Fix #41

CHANGELOG.md

@@ -1,6 +1,11 @@
 # Changelog
 
-## [3.0.0]
+## [Next]
+### Fixed
+- Make `Promise.is` work with promises from old versions of the library
+
+## [3.0.0] - 2020-08-17
+### Changed
 - `Promise.delay` now uses `os.clock`
 - Made `Promise.delay` behavior more consistent when creating new timers in the callback of a timer.
 

lib/init.lua

@@ -667,7 +667,11 @@ function Promise.is(object)
 	elseif objectMetatable == nil then
 		-- No metatable, but we should still chain onto tables with andThen methods
 		return type(object.andThen) == "function"
-	elseif type(objectMetatable) == "table" and type(rawget(objectMetatable, "andThen")) == "function" then
+	elseif
+		type(objectMetatable) == "table"
+		and type(rawget(objectMetatable, "__index")) == "table"
+		and type(rawget(rawget(objectMetatable, "__index"), "andThen")) == "function"
+	then
 		-- Maybe this came from a different or older Promise library.
 		return true
 	end

lib/init.spec.lua

@@ -1498,4 +1498,36 @@ return function()
 			expect(promise._values[1]).to.equal("foo")
 		end)
 	end)
+
+	describe("Promise.is", function()
+		it("should work with current version", function()
+			local promise = Promise.resolve(1)
+
+			expect(Promise.is(promise)).to.equal(true)
+		end)
+
+		it("should work with any object with an andThen", function()
+			local obj = {
+				andThen = function()
+					return 1
+				end
+			}
+
+			expect(Promise.is(obj)).to.equal(true)
+		end)
+
+		it("should work with older promises", function()
+			local OldPromise = {}
+			OldPromise.prototype = {}
+			OldPromise.__index = OldPromise.prototype
+
+			function OldPromise.prototype:andThen()
+
+			end
+
+			local oldPromise = setmetatable({}, OldPromise)
+
+			expect(Promise.is(oldPromise)).to.equal(true)
+		end)
+	end)
 end