On Wed, 21 Apr 2021, Jason Merrill wrote:
> On 4/12/21 1:20 PM, Patrick Palka wrote:
> > Here we're crashing during deduction for a template placeholder from a
> > dependent initializer because one of the initializer's elements has an
> > empty TREE_TYPE, something which resolve_args and later unify_one_argument
> > don't expect. And if the deduction from a dependent initializer
> > otherwise fails, we prematurely issue