diff --git a/spec/cli/feat_spec.lua b/spec/cli/feat_spec.lua
index 286033387..7741cc8f3 100644
--- a/spec/cli/feat_spec.lua
+++ b/spec/cli/feat_spec.lua
@@ -43,8 +43,8 @@ local test_cases = {
                status = 1,
                match = {
                   "2 errors:",
-                  ":9:22: wrong number of arguments (given 3, expects 2)",
-                  ":19:22: wrong number of arguments (given 3, expects at least 1 and at most 2)",
+                  ":9:22: wrong number of arguments (given 3, expects at most 2)",
+                  ":19:22: wrong number of arguments (given 3, expects at most 2)",
                }
             }
          }
diff --git a/spec/pragma/arity_spec.lua b/spec/pragma/arity_spec.lua
new file mode 100644
index 000000000..02cf8ccc7
--- /dev/null
+++ b/spec/pragma/arity_spec.lua
@@ -0,0 +1,237 @@
+local util = require("spec.util")
+
+describe("pragma arity", function()
+   describe("on", function()
+      it("rejects function calls with missing arguments", util.check_type_error([[
+         --#pragma arity on
+
+         local function f(x: integer, y: integer)
+            print(x + y)
+         end
+
+         print(f(10))
+      ]], {
+         { msg = "wrong number of arguments (given 1, expects 2)" }
+      }))
+
+      it("accepts optional arguments", util.check([[
+         --#pragma arity on
+
+         local function f(x: integer, y?: integer)
+            print(x + (y or 20))
+         end
+
+         print(f(10))
+      ]]))
+   end)
+
+   describe("off", function()
+      it("accepts function calls with missing arguments", util.check([[
+         --#pragma arity off
+
+         local function f(x: integer, y: integer)
+            print(x + (y or 20))
+         end
+
+         print(f(10))
+      ]]))
+
+      it("ignores optional argument annotations", util.check([[
+         --#pragma arity off
+
+         local function f(x: integer, y?: integer)
+            print(x + y)
+         end
+
+         print(f(10))
+      ]]))
+   end)
+
+   describe("no propagation from required module upwards:", function()
+      it("on then off, with error in 'on'", function()
+         util.mock_io(finally, {
+            ["r.tl"] = [[
+               --#pragma arity off
+               local function f(x: integer, y: integer, z: integer)
+                  print(x + (y or 20))
+               end
+               print(f(10))
+            ]]
+         })
+         util.check_type_error([[
+            --#pragma arity on
+
+            local function f(x: integer, y: integer)
+               print(x + y)
+            end
+
+            print(f(10))
+
+            local r = require("r")
+
+            local function g(x: integer, y: integer, z: integer, w: integer)
+               print(x + y)
+            end
+
+            print(g(10, 20))
+         ]], {
+            { filename = "foo.tl", y = 7, msg = "wrong number of arguments (given 1, expects 2)" },
+            { filename = "foo.tl", y = 15, msg = "wrong number of arguments (given 2, expects 4)" },
+         })()
+      end)
+
+      it("on then on, with errors in both", function()
+         util.mock_io(finally, {
+            ["r.tl"] = [[
+               --#pragma arity on
+               local function f(x: integer, y: integer, z: integer)
+                  print(x + (y or 20))
+               end
+               print(f(10))
+            ]]
+         })
+         util.check_type_error([[
+            --#pragma arity on
+
+            local function f(x: integer, y: integer)
+               print(x + y)
+            end
+
+            print(f(10))
+
+            local r = require("r")
+
+            local function g(x: integer, y: integer, z: integer, w: integer)
+               print(x + y)
+            end
+
+            print(g(10, 20))
+         ]], {
+            { filename = "r.tl", y = 5, msg = "wrong number of arguments (given 1, expects 3)" },
+            { filename = "foo.tl", y = 7, msg = "wrong number of arguments (given 1, expects 2)" },
+            { filename = "foo.tl", y = 15, msg = "wrong number of arguments (given 2, expects 4)" },
+         })()
+      end)
+
+      it("off then on, with error in 'on'", function()
+         util.mock_io(finally, {
+            ["r.tl"] = [[
+               --#pragma arity on
+
+               local function f(x: integer, y: integer)
