Warn on nullary remote calls, related to #9510

lib/elixir/pages/references/syntax-reference.md

@@ -433,7 +433,7 @@ end
 
 ### Optional parentheses
 
-Elixir provides optional parentheses:
+Elixir provides optional parentheses on local and remote calls with one or more arguments:
 
 ```elixir
 quote do

lib/elixir/src/elixir_expand.erl

@@ -396,12 +396,16 @@ expand({Atom, Meta, Args}, S, E) when is_atom(Atom), is_list(Meta), is_list(Args
 
 %% Remote calls
 
-expand({{'.', DotMeta, [Left, Right]}, Meta, Args}, S, E)
+expand({{'.', DotMeta, [Left, Right]}, Meta, Args} = Expr, S, E)
     when (is_tuple(Left) orelse is_atom(Left)), is_atom(Right), is_list(Meta), is_list(Args) ->
   {ELeft, SL, EL} = expand(Left, elixir_env:prepare_write(S), E),
+  NoParens = lists:keyfind(no_parens, 1, Meta),
+
+  (is_atom(ELeft) and (Args =:= []) and (NoParens =:= {no_parens, true})) andalso
+    elixir_errors:file_warn(Meta, E, ?MODULE, {remote_nullary_no_parens, Expr}),
 
   elixir_dispatch:dispatch_require(Meta, ELeft, Right, Args, S, EL, fun(AR, AF, AA) ->
-    expand_remote(AR, DotMeta, AF, Meta, AA, S, SL, EL)
+    expand_remote(AR, DotMeta, AF, Meta, NoParens, AA, S, SL, EL)
   end);
 
 %% Anonymous calls
@@ -844,17 +848,17 @@ expand_local(Meta, Name, Args, _S, #{function := nil} = E) ->
 
 %% Remote
 
-expand_remote(Receiver, DotMeta, Right, Meta, Args, S, SL, #{context := Context} = E) when is_atom(Receiver) or is_tuple(Receiver) ->
+expand_remote(Receiver, DotMeta, Right, Meta, NoParens, Args, S, SL, #{context := Context} = E) when is_atom(Receiver) or is_tuple(Receiver) ->
   assert_no_clauses(Right, Meta, Args, E),
 
-  case {Context, lists:keyfind(no_parens, 1, Meta)} of
-    {guard, NoParens} when is_tuple(Receiver) ->
+  if
+    Context =:= guard, is_tuple(Receiver) ->
       (NoParens /= {no_parens, true}) andalso
         function_error(Meta, E, ?MODULE, {parens_map_lookup, Receiver, Right, guard_context(S)}),
 
       {{{'.', DotMeta, [Receiver, Right]}, Meta, []}, SL, E};
 
-    _ ->
+    true ->
       AttachedDotMeta = attach_context_module(Receiver, DotMeta, E),
 
       is_atom(Receiver) andalso
@@ -871,7 +875,7 @@ expand_remote(Receiver, DotMeta, Right, Meta, Args, S, SL, #{context := Context}
           file_error(Meta, E, elixir_rewrite, Error)
       end
   end;
-expand_remote(Receiver, DotMeta, Right, Meta, Args, _, _, E) ->
+expand_remote(Receiver, DotMeta, Right, Meta, _NoParens, Args, _, _, E) ->
   Call = {{'.', DotMeta, [Receiver, Right]}, Meta, Args},
   file_error(Meta, E, ?MODULE, {invalid_call, Call}).
 
@@ -1159,6 +1163,9 @@ assert_no_underscore_clause_in_cond(_Other, _E) ->
 guard_context(#elixir_ex{prematch={_, _, {bitsize, _}}}) -> "bitstring size specifier";
 guard_context(_) -> "guards".
 
+format_error({remote_nullary_no_parens, Expr}) ->
+  String = 'Elixir.String':replace_suffix('Elixir.Macro':to_string(Expr), <<"()">>, <<>>),
+  io_lib:format("parentheses are required for function calls with no arguments, got: ~ts", [String]);
 format_error({useless_literal, Term}) ->
   io_lib:format("code block contains unused literal ~ts "
                 "(remove the literal or assign it to _ to avoid warnings)",

lib/elixir/test/elixir/kernel/cli_test.exs

@@ -65,13 +65,15 @@ defmodule Kernel.CLITest do
     {output, 0} = System.cmd(elixir_executable(), ["--eval", "IO.puts :hello_world123"])
     assert output =~ "hello_world123"
 
-    {output, 0} = System.cmd(iex_executable(), ["--eval", "IO.puts :hello_world123; System.halt"])
+    {output, 0} =
+      System.cmd(iex_executable(), ["--eval", "IO.puts :hello_world123; System.halt()"])
+
     assert output =~ "hello_world123"
 
     {output, 0} = System.cmd(elixir_executable(), ["-e", "IO.puts :hello_world123"])
     assert output =~ "hello_world123"
 
-    {output, 0} = System.cmd(iex_executable(), ["-e", "IO.puts :hello_world123; System.halt"])
+    {output, 0} = System.cmd(iex_executable(), ["-e", "IO.puts :hello_world123; System.halt()"])
     assert output =~ "hello_world123"
   end
 

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -1175,6 +1175,18 @@ defmodule Kernel.WarningTest do
     purge(Sample)
   end
 
+  test "parens on nullary remote call" do
+    assert_warn_eval(
+      [
+        "nofile:1:8",
+        "parentheses are required for function calls with no arguments, got: System.version"
+      ],
+      "System.version"
+    )
+  after
+    purge(Sample)
+  end
+
   test "parens with module attribute" do
     assert_warn_eval(
       [

lib/elixir/test/elixir/module/types/expr_test.exs

@@ -52,14 +52,6 @@ defmodule Module.Types.ExprTest do
     assert quoted_expr({:a, 123}) == {:ok, {:tuple, 2, [{:atom, :a}, :integer]}}
   end
 
-  # Use module attribute to avoid formatter adding parentheses
-  @mix_module Mix
-
-  test "module call" do
-    assert quoted_expr(@mix_module.shell) == {:ok, :dynamic}
-    assert quoted_expr(@mix_module.shell.info) == {:ok, {:var, 0}}
-  end
-
   describe "binary" do
     test "literal" do
       assert quoted_expr(<<"foo"::binary>>) == {:ok, :binary}