On Wed, 29 Mar 2023, Jason Merrill wrote:
> On 3/28/23 13:37, Patrick Palka wrote:
> > Now that we resolve non-dependent variable template-ids ahead of time,
> > cp_finish_decl needs to handle a new invalid situation: we can end up
> > trying to instantiate a variable template with deduced return type
> > before we fully parsed (and attached) its initializer.
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this OK for
> > trunK?
> >
> > PR c++/109300
> >
> > gcc/cp/ChangeLog:
> >
> > * decl.cc (cp_finish_decl): Diagnose ordinary auto deduction
> > with no initializer instead of asserting.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp1y/var-templ79.C: New test.
> > ---
> > gcc/cp/decl.cc | 15 ++++++++++++++-
> > gcc/testsuite/g++.dg/cpp1y/var-templ79.C | 5 +++++
> > 2 files changed, 19 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> >
> > diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
> > index 20b980f68c8..2c91693b99d 100644
> > --- a/gcc/cp/decl.cc
> > +++ b/gcc/cp/decl.cc
> > @@ -8276,7 +8276,20 @@ cp_finish_decl (tree decl, tree init, bool
> > init_const_expr_p,
> > return;
> > }
> > - gcc_assert (CLASS_PLACEHOLDER_TEMPLATE (auto_node));
> > + if (CLASS_PLACEHOLDER_TEMPLATE (auto_node))
> > + /* Class deduction with no initializer is OK. */;
> > + else
> > + {
> > + /* Ordinary auto deduction without an initializer, a situation
> > + which grokdeclarator already catches and rejects for the most
> > + part. But we can still get here if we're instantiating a
> > + variable template before we've fully parsed (and attached)
> > its
> > + initializer, e.g. template<class> auto x = x<int>; */
>
> In the case of recursively dependent instantiation I'd hope to have an
> error_mark_node initializer, rather than none?
Do you mean setting the initializer to error_mark_node after the fact, e.g.
@@ -8288,7 +8297,7 @@ cp_finish_decl (tree decl, tree init, bool
init_const_expr_p,
error_at (DECL_SOURCE_LOCATION (decl),
"declaration of %q#D has no initializer", decl);
TREE_TYPE (decl) = error_mark_node;
- return;
+ init = error_mark_node;
}
}
d_init = init;
or before the fact, i.e. setting DECL_INITIAL to error_mark_node as a
sentinel value for detecting recursion before we begin parsing a variable
initializer? The former should work I suppose, but the latter is
problematic because we also call cp_finish_decl with init=error_mark_node
when the initializer is generally invalid, so by overloading the meaning
of error_mark_node here and checking for it from cp_finish_decl we would
end up emitting a bogus extra diagnostic in a bunch of cases e.g.
g++.dg/pr53055.C:
int i = p ->* p ; // invalid initializer
I guess we would need to use a different sentinel value for detecting
recursion, or expose and inspect the 'lambda_scope' stack which already
keeps track of whether we're in the middle of a variable initializer...
Dunno if it's worth it just for sake of a better diagnostic for this
corner case, I notice e.g. Clang doesn't give a great diagnostic either:
src/gcc/testsuite/g++.dg/cpp1y/var-templ79.C:5:6: error: declaration of
variable 'x' with deduced type 'auto' requires an initializer
auto x = x<int>; // { dg-error "" }
^
>
> > + error_at (DECL_SOURCE_LOCATION (decl),
> > + "declaration of %q#D has no initializer", decl);
> > + TREE_TYPE (decl) = error_mark_node;
> > + return;
> > + }
> > }
> > d_init = init;
> > if (d_init)
> > diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> > b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> > new file mode 100644
> > index 00000000000..3c0d276153a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
> > @@ -0,0 +1,5 @@
> > +// PR c++/109300
> > +// { dg-do compile { target c++14 } }
> > +
> > +template<class>
> > +auto x = x<int>; // { dg-error "" }
>
>