+                  print(x + y)
+               end
+
+               print(f(10))
+            ]]
+         })
+         util.check_type_error([[
+            --#pragma arity off
+
+            local r = require("r")
+
+            local function f(x: integer, y: integer)
+               print(x + y)
+            end
+
+            print(f(10))
+         ]], {
+            { y = 7, filename = "r.tl", msg = "wrong number of arguments (given 1, expects 2)" }
+         })()
+      end)
+   end)
+
+   describe("does propagate downwards into required module:", function()
+      it("can trigger errors in required modules", function()
+         util.mock_io(finally, {
+            ["r.tl"] = [[
+               local function f(x: integer, y: integer, z: integer)
+                  print(x + (y or 20))
+               end
+               print(f(10))
+
+               return {
+                  f = f
+               }
+            ]]
+         })
+         util.check_type_error([[
+            --#pragma arity on
+
+            local function f(x: integer, y: integer)
+               print(x + y)
+            end
+
+            print(f(10))
+
+            local r = require("r")
+
+            local function g(x: integer, y: integer, z: integer, w: integer)
+               print(x + y)
+            end
+
+            print(g(10, 20))
+
+            r.f(10)
+         ]], {
+            { filename = "r.tl", y = 4, msg = "wrong number of arguments (given 1, expects 3)" },
+            { filename = "foo.tl", y = 7, msg = "wrong number of arguments (given 1, expects 2)" },
+            { filename = "foo.tl", y = 15, msg = "wrong number of arguments (given 2, expects 4)" },
+            { filename = "foo.tl", y = 17, msg = "wrong number of arguments (given 1, expects 3)" },
+         })()
+      end)
+
+      it("can be used to load modules with different settings", function()
+         util.mock_io(finally, {
+            ["r.tl"] = [[
+               local function f(x: integer, y: integer, z: integer)
+                  print(x + (y or 20))
+               end
+               print(f(10))
+
+               return {
+                  f = f
+               }
+            ]]
+         })
+         util.check_type_error([[
+            --#pragma arity on
+
+            local function f(x: integer, y: integer)
+               print(x + y)
+            end
+
+            print(f(10))
+
+            --#pragma arity off
+            local r = require("r")
+            --#pragma arity on
+
+            local function g(x: integer, y: integer, z: integer, w: integer)
+               print(x + y)
+            end
+
+            print(g(10, 20))
+
+            r.f(10) -- no error here!
+         ]], {
+            { filename = "foo.tl", y = 7, msg = "wrong number of arguments (given 1, expects 2)" },
+            { filename = "foo.tl", y = 17, msg = "wrong number of arguments (given 2, expects 4)" },
+         })()
+      end)
+   end)
+
+   describe("invalid", function()
+      it("rejects invalid value", util.check_type_error([[
+         --#pragma arity invalid_value
+
+         local function f(x: integer, y?: integer)
+            print(x + y)
+         end
+
+         print(f(10))
+      ]], {
+         { y = 1, msg = "invalid value for pragma 'arity': invalid_value" }
+      }))
+   end)
+end)
diff --git a/spec/pragma/invalid_spec.lua b/spec/pragma/invalid_spec.lua
new file mode 100644
index 000000000..105770481
--- /dev/null
+++ b/spec/pragma/invalid_spec.lua
@@ -0,0 +1,29 @@
+local util = require("spec.util")
+
+describe("invalid pragma", function()
+   it("rejects invalid pragma", util.check_syntax_error([[
+      --#invalid_pragma on
+   ]], {
+      { y = 1, msg = "invalid token '--#invalid_pragma'" }
+   }))
+
+   it("pragmas currently do not accept punctuation", util.check_syntax_error([[
+      --#pragma something(other)
+   ]], {
+      { y = 1, msg = "invalid token '('" },
+      { y = 1, msg = "invalid token ')'" },
+   }))
+
+   it("pragma arguments need to be in a single line", util.check_syntax_error([[
+      --#pragma arity
+        on
+
+      local function f(x: integer, y: integer)
+         print(x + y)
+      end
+
+      print(f(10))
+   ]], {
+      { msg = "expected pragma value" }
+   }))
+end)
diff --git a/tl.lua b/tl.lua
index d8e10b3e5..fafb1a100 100644
--- a/tl.lua
+++ b/tl.lua
@@ -804,6 +804,8 @@ end
 
