Re: C++ PATCH to fix static init with () in a template (PR c++/84582)

[Jason Merrill <jason at redhat dot com>]

Ok.

On Mar 1, 2018 4:40 PM, "Marek Polacek" <polacek@redhat.com> wrote:

> On Thu, Mar 01, 2018 at 01:56:50PM -0500, Jason Merrill wrote:
> > On Thu, Mar 1, 2018 at 8:17 AM, Marek Polacek <polacek@redhat.com>
> wrote:
> > > On Wed, Feb 28, 2018 at 04:50:39PM -0500, Jason Merrill wrote:
> > >> On Wed, Feb 28, 2018 at 4:19 PM, Marek Polacek <polacek@redhat.com>
> wrote:
> > >> > On Wed, Feb 28, 2018 at 10:51:17AM -0500, Jason Merrill wrote:
> > >> >> On Wed, Feb 28, 2018 at 9:32 AM, Marek Polacek <polacek@redhat.com>
> wrote:
> > >> >> > On Tue, Feb 27, 2018 at 04:16:31PM -0500, Jason Merrill wrote:
> > >> >> >> On 02/27/2018 02:13 PM, Marek Polacek wrote:
> > >> >> >> > My recent change introducing cxx_constant_init caused this
> code
> > >> >> >> >
> > >> >> >> > template <class> class A {
> > >> >> >> >    static const long b = 0;
> > >> >> >> >    static const unsigned c = (b);
> > >> >> >> > };
> > >> >> >> >
> > >> >> >> > to be rejected.  The reason is that force_paren_expr turns
> "b" into "*(const
> > >> >> >> > long int &) &b", where the former is not value-dependent but
> the latter is
> > >> >> >> > value-dependent.  So when we get to maybe_constant_init_1:
> > >> >> >> > 5147   if (!is_nondependent_static_init_expression (t))
> > >> >> >> > 5148     /* Don't try to evaluate it.  */;
> > >> >> >> > it's not evaluated and we get the non-constant initialization
> error.
> > >> >> >> > (Before we'd always evaluated the expression.)
> > >> >> >> >
> > >> >> >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> > >> >> >> >
> > >> >> >> > 2018-02-27  Marek Polacek  <polacek@redhat.com>
> > >> >> >> >
> > >> >> >> >     PR c++/84582
> > >> >> >> >     * semantics.c (force_paren_expr): Avoid creating a static
> cast
> > >> >> >> >     when processing a template.
> > >> >> >> >
> > >> >> >> >     * g++.dg/cpp1z/static1.C: New test.
> > >> >> >> >     * g++.dg/template/static37.C: New test.
> > >> >> >> >
> > >> >> >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> > >> >> >> > index 35569d0cb0d..b48de2df4e2 100644
> > >> >> >> > --- gcc/cp/semantics.c
> > >> >> >> > +++ gcc/cp/semantics.c
> > >> >> >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
> > >> >> >> >       expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> > >> >> >> >     else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
> > >> >> >> >       /* We can't bind a hard register variable to a
> reference.  */;
> > >> >> >> > -  else
> > >> >> >> > +  else if (!processing_template_decl)
> > >> >> >>
> > >> >> >> Hmm, this means that we forget about the parentheses in a
> template.  I'm
> > >> >> >> surprised that this didn't break anything in the testsuite.  In
> particular,
> > >> >> >> auto-fn15.C.  I've attached an addition to auto-fn15.C to catch
> this issue.
> > >> >> >
> > >> >> > Thanks, you're right.  I'll use it.
> > >> >> >
> > >> >> >> Can we use PAREN_EXPR instead of the static_cast in a template?
> > >> >> >
> > >> >> > I don't think so, it would fix the issue you pointed out in
> auto-fn15.C but
> > >> >> > it wouldn't fix the original test.  The problem with using
> PAREN_EXPR in a
> > >> >> > template is that instantiate_non_dependent_expr will turn in
> into the
> > >> >> > static cast anyway; tsubst_copy_and_build has
> > >> >> >     case PAREN_EXPR:
> > >> >> >       RETURN (finish_parenthesized_expr (RECUR (TREE_OPERAND (t,
> 0))));
> > >> >> > so it calls force_paren_expr and this time we're not in a
> template.  And
> > >> >> > then when calling cxx_constant_init we have the same issue.
> > >> >>
> > >> >> Then maybe we need something like fold_non_dependent_expr, which
> > >> >> checks for dependency before substitution and then immediately
> > >> >> evaluates the result.
> > >> >
> > >> > I hope you meant something like this.  Further testing also
> revealed that
> > >> > maybe_undo_parenthesized_ref should be able to unwrap PAREN_EXPR
> (so that
> > >> > (fn1)(); in paren2.C is handled correctly), and that lvalue_kind
> should look
> > >> > into PAREN_EXPR so as to give the correct answer regarding
> lvalueness: we
> > >> > should accept
> > >> >
> > >> > template<typename T>
> > >> > void foo (int i)
> > >> > {
> > >> >   ++(i);
> > >> > }
> > >> >
> > >> > Apologies if I'm on the wrong track.
