string.gsub: accept functions with no returns

Note that we're doing a little hack here, but notating the
return type as vararg, as a stand-in for something like a `?`
in the return type. I guess we need `min_arity` calculations
in the return types as well.

Also fix the declaration of `gsub` because Lua does not accept
functions that return booleans, but it does accept numbers.

spec/stdlib/string_spec.lua

@@ -22,4 +22,75 @@ describe("string", function()
       ]]))
    end)
 
+   describe("gsub", function()
+      it("accepts a string, returns a string", util.check([[
+         local s = "hello"
+         local hi: string = s:gsub("ello", "i")
+      ]]))
+
+      it("accepts a string, returns a string and integer", util.check([[
+         local s = "hello world"
+         local wordword, count: string, integer = s:gsub("%w+", "word")
+      ]]))
+
+      it("accepts a string and integer, returns a string and integer", util.check([[
+         local s = "hello world"
+         local helloword, count: string, integer = s:gsub("%w+", "word", 6)
+      ]]))
+
+      it("accepts a map, returns a string and integer", util.check([[
+         local s = "hello world"
+         local map = {
+            ["hello"] = "hola",
+            ["world"] = "mundo",
+         }
+         local holamundo, count: string, integer = s:gsub("%w+", map)
+      ]]))
+
+      it("accepts a map and integer, returns a string and integer", util.check([[
+         local s = "hello world"
+         local map = {
+            ["hello"] = "hola",
+            ["world"] = "mundo",
+         }
+         local hellomundo, count: string, integer = s:gsub("%w+", map, 6)
+      ]]))
+
+      it("accepts a function to strings, returns a string", util.check([[
+         local s = "hello world"
+         local function f(x: string): string
+            return x:upper()
+         end
+         local ret: string = s:gsub("%w+", f)
+      ]]))
+
+      it("accepts a function to integers, returns a string", util.check([[
+         local s = "hello world"
+         local function f(x: string): integer
+            return #x
+         end
+         local ret: string = s:gsub("%w+", f)
+      ]]))
+
+      it("accepts a function to numbers, returns a string", util.check([[
+         local s = "hello world"
+         local function f(x: string): number
+            return #x * 1.5
+         end
+         local ret: string = s:gsub("%w+", f)
+      ]]))
+
+      it("accepts a function that returns nothing", util.check([[
+         local function parse_integers(s: string, i0: integer) : {integer}
+             local t, p = {}, i0 or 1
+             local function f(x: string)
+                 t[p] = math.tointeger(x)
+                 p = p + 1
+             end
+             s:gsub("[-%d]+", f)
+             return t
+         end
+      ]]))
+   end)
+
 end)

tl.lua

@@ -317,7 +317,7 @@ do
 
       gsub: function(string, string, string, ? integer): string, integer
       gsub: function(string, string, {string:string}, ? integer): string, integer
-      gsub: function(string, string, function(string...): (string | integer | boolean), ? integer): string, integer
+      gsub: function(string, string, function(string...): ((string | integer | number)...), ? integer): string, integer
 
       len: function(string): integer
       lower: function(string): string

tl.tl

@@ -317,7 +317,7 @@ do
 
       gsub: function(string, string, string, ? integer): string, integer
       gsub: function(string, string, {string:string}, ? integer): string, integer
-      gsub: function(string, string, function(string...): (string | integer | boolean), ? integer): string, integer
+      gsub: function(string, string, function(string...): ((string | integer | number)...), ? integer): string, integer
 
       len: function(string): integer
       lower: function(string): string