 
 
+
+
 
 
 
@@ -838,6 +840,9 @@ do
 
 
 
+
+
+
 
 
 
@@ -874,6 +879,9 @@ do
       ["number hexfloat"] = "number",
       ["number power"] = "number",
       ["number powersign"] = "$ERR invalid_number$",
+      ["pragma"] = "pragma",
+      ["pragma any"] = nil,
+      ["pragma word"] = "pragma_identifier",
    }
 
    local keywords = {
@@ -1267,11 +1275,39 @@ do
          elseif state == "got --" then
             if c == "[" then
                state = "got --["
+            elseif c == "#" then
+               state = "pragma"
             else
                fwd = false
                state = "comment short"
                drop_token()
             end
+         elseif state == "pragma" then
+            if not lex_word[c] then
+               end_token_prev("pragma")
+               if tokens[nt].tk ~= "--#pragma" then
+                  add_syntax_error()
+               end
+               fwd = false
+               state = "pragma any"
+            end
+         elseif state == "pragma any" then
+            if c == "\n" then
+               state = "any"
+            elseif lex_word[c] then
+               state = "pragma word"
+               begin_token()
+            elseif not lex_space[c] then
+               begin_token()
+               end_token_here("$ERR invalid$")
+               add_syntax_error()
+            end
+         elseif state == "pragma word" then
+            if not lex_word[c] then
+               end_token_prev("pragma_identifier")
+               fwd = false
+               state = (c == "\n") and "any" or "pragma any"
+            end
          elseif state == "got 0" then
             if c == "x" or c == "X" then
                state = "number hex"
@@ -1920,6 +1956,7 @@ end
 
 
 
+
 
 
 local TruthyFact = {}
@@ -2100,6 +2137,10 @@ local Node = {ExpectedContext = {}, }
 
 
 
+
+
+
+
 
 
 
@@ -4220,7 +4261,27 @@ do
       return parse_function(ps, i, "record")
    end
 
+   local function parse_pragma(ps, i)
+      i = i + 1
+      local pragma = new_node(ps, i, "pragma")
+
+      if ps.tokens[i].kind ~= "pragma_identifier" then
+         return fail(ps, i, "expected pragma name")
+      end
+      pragma.pkey = ps.tokens[i].tk
+      i = i + 1
+
+      if ps.tokens[i].kind ~= "pragma_identifier" then
+         return fail(ps, i, "expected pragma value")
+      end
+      pragma.pvalue = ps.tokens[i].tk
+      i = i + 1
+
+      return i, pragma
+   end
+
    local parse_statement_fns = {
+      ["--#pragma"] = parse_pragma,
       ["::"] = parse_label,
       ["do"] = parse_do,
       ["if"] = parse_if,
@@ -4589,6 +4650,7 @@ local no_recurse_node = {
    ["break"] = true,
    ["label"] = true,
    ["number"] = true,
+   ["pragma"] = true,
    ["string"] = true,
    ["boolean"] = true,
    ["integer"] = true,
@@ -5547,6 +5609,8 @@ function tl.pretty_print_ast(ast, gen_target, mode)
             return out
          end,
       },
+      ["pragma"] = {},
+
 
       ["variable"] = emit_exactly_visitor_cbs,
       ["identifier"] = emit_exactly_visitor_cbs,
@@ -6771,21 +6835,33 @@ function tl.search_module(module_name, search_dtl)
    return nil, nil, tried
 end
 
