, and some objects error on
+	-- unknown keys. Promises from another copy of this package have their own metatable.
+	local metatable = getmetatable(value)
+	return type(metatable) == "table" and rawget(metatable, "ClassName") == "Promise"
 end
 
 --[=[

src/promise/src/Shared/Promise.spec.lua

@@ -26,6 +26,23 @@ describe("Promise.isPromise", function()
 		expect(Promise.isPromise({})).toEqual(false)
 		expect(Promise.isPromise({ ClassName = "NotAPromise" })).toEqual(false)
 	end)
+
+	it("returns false without indexing objects that error on unknown keys", function()
+		local strict = setmetatable({}, {
+			__index = function(_, key)
+				error(string.format("%s is not a valid member", tostring(key)))
+			end,
+		})
+
+		expect(Promise.isPromise(strict)).toEqual(false)
+	end)
+
+	it("returns true for a promise from another copy of the package", function()
+		local OtherPromise = { ClassName = "Promise" }
+		OtherPromise.__index = OtherPromise
+
+		expect(Promise.isPromise(setmetatable({}, OtherPromise))).toEqual(true)
+	end)
 end)
 
 describe("Promise.new", function()