Solve constraints in type_check_list_op_in/3

[erszcz]

At the location of op_type_too_precise we usually get a type variable. This means that if we solved the available constraints and got a substitution for it, we could tell more about the operation in question.

Gradualizer/src/typechecker.erl

Lines 3130 to 3158 in 1498d17

	-spec type_check_list_op_in(env(), type(), expr()) -> {env(), constraints:t()}.
	type_check_list_op_in(Env, ResTy, {op, P, Op, Arg1, Arg2} = Expr) ->
	Target =
	%% '--' always produces a proper list, but '++' gives you an improper
	%% list if the second argument is improper.
	case Op of
	'--' -> type(list, [top()]);
	'++' -> type(maybe_improper_list, [top()
	,top()])
	end,
	{ResTy1, Cs} = glb(Target, ResTy, Env),
	case ResTy1 of
	{type, _, none, []} ->
	% Tested by test/should_fail/list_op.erl subtract/1
	throw(type_error(Expr, type(list), ResTy));
	{type, _, any, []} ->
	{_, VB, Cs1} = type_check_list_op(Env, Arg1, Arg2),
	{VB, constraints:combine(Cs, Cs1)};
	_ ->
	case list_op_arg_types(Op, ResTy1) of
	{ArgTy1, ArgTy2} ->
	{VarBinds1, Cs1} = type_check_expr_in(Env, ArgTy1, Arg1),
	{VarBinds2, Cs2} = type_check_expr_in(Env, ArgTy2, Arg2),
	{union_var_binds(VarBinds1, VarBinds2, Env),
	constraints:combine([Cs, Cs1, Cs2])};
	false ->
	throw(type_error(op_type_too_precise, Op, P, ResTy1))
	end
	end.