-local function require_module(w, module_name, feat_lax, env)
+local function require_module(w, module_name, opts, env)
    local mod = env.modules[module_name]
    if mod then
       return mod, env.module_filenames[module_name]
    end
 
    local found, fd = tl.search_module(module_name, true)
-   if found and (feat_lax or found:match("tl$")) then
+   if found and (opts.feat_lax == "on" or found:match("tl$")) then
 
       env.module_filenames[module_name] = found
       env.modules[module_name] = a_type(w, "typedecl", { def = a_type(w, "circular_require", {}) })
 
+      local save_defaults = env.defaults
+      local defaults = {
+         feat_lax = opts.feat_lax or save_defaults.feat_lax,
+         feat_arity = opts.feat_arity or save_defaults.feat_arity,
+         gen_compat = opts.gen_compat or save_defaults.gen_compat,
+         gen_target = opts.gen_target or save_defaults.gen_target,
+         run_internal_compiler_checks = opts.run_internal_compiler_checks or save_defaults.run_internal_compiler_checks,
+      }
+      env.defaults = defaults
+
       local found_result, err = tl.process(found, env, fd)
       assert(found_result, err)
 
+      env.defaults = save_defaults
+
       env.modules[module_name] = found_result.type
 
       return found_result.type, found
@@ -6991,7 +7067,11 @@ tl.new_env = function(opts)
 
    if opts.predefined_modules then
       for _, name in ipairs(opts.predefined_modules) do
-         local module_type = require_module(w, name, env.defaults.feat_lax == "on", env)
+         local tc_opts = {
+            feat_lax = env.defaults.feat_lax,
+            feat_arity = env.defaults.feat_arity,
+         }
+         local module_type = require_module(w, name, tc_opts, env)
 
          if module_type.typename == "invalid" then
             return nil, string.format("Error: could not predefine module '%s'", name)
@@ -7264,9 +7344,15 @@ do
 
    local function show_arity(f)
       local nfargs = #f.args.tuple
-      return f.min_arity < nfargs and
-      "at least " .. f.min_arity .. (f.args.is_va and "" or " and at most " .. nfargs) or
-      tostring(nfargs or 0)
+      if f.min_arity < nfargs then
+         if f.min_arity > 0 then
+            return "at least " .. f.min_arity .. (f.args.is_va and "" or " and at most " .. nfargs)
+         else
+            return (f.args.is_va and "any number" or "at most " .. nfargs)
+         end
+      else
+         return tostring(nfargs or 0)
+      end
    end
 
    local function drop_constant_value(t)
@@ -8918,7 +9004,11 @@ a.types[i], b.types[i]), }
 
       if self.feat_lax and is_unknown(func) then
          local unk = func
-         func = a_function(func, { min_arity = 0, args = a_vararg(func, { unk }), rets = a_vararg(func, { unk }) })
+         func = a_function(func, {
+            min_arity = 0,
+            args = a_vararg(func, { unk }),
+            rets = a_vararg(func, { unk }),
+         })
       end
 
       func = self:to_structural(func)
@@ -9561,9 +9651,9 @@ a.types[i], b.types[i]), }
       end
    end
 
-   function TypeChecker:add_function_definition_for_recursion(node, fnargs)
+   function TypeChecker:add_function_definition_for_recursion(node, fnargs, feat_arity)
       self:add_var(nil, node.name.tk, a_function(node, {
-         min_arity = node.min_arity,
+         min_arity = feat_arity and node.min_arity or 0,
          typeargs = node.typeargs,
          args = fnargs,
          rets = self.get_rets(node.rets),
@@ -10281,7 +10371,7 @@ a.types[i], b.types[i]), }
          local arg2 = node.e2[2]
          local msgh = table.remove(b.tuple, 1)
          local msgh_type = a_function(arg2, {
-            min_arity = 1,
+            min_arity = self.feat_arity and 1 or 0,
             args = a_type(arg2, "tuple", { tuple = { a_type(arg2, "any", {}) } }),
             rets = a_type(arg2, "tuple", { tuple = {} }),
          })
@@ -10369,7 +10459,11 @@ a.types[i], b.types[i]), }
          end
 
