[m-rev.] for post-commit review: fix Mantis bug 488

[Julien Fischer]

Hi Zoltan,

On Mon, 28 Oct 2019, Zoltan Somogyi wrote:

> Diagnose variables as pred/func names in more contexts.
> 
> compiler/parse_item.m:
>     Expand the set of situations in which we look for predicate and function
>     name that is a variable. We now look in:
>
>     - type declarations of predicates and functions, with and without
>       attached mode and/or determinism information,
>     - mode declarations of predicates and functions,
>     - clause heads.

There's another situation: predicate / function method declarations in type classes.

With this change:

     :- typeclass foo(T) where [
        pred MyMethod(T::in, int::out) is semidet
     ].

produces the message:

    foo.m:005: Error: you cannot declare a predicate whose name is a variable such
    foo.m:005:   as `MyMethod'

The one change I would make there is that in the above case the error message
should say "predicate method" rather than just "predicate".

For type classes there's another problem:

    :- typeclass MyClass(T) where [].

compiles *without* complaint.  (Attempting to define an instance for MyClass
result *will* result in an error message, albeit a misleading one.)

> compiler/make_hlds_passes.m:
>     Delete the old test for variables as predicate or function names in
>     clauses. It is best for consistency if parse_item.m handles
>     all occurrences of this test.
> 
> tests/invalid/var_as_pred_name.{m,err_exp}:
>     Test for the handling of variables as both predicate and function names in
>
>     - a plain type declaration,
>     - a mode type declaration without determinism,
>     - a mode type declaration with determinism,
>     - a combined type and mode declaration without determinism,
>     - a combined type and mode declaration with determinism,
>     - a clause.

I would extend this teszt to cover type class method declarations since
this change has affected them too  (They share pretty much the same
code.)

The diff looks fine otherwise.

Julien.