> > >> >
> > >> > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> > >> >
> > >> > 2018-02-28  Marek Polacek  <polacek@redhat.com>
> > >> >             Jason Merrill  <jason@redhat.com>
> > >> >
> > >> >         PR c++/84582
> > >> >         * semantics.c (force_paren_expr): Avoid creating the static
> cast
> > >> >         when in a template.  Create a PAREN_EXPR when in a template.
> > >> >         (maybe_undo_parenthesized_ref): Unwrap PAREN_EXPR.
> > >> >         * typeck2.c (store_init_value): Call
> fold_non_dependent_expr instead
> > >> >         of instantiate_non_dependent_expr.
> > >> >         * tree.c (lvalue_kind): Handle PAREN_EXPR like
> NON_DEPENDENT_EXPR.
> > >> >
> > >> >         * g++.dg/cpp1y/auto-fn15.C: Extend testing.
> > >> >         * g++.dg/cpp1z/static1.C: New test.
> > >> >         * g++.dg/template/static37.C: New test.
> > >> >
> > >> > diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> > >> > index 35569d0cb0d..722e3718a14 100644
> > >> > --- gcc/cp/semantics.c
> > >> > +++ gcc/cp/semantics.c
> > >> > @@ -1697,7 +1697,7 @@ force_paren_expr (tree expr)
> > >> >      expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> > >> >    else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
> > >> >      /* We can't bind a hard register variable to a reference.  */;
> > >> > -  else
> > >> > +  else if (!processing_template_decl)
> > >> >      {
> > >> >        cp_lvalue_kind kind = lvalue_kind (expr);
> > >> >        if ((kind & ~clk_class) != clk_none)
> > >> > @@ -1713,6 +1713,8 @@ force_paren_expr (tree expr)
> > >> >             REF_PARENTHESIZED_P (expr) = true;
> > >> >         }
> > >> >      }
> > >> > +  else
> > >> > +    expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
> > >>
> > >> There's already a branch for building PAREN_EXPR, let's just replace
> > >> its condition.
> > >
> > > Sure.
> > >
> > >> > -      value = instantiate_non_dependent_expr (value);
> > >> > +      value = fold_non_dependent_expr (value);
> > >>
> > >> I was thinking that we want a parallel fold_non_dependent_init (that
> > >> hopefully shares most of the implementation).  Then we shouldn't need
> > >> the call to maybe_constant_init anymore.
> > >
> > > If you mean fold_non_dependent_init that would be like
> fold_non_dependent_expr
> > > but with maybe_constant_init and not maybe_constant_value
> >
> > And is_nondependent_static_init_expression, and different arguments to
> > cxx_eval_outermost_constant_expression, yes.
>
> Ah.  Maybe it'll be useful sometime in the future.
>
> > > then that would break e.g.
> > >
> > > const double d = 9.0;   // missing constexpr
> > > constexpr double j = d; // should give error
> > >
> > > because maybe_constant_value checks is_nondependent_constant_expression,
> and
> > > "d" in the example above is not a constant expression, so we don't
> evaluate,
> > > and "d" stays "d", so require_constant_expression gives the error.  On
> the
> > > other hand, maybe_constant_init checks is_nondependent_static_init_
> expression,
> > > and "d" is that, so we evaluate "d" to "9.0".  Then
> require_constant_expression
> > > doesn't complain.
> >
> > Ah, I see.  You're right, let's stick with fold_non_dependent_expr.
>
> Thanks, so this is the final patch then:
>
> Bootstrapped/regtested on x86_64-linux, ok for trunk?
>
> 2018-03-01  Marek Polacek  <polacek@redhat.com>
>             Jason Merrill  <jason@redhat.com>
>
>         PR c++/84582
>         * semantics.c (force_paren_expr): Create a PAREN_EXPR when in
>         a template.
>         (maybe_undo_parenthesized_ref): Unwrap PAREN_EXPR.
>         * typeck2.c (store_init_value): Call fold_non_dependent_expr
> instead
>         of instantiate_non_dependent_expr.
>         * tree.c (lvalue_kind): Handle PAREN_EXPR like NON_DEPENDENT_EXPR.
>
>         * g++.dg/cpp1y/auto-fn15.C: Extend testing.
>         * g++.dg/cpp1z/static1.C: New test.
>         * g++.dg/template/static37.C: New test.