          local module_name = assert(node.e2[1].conststr)
-         local t, module_filename = require_module(node, module_name, self.feat_lax, self.env)
+         local tc_opts = {
+            feat_lax = self.feat_lax and "on" or "off",
+            feat_arity = self.feat_arity and "on" or "off",
+         }
+         local t, module_filename = require_module(node, module_name, tc_opts, self.env)
 
          if t.typename == "invalid" then
             if not module_filename then
@@ -11528,7 +11622,7 @@ self:expand_type(node, values, elements) })
             assert(args.typename == "tuple")
 
             self:add_internal_function_variables(node, args)
-            self:add_function_definition_for_recursion(node, args)
+            self:add_function_definition_for_recursion(node, args, self.feat_arity)
          end,
          after = function(self, node, children)
             local args = children[2]
@@ -11539,7 +11633,7 @@ self:expand_type(node, values, elements) })
             self:end_function_scope(node)
 
             local t = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11568,7 +11662,7 @@ self:expand_type(node, values, elements) })
             self:check_macroexp_arg_use(node.macrodef)
 
             local t = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.macrodef.min_arity,
+               min_arity = self.feat_arity and node.macrodef.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11601,7 +11695,7 @@ self:expand_type(node, values, elements) })
             assert(args.typename == "tuple")
 
             self:add_internal_function_variables(node, args)
-            self:add_function_definition_for_recursion(node, args)
+            self:add_function_definition_for_recursion(node, args, self.feat_arity)
          end,
          after = function(self, node, children)
             local args = children[2]
@@ -11615,7 +11709,7 @@ self:expand_type(node, values, elements) })
             end
 
             self:add_global(node, node.name.tk, self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11686,7 +11780,7 @@ self:expand_type(node, values, elements) })
             end
 
             local fn_type = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                is_method = node.is_method,
                typeargs = node.typeargs,
                args = args,
@@ -11760,7 +11854,7 @@ self:expand_type(node, values, elements) })
 
             self:end_function_scope(node)
             return self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11786,7 +11880,7 @@ self:expand_type(node, values, elements) })
 
             self:end_function_scope(node)
             return self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = rets,
@@ -12274,6 +12368,22 @@ self:expand_type(node, values, elements) })
             return node.newtype
          end,
       },
+      ["pragma"] = {
+         after = function(self, node, _children)
+            if node.pkey == "arity" then
+               if node.pvalue == "on" then
+                  self.feat_arity = true
+               elseif node.pvalue == "off" then
+                  self.feat_arity = false
+               else
+                  return self.errs:invalid_at(node, "invalid value for pragma 'arity': " .. node.pvalue)
+               end
+            else
+               return self.errs:invalid_at(node, "invalid pragma: " .. node.pkey)
+            end
+            return NONE
+         end,
+      },
       ["error_node"] = {
          after = function(_self, node, _children)
             return a_type(node, "invalid", {})
@@ -12461,6 +12571,15 @@ self:expand_type(node, values, elements) })
    local visit_type
    visit_type = {
       cbs = {
+         ["function"] = {
+            before = visit_type_with_typeargs.before,
+            after = function(self, typ, children)
+               if self.feat_arity == false then
+                  typ.min_arity = 0
+               end
+               return visit_type_with_typeargs.after(self, typ, children)
+            end,
+         },
          ["record"] = {
             before = function(self, typ)
                self:begin_scope()
@@ -12629,7 +12748,6 @@ self:expand_type(node, values, elements) })
 
    visit_type.cbs["interface"] = visit_type.cbs["record"]
 
-   visit_type.cbs["function"] = visit_type_with_typeargs
    visit_type.cbs["typedecl"] = visit_type_with_typeargs
    visit_type.cbs["typealias"] = visit_type_with_typeargs
 
diff --git a/tl.tl b/tl.tl
index cec12f333..5f150c924 100644
--- a/tl.tl
+++ b/tl.tl
@@ -793,6 +793,8 @@ local enum TokenKind
    "identifier"
    "number"
    "integer"
+   "pragma"
+   "pragma_identifier"
    "$ERR unfinished_comment$"
    "$ERR invalid_string$"
    "$ERR invalid_number$"
@@ -840,6 +842,9 @@ do
       "number hexfloat"
       "number power"
       "number powersign"
+      "pragma"
+      "pragma word"
+      "pragma any"
    end
 
