On Tue, Mar 07, 2023 at 09:53:28AM -0500, Jason Merrill wrote:
> On 3/6/23 17:01, Marek Polacek wrote:
> > On Mon, Mar 06, 2023 at 11:12:56AM -0500, Jason Merrill wrote:
> > > On 3/3/23 12:51, Marek Polacek wrote:
> > > > Similarly to PR107938, this also started with r11-557, whereby
> > > > cp_finish_decl
> > > > can call check_initializer even in a template for a constexpr
> > > > initializer.
> > > >
> > > > Here we are rejecting
> > > >
> > > > extern const Q q;
> > > >
> > > > template<int>
> > > > constexpr auto p = q(0);
> > > >
> > > > even though q has a constexpr operator(). It's deemed non-const by
> > > > decl_maybe_constant_var_p because even though 'q' is const it is not
> > > > of integral/enum type. I think the fix is for p_c_e to treat q(0) as
> > > > potentially-constant, as below.
> > > >
> > > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?
> > > >
> > > > PR c++/107939
> > > >
> > > > gcc/cp/ChangeLog:
> > > >
> > > > * constexpr.cc (is_constexpr_function_object): New.
> > > > (potential_constant_expression_1): Treat an object with
> > > > constexpr
> > > > operator() as potentially-constant.
> > > >
> > > > gcc/testsuite/ChangeLog:
> > > >
> > > > * g++.dg/cpp1y/var-templ74.C: Remove dg-error.
> > > > * g++.dg/cpp1y/var-templ77.C: New test.
> > > > ---
> > > > gcc/cp/constexpr.cc | 23 ++++++++++++++++++++++-
> > > > gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +-
> > > > gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 14 ++++++++++++++
> > > > 3 files changed, 37 insertions(+), 2 deletions(-)
> > > > create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C
> > > >
> > > > diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
> > > > index acf9847a4d1..7d786f332b4 100644
> > > > --- a/gcc/cp/constexpr.cc
> > > > +++ b/gcc/cp/constexpr.cc
> > > > @@ -8929,6 +8929,24 @@ check_for_return_continue (tree *tp, int
> > > > *walk_subtrees, void *data)
> > > > return NULL_TREE;
> > > > }
> > > > +/* Return true iff TYPE is a class with constexpr operator(). */
> > > > +
> > > > +static bool
> > > > +is_constexpr_function_object (tree type)
> > > > +{
> > > > + if (!CLASS_TYPE_P (type))
> > > > + return false;
> > > > +
> > > > + for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f))
> > > > + if (TREE_CODE (f) == FUNCTION_DECL
> > > > + && DECL_OVERLOADED_OPERATOR_P (f)
> > > > + && DECL_OVERLOADED_OPERATOR_IS (f, CALL_EXPR)
> > > > + && DECL_DECLARED_CONSTEXPR_P (f))
> > > > + return true;
> > > > +
> > > > + return false;
> > > > +}
> > > > +
> > > > /* Return true if T denotes a potentially constant expression. Issue
> > > > diagnostic as appropriate under control of FLAGS. If WANT_RVAL
> > > > is true,
> > > > an lvalue-rvalue conversion is implied. If NOW is true, we want
> > > > to
> > > > @@ -9160,7 +9178,10 @@ potential_constant_expression_1 (tree t, bool
> > > > want_rval, bool strict, bool now,
> > > > }
> > > > else if (fun)
> > > > {
> > > > - if (RECUR (fun, rval))
> > > > + if (VAR_P (fun)
> > > > + && is_constexpr_function_object (TREE_TYPE (fun)))
> > > > + /* Could be an object with constexpr operator(). */;
> > >
> > > I guess if fun is not a function pointer, we don't know if we're using it
> > > as
> > > an lvalue or rvalue
> >
> > Presumably the operator function could return this, making it an lvalue?
> > I'm not sure I'm really clear on this.
>
> I mean just calling the operator uses the variable as an lvalue, by passing
> its address as 'this'.
Ah yeah, right. Unless there's the && ref-qual etc.
> > > , so we want to pass 'any' for want_rval, which should
> > > make this work;
> >
> > Yes, want_rval==false means that p_c_e/VAR_DECL will not issue the
> > hard error.
> >
> > > I don't think we need to be specific about constexpr op(),
> > > as a constexpr conversion operator to fn* could also do the trick.
> >
> > Ah, those surrogate classes. I couldn't reproduce the problem with
> > them, though I'm adding a test for it anyway.
> >
> > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?
>
> OK, thanks.
Thanks.
Marek
