Crash with Uncaught error: function_clause

[ilya-klyuchnikov]

Trying to run it on our code base:

===> Uncaught error: function_clause
===> Stack trace to the error location:
[{gradualizer_lib,pick_value,
  [{type,0,term,[]},
   {env,
     ..., omitting environment
    false}],
  [{file,
    "gradualizer/src/gradualizer_lib.erl"},
   {line,162}]},
 {gradualizer_lib,'-pick_values/2-lc$^0/1-0-',2,
  [{file,
    "gradualizer/src/gradualizer_lib.erl"},
   {line,156}]},
 {typechecker,check_clauses_intersection_throw_if_seen,5,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,3892}]},
 {typechecker,check_clauses_intersection,5,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,3830}]},
 {typechecker,check_clauses_fun,3,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,3744}]},
 {typechecker,type_check_function,2,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,4647}]},
 {typechecker,type_check_form,5,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,5687}]},
 {typechecker,'-type_check_form_with_timeout/5-fun-0-',6,
  [{file,
    "gradualizer/src/typechecker.erl"},
   {line,5645}]}]

Looking into gradualizer_lib:pick_value - it doesn't expect term() type.

[erszcz]

Thanks, @ilya-klyuchnikov!

Can you share a (possibly minimised) snippet on which this happens? Generally, a denormalised type like term() should not reach pick_value because of these checks, specifically all_refinable:

Gradualizer/src/typechecker.erl

Lines 3954 to 3957 in 01a1931

	case exhaustiveness_checking(Env) andalso
	all_refinable(ArgTys, Env) andalso
	no_clause_has_guards(Clauses) andalso
	some_type_not_none(RefinedArgTys)

But, apparently, something's falling through the cracks 🤔

[erszcz]

For more context, simple examples like these don't trigger this:

$ cat test.erl
-module(test).

-spec f(term()) -> ok.
f(T) ->
    case T of
        1 -> ok
    end.

-spec g({a | b, term()}) -> ok.
g(T) ->
    case T of
        {a, 1} -> ok
    end.

And while a trivial patch like this should fix the problem, I'd be hesitant to merge it without understanding what expression leads to the crash in the first place.

[ilya-klyuchnikov]

I am trying to run gradualizer on a fairly large project. How do I find out what it analyzes when it fails?

[erszcz]

Please add

{gradualizer_opts, [
    verbose
]}.

to rebar.config or, if using the escript, --verbose to the list of command line arguments. Verbose logging should result in a log like this (but with lots of output for a large codebase):

Checking module rebar3_example
Spawning async task...
Checking function c/1
Pushing {clauses_controls,true}
6:1: Checking clause :: boolean()
7:5: Checking that X :: boolean()
Exhaustiveness checking: true
Popping clauses controls
Task returned from function c/1 with []

which should make it possible to pinpoint where the problem is. It might help to run Gradualizer app by app in a big umbrella repo, to limit the scope.

This use case is certainly not polished for large code bases yet, but with the available resources we have to pick our battles.

[ilya-klyuchnikov]

A minimized repro:

-module(gradualizer_repro).

-compile([export_all, nowarn_export_all]).

-spec to_i(term()) -> integer().
to_i(_) -> error(example).

-spec to_b(term()) -> boolean().
to_b(_) -> error(example).

-spec to_x
    (term(), to_i | i, Default) -> integer() | Default;
    (term(), to_b | b, Default) -> boolean() | Default.
to_x(Value, _, Default) when Value =:= Default ->
    Default;
to_x(Value, to_i, _) -> to_i(Value);
to_x(Value, i, _) -> to_i(Value);
to_x(Value, to_b, _) -> to_b(Value);
to_x(Value, b, _) -> to_b(Value).

[erszcz]

Thanks for the example, @ilya-klyuchnikov! It's most likely the case that spurious types reach gradualizer:pick_value/2 due to changes introduced in #461, which introduces another call to pick_value than the one that's guarded with exhaustiveness checks. I'll have to investigate further how to prevent this from happening.