    local last_token_kind <total>: {LexState:TokenKind} = {
@@ -874,6 +879,9 @@ do
       ["number hexfloat"] = "number",
       ["number power"] = "number",
       ["number powersign"] = "$ERR invalid_number$",
+      ["pragma"] = "pragma",
+      ["pragma any"] = nil, -- never in a token
+      ["pragma word"] = "pragma_identifier", -- never in a token
    }
 
    local keywords: {string:boolean} = {
@@ -1267,11 +1275,39 @@ do
          elseif state == "got --" then
             if c == "[" then
                state = "got --["
+            elseif c == "#" then
+               state = "pragma"
             else
                fwd = false
                state = "comment short"
                drop_token()
             end
+         elseif state == "pragma" then
+            if not lex_word[c] then
+               end_token_prev("pragma")
+               if tokens[nt].tk ~= "--#pragma" then
+                  add_syntax_error()
+               end
+               fwd = false
+               state = "pragma any"
+            end
+         elseif state == "pragma any" then
+            if c == "\n" then
+               state = "any"
+            elseif lex_word[c] then
+               state = "pragma word"
+               begin_token()
+            elseif not lex_space[c] then
+               begin_token()
+               end_token_here("$ERR invalid$")
+               add_syntax_error()
+            end
+         elseif state == "pragma word" then
+            if not lex_word[c] then
+               end_token_prev("pragma_identifier")
+               fwd = false
+               state = (c == "\n") and "any" or "pragma any"
+            end
          elseif state == "got 0" then
             if c == "x" or c == "X" then
                state = "number hex"
@@ -1902,6 +1938,7 @@ local enum NodeKind
    "macroexp"
    "local_macroexp"
    "interface"
+   "pragma"
    "error_node"
 end
 
@@ -2100,6 +2137,10 @@ local record Node
    itemtype: Type
    decltuple: TupleType
 
+   -- pragma
+   pkey: string
+   pvalue: string
+
    opt: boolean
 
    debug_type: Type
@@ -4220,7 +4261,27 @@ local function parse_record_function(ps: ParseState, i: integer): integer, Node
    return parse_function(ps, i, "record")
 end
 
+local function parse_pragma(ps: ParseState, i: integer): integer, Node
+   i = i + 1 -- skip "--#pragma"
+   local pragma = new_node(ps, i, "pragma")
+
+   if ps.tokens[i].kind ~= "pragma_identifier" then
+      return fail(ps, i, "expected pragma name")
+   end
+   pragma.pkey = ps.tokens[i].tk
+   i = i + 1
+
+   if ps.tokens[i].kind ~= "pragma_identifier" then
+      return fail(ps, i, "expected pragma value")
+   end
+   pragma.pvalue = ps.tokens[i].tk
+   i = i + 1
+
+   return i, pragma
+end
+
 local parse_statement_fns: {string : function(ParseState, integer):(integer, Node)} = {
+   ["--#pragma"] = parse_pragma,
    ["::"] = parse_label,
    ["do"] = parse_do,
    ["if"] = parse_if,
@@ -4589,6 +4650,7 @@ local no_recurse_node: {NodeKind : boolean} = {
    ["break"] = true,
    ["label"] = true,
    ["number"] = true,
+   ["pragma"] = true,
    ["string"] = true,
    ["boolean"] = true,
    ["integer"] = true,
@@ -5547,6 +5609,8 @@ function tl.pretty_print_ast(ast: Node, gen_target: GenTarget, mode?: boolean |
             return out
          end,
       },
+      ["pragma"] = {
+      },
 
       ["variable"] = emit_exactly_visitor_cbs,
       ["identifier"] = emit_exactly_visitor_cbs,
@@ -6771,21 +6835,33 @@ function tl.search_module(module_name: string, search_dtl: boolean): string, FIL
    return nil, nil, tried
 end
 