>
> diff --git gcc/cp/semantics.c gcc/cp/semantics.c
> index 87c5c669a55..1ac1d23e761 100644
> --- gcc/cp/semantics.c
> +++ gcc/cp/semantics.c
> @@ -1693,7 +1693,8 @@ force_paren_expr (tree expr)
>    if (TREE_CODE (expr) == COMPONENT_REF
>        || TREE_CODE (expr) == SCOPE_REF)
>      REF_PARENTHESIZED_P (expr) = true;
> -  else if (type_dependent_expression_p (expr))
> +  else if (type_dependent_expression_p (expr)
> +          || processing_template_decl)
>      expr = build1 (PAREN_EXPR, TREE_TYPE (expr), expr);
>    else if (VAR_P (expr) && DECL_HARD_REGISTER (expr))
>      /* We can't bind a hard register variable to a reference.  */;
> @@ -1724,9 +1725,10 @@ force_paren_expr (tree expr)
>  tree
>  maybe_undo_parenthesized_ref (tree t)
>  {
> -  if (cxx_dialect >= cxx14
> -      && INDIRECT_REF_P (t)
> -      && REF_PARENTHESIZED_P (t))
> +  if (cxx_dialect < cxx14)
> +    return t;
> +
> +  if (INDIRECT_REF_P (t) && REF_PARENTHESIZED_P (t))
>      {
>        t = TREE_OPERAND (t, 0);
>        while (TREE_CODE (t) == NON_LVALUE_EXPR
> @@ -1737,6 +1739,8 @@ maybe_undo_parenthesized_ref (tree t)
>                   || TREE_CODE (t) == STATIC_CAST_EXPR);
>        t = TREE_OPERAND (t, 0);
>      }
> +  else if (TREE_CODE (t) == PAREN_EXPR)
> +    t = TREE_OPERAND (t, 0);
>
>    return t;
>  }
> diff --git gcc/cp/tree.c gcc/cp/tree.c
> index 9b9e36a1173..19f1c0629c9 100644
> --- gcc/cp/tree.c
> +++ gcc/cp/tree.c
> @@ -239,6 +239,7 @@ lvalue_kind (const_tree ref)
>        return lvalue_kind (BASELINK_FUNCTIONS (CONST_CAST_TREE (ref)));
>
>      case NON_DEPENDENT_EXPR:
> +    case PAREN_EXPR:
>        return lvalue_kind (TREE_OPERAND (ref, 0));
>
>      case VIEW_CONVERT_EXPR:
> diff --git gcc/cp/typeck2.c gcc/cp/typeck2.c
> index 153b46cca77..583c65d4d0a 100644
> --- gcc/cp/typeck2.c
> +++ gcc/cp/typeck2.c
> @@ -822,7 +822,7 @@ store_init_value (tree decl, tree init, vec<tree,
> va_gc>** cleanups, int flags)
>    if (decl_maybe_constant_var_p (decl) || TREE_STATIC (decl))
>      {
>        bool const_init;
> -      value = instantiate_non_dependent_expr (value);
> +      value = fold_non_dependent_expr (value);
>        if (DECL_DECLARED_CONSTEXPR_P (decl)
>           || (DECL_IN_AGGR_P (decl) && !DECL_VAR_DECLARED_INLINE_P (decl)))
>         {
> diff --git gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
> gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
> index ba9f3579f62..0db428f7270 100644
> --- gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
> +++ gcc/testsuite/g++.dg/cpp1y/auto-fn15.C
> @@ -22,6 +22,8 @@ template <class T>
>  decltype(auto) h5(T t) { return t.i; }
>  template <class T>
>  decltype(auto) h6(T t) { return (t.i); }
> +template <class T>
> +decltype(auto) h7(T t) { return (i); }
>
>  int main()
>  {
> @@ -48,4 +50,5 @@ int main()
>    same_type<decltype(h4()),int&>();
>    same_type<decltype(h5(a)),int>();
>    same_type<decltype(h6(a)),int&>();
> +  same_type<decltype(h7(a)),int&>();
>  }
> diff --git gcc/testsuite/g++.dg/cpp1z/static1.C
> gcc/testsuite/g++.dg/cpp1z/static1.C
> index e69de29bb2d..cb872997c5a 100644
> --- gcc/testsuite/g++.dg/cpp1z/static1.C
> +++ gcc/testsuite/g++.dg/cpp1z/static1.C
> @@ -0,0 +1,19 @@
> +// PR c++/84582
> +// { dg-options -std=c++17 }
> +
> +class C {
> +  static inline const long b = 0;
> +  static inline const unsigned c = (b);
> +};
> +class D {
> +  static inline const long b = 0;
> +  static inline const unsigned c = b;
> +};
> +template <class> class A {
> +  static inline const long b = 0;
> +  static inline const unsigned c = (b);
> +};
> +template <class> class B {
> +  static inline const long b = 0;
> +  static inline const unsigned c = b;
> +};
> diff --git gcc/testsuite/g++.dg/template/static37.C
> gcc/testsuite/g++.dg/template/static37.C
> index e69de29bb2d..90bc65d2fbc 100644
> --- gcc/testsuite/g++.dg/template/static37.C
> +++ gcc/testsuite/g++.dg/template/static37.C
> @@ -0,0 +1,18 @@
> +// PR c++/84582
> +
> +class C {
> +  static const long b = 0;
> +  static const unsigned c = (b);
> +};
> +class D {
> +  static const long b = 0;
> +  static const unsigned c = b;
> +};
> +template <class> class A {
> +  static const long b = 0;
> +  static const unsigned c = (b);
> +};
> +template <class> class B {
> +  static const long b = 0;
> +  static const unsigned c = b;
> +};
>
>         Marek
>