stdilb: Check for bad type constraints in function types

The parser recognizes the 'is_subtype(V, T)' syntax for constraints,
and of course the new 'V :: T' syntax, but other variants result in an
error message. Up to now, the parser and linter have let badly formed
constraints through, and relied upon Dialyzer to emit warnings.

is_subtype/2 cannot easily be taken out from the parser. Not only
would we need find a way to emit a (linter) warning, but there also
needs to be an option for suppressing the linter warning as
compilation with +warnings_as_errors has to work. (Notice that the
abstract format representation for 'V :: T' is the same as for
'is_subtype(V, T)'.)

This correction was triggered by an email from Robert, and Kostis
created pull request 1214 to provide a fix. However, Kostis' fix
disallowed is_subtype() altogether, which breaks backward
compatibility.

As of Erlang/OTP 19.0 (ticket OTP-11879), the 'is_subtype(V, T)' is no
longer documented.

2 files changed, 58 insertions, 8 deletions

diff --git a/lib/stdlib/test/erl_lint_SUITE.erl b/lib/stdlib/test/erl_lint_SUITE.erl
index 4ee3950882..1dc7c58337 100644
--- a/lib/stdlib/test/erl_lint_SUITE.erl
+++ b/lib/stdlib/test/erl_lint_SUITE.erl
@@ -64,7 +64,7 @@
          predef/1,
          maps/1,maps_type/1,maps_parallel_match/1,
          otp_11851/1,otp_11879/1,otp_13230/1,
-         record_errors/1]).
+         record_errors/1, otp_xxxxx/1]).
 
 suite() ->
     [{ct_hooks,[ts_install_cth]},
@@ -84,7 +84,7 @@ all() ->
      too_many_arguments, basic_errors, bin_syntax_errors, predef,
      maps, maps_type, maps_parallel_match,
      otp_11851, otp_11879, otp_13230,
-     record_errors].
+     record_errors, otp_xxxxx].
 
 groups() -> 
     [{unused_vars_warn, [],
@@ -3869,6 +3869,55 @@ record_errors(Config) when is_list(Config) ->
 		    {3,erl_lint,{redefine_field,r,a}}],[]}}],
     run(Config, Ts).
 
+otp_xxxxx(Config) ->
+    Ts = [{constraint1,
+           <<"-export([t/1]).
+              -spec t(X) -> X when is_subtype(integer()).
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           {errors,
+            [{2,erl_parse,"unsupported constraint " ++ ["is_subtype"]}],
+            []}},
+          {constraint2,
+           <<"-export([t/1]).
+              -spec t(X) -> X when bad_atom(X, integer()).
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           {errors,
+            [{2,erl_parse,"unsupported constraint " ++ ["bad_atom"]}],
+            []}},
+          {constraint3,
+           <<"-export([t/1]).
+              -spec t(X) -> X when is_subtype(bad_variable, integer()).
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           {errors,[{2,erl_parse,"bad type variable"}],[]}},
+          {constraint4,
+           <<"-export([t/1]).
+              -spec t(X) -> X when is_subtype(atom(), integer()).
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           {errors,[{2,erl_parse,"bad type variable"}],[]}},
+          {constraint5,
+           <<"-export([t/1]).
+              -spec t(X) -> X when is_subtype(X, integer()).
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           []},
+          {constraint6,
+           <<"-export([t/1]).
+              -spec t(X) -> X when X :: integer().
+              t(a) -> foo:bar().
+             ">>,
+           [],
+           []}],
+    run(Config, Ts).
+
 run(Config, Tests) ->
     F = fun({N,P,Ws,E}, BadL) ->
                 case catch run_test(Config, P, Ws) of
diff --git a/lib/stdlib/test/erl_pp_SUITE.erl b/lib/stdlib/test/erl_pp_SUITE.erl
index 13c5662741..31ea3210a8 100644
--- a/lib/stdlib/test/erl_pp_SUITE.erl
+++ b/lib/stdlib/test/erl_pp_SUITE.erl
@@ -825,12 +825,13 @@ type_examples() ->
      %% is_subtype(V, T) syntax, we need a few examples of the syntax.
      {ex31,<<"-spec t1(FooBar :: t99()) -> t99();"
 	     "(t2()) -> t2();"
-	     "('\\'t::4'()) -> '\\'t::4'() when is_subtype('\\'t::4'(), t24);"
-	     "(t23()) -> t23() when is_subtype(t23(), atom()),"
-	     "                      is_subtype(t23(), t14());"
-	     "(t24()) -> t24() when is_subtype(t24(), atom()),"
-	     "                      is_subtype(t24(), t14()),"
-	     "                      is_subtype(t24(), '\\'t::4'()).">>},
+	     "('\\'t::4'()) -> {'\\'t::4'(), B}"
+             "             when is_subtype(B, '\\'t::4'());"
+	     "(t23()) -> C when is_subtype(C, atom()),"
+	     "                      is_subtype(C, t14());"
+	     "(t24()) -> D when is_subtype(D, atom()),"
+	     "                      is_subtype(D, t14()),"
+	     "                      is_subtype(D, '\\'t::4'()).">>},
      {ex32,<<"-spec mod:t2() -> any(). ">>},
      {ex33,<<"-opaque attributes_data() :: "
        "[{'column', column()} | {'line', info_line()} |"