-local function require_module(w: Where, module_name: string, feat_lax: boolean, env: Env): Type, string
+local function require_module(w: Where, module_name: string, opts: TypeCheckOptions, env: Env): Type, string
    local mod = env.modules[module_name]
    if mod then
       return mod, env.module_filenames[module_name]
    end
 
    local found, fd = tl.search_module(module_name, true)
-   if found and (feat_lax or found:match("tl$") as boolean) then
+   if found and (opts.feat_lax == "on" or found:match("tl$") as boolean) then
 
       env.module_filenames[module_name] = found
       env.modules[module_name] = a_typedecl(w, a_type(w, "circular_require", {}))
 
+      local save_defaults = env.defaults
+      local defaults <total>: TypeCheckOptions = {
+         feat_lax = opts.feat_lax or save_defaults.feat_lax,
+         feat_arity = opts.feat_arity or save_defaults.feat_arity,
+         gen_compat = opts.gen_compat or save_defaults.gen_compat,
+         gen_target = opts.gen_target or save_defaults.gen_target,
+         run_internal_compiler_checks = opts.run_internal_compiler_checks or save_defaults.run_internal_compiler_checks,
+      }
+      env.defaults = defaults
+
       local found_result, err: Result, string = tl.process(found, env, fd)
       assert(found_result, err)
 
+      env.defaults = save_defaults
+
       env.modules[module_name] = found_result.type
 
       return found_result.type, found
@@ -6991,7 +7067,11 @@ tl.new_env = function(opts?: EnvOptions): Env, string
 
    if opts.predefined_modules then
       for _, name in ipairs(opts.predefined_modules) do
-         local module_type = require_module(w, name, env.defaults.feat_lax == "on", env)
+         local tc_opts = {
+            feat_lax = env.defaults.feat_lax,
+            feat_arity = env.defaults.feat_arity,
+         }
+         local module_type = require_module(w, name, tc_opts, env)
 
          if module_type is InvalidType then
             return nil, string.format("Error: could not predefine module '%s'", name)
@@ -7264,9 +7344,15 @@ do
 
    local function show_arity(f: FunctionType): string
       local nfargs = #f.args.tuple
-      return f.min_arity < nfargs
-             and "at least " .. f.min_arity .. (f.args.is_va and "" or " and at most " .. nfargs)
-             or  tostring(nfargs or 0)
+      if f.min_arity < nfargs then
+         if f.min_arity > 0 then
+            return "at least " .. f.min_arity .. (f.args.is_va and "" or " and at most " .. nfargs)
+         else
+            return (f.args.is_va and "any number" or "at most " .. nfargs)
+         end
+      else
+         return tostring(nfargs or 0)
+      end
    end
 
    local function drop_constant_value(t: Type): Type
@@ -8918,7 +9004,11 @@ do
       -- resolve unknown in lax mode, produce a general unknown function
       if self.feat_lax and is_unknown(func) then
          local unk = func
-         func = a_function(func, { min_arity = 0, args = a_vararg(func, { unk }), rets = a_vararg(func, { unk }) })
+         func = a_function(func, {
+            min_arity = 0,
+            args = a_vararg(func, { unk }),
+            rets = a_vararg(func, { unk })
+         })
       end
       -- unwrap if tuple, resolve if nominal
       func = self:to_structural(func)
@@ -9561,9 +9651,9 @@ do
       end
    end
 
-   function TypeChecker:add_function_definition_for_recursion(node: Node, fnargs: TupleType)
+   function TypeChecker:add_function_definition_for_recursion(node: Node, fnargs: TupleType, feat_arity: boolean)
       self:add_var(nil, node.name.tk, a_function(node, {
-         min_arity = node.min_arity,
+         min_arity = feat_arity and node.min_arity or 0,
          typeargs = node.typeargs,
          args = fnargs,
          rets = self.get_rets(node.rets),
@@ -10281,7 +10371,7 @@ do
          local arg2 = node.e2[2]
          local msgh = table.remove(b.tuple, 1)
          local msgh_type = a_function(arg2, {
-            min_arity = 1,
+            min_arity = self.feat_arity and 1 or 0,
             args = a_tuple(arg2, { a_type(arg2, "any", {}) }),
             rets = a_tuple(arg2, {})
          })
@@ -10369,7 +10459,11 @@ do
          end
 
          local module_name = assert(node.e2[1].conststr)
-         local t, module_filename = require_module(node, module_name, self.feat_lax, self.env)
+         local tc_opts: TypeCheckOptions = {
+            feat_lax = self.feat_lax and "on" or "off",
+            feat_arity = self.feat_arity and "on" or "off",
+         }
+         local t, module_filename = require_module(node, module_name, tc_opts, self.env)
 
          if t.typename == "invalid" then
             if not module_filename then
@@ -11528,7 +11622,7 @@ do
             assert(args is TupleType)
 
             self:add_internal_function_variables(node, args)
-            self:add_function_definition_for_recursion(node, args)
+            self:add_function_definition_for_recursion(node, args, self.feat_arity)
          end,
          after = function(self: TypeChecker, node: Node, children: {Type}): Type
             local args = children[2]
@@ -11539,7 +11633,7 @@ do
             self:end_function_scope(node)
 
             local t = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11568,7 +11662,7 @@ do
             self:check_macroexp_arg_use(node.macrodef)
 
             local t = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.macrodef.min_arity,
+               min_arity = self.feat_arity and node.macrodef.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11601,7 +11695,7 @@ do
             assert(args is TupleType)
 
             self:add_internal_function_variables(node, args)
-            self:add_function_definition_for_recursion(node, args)
+            self:add_function_definition_for_recursion(node, args, self.feat_arity)
          end,
          after = function(self: TypeChecker, node: Node, children: {Type}): Type
             local args = children[2]
@@ -11615,7 +11709,7 @@ do
             end
 
             self:add_global(node, node.name.tk, self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11686,7 +11780,7 @@ do
             end
 
             local fn_type = self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                is_method = node.is_method,
                typeargs = node.typeargs,
                args = args,
@@ -11760,7 +11854,7 @@ do
 
             self:end_function_scope(node)
             return self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = self.get_rets(rets),
@@ -11786,7 +11880,7 @@ do
 
             self:end_function_scope(node)
             return self:ensure_fresh_typeargs(a_function(node, {
-               min_arity = node.min_arity,
+               min_arity = self.feat_arity and node.min_arity or 0,
                typeargs = node.typeargs,
                args = args,
                rets = rets,
@@ -12274,6 +12368,22 @@ do
             return node.newtype
          end,
       },
+      ["pragma"] = {
+         after = function(self: TypeChecker, node: Node, _children: {Type}): Type
+            if node.pkey == "arity" then
+               if node.pvalue == "on" then
+                  self.feat_arity = true
+               elseif node.pvalue == "off" then
+                  self.feat_arity = false
+               else
+                  return self.errs:invalid_at(node, "invalid value for pragma 'arity': " .. node.pvalue)
+               end
+            else
+               return self.errs:invalid_at(node, "invalid pragma: " .. node.pkey)
+            end
+            return NONE
+         end,
+      },
       ["error_node"] = {
          after = function(_self: TypeChecker, node: Node, _children: {Type}): Type
             return an_invalid(node)
@@ -12461,6 +12571,15 @@ do
    local visit_type: Visitor<TypeChecker, TypeName,Type,Type>
    visit_type = {
       cbs = {
+         ["function"] = {
+            before = visit_type_with_typeargs.before,
+            after = function(self: TypeChecker, typ: FunctionType, children: {Type}): Type
+               if self.feat_arity == false then
+                  typ.min_arity = 0
+               end
+               return visit_type_with_typeargs.after(self, typ, children)
+            end
+         },
          ["record"] = {
             before = function(self: TypeChecker, typ: RecordType)
                self:begin_scope()
@@ -12629,7 +12748,6 @@ do
 
    visit_type.cbs["interface"] = visit_type.cbs["record"]
 
-   visit_type.cbs["function"] = visit_type_with_typeargs
    visit_type.cbs["typedecl"] = visit_type_with_typeargs
    visit_type.cbs["typealias"] = visit_type_with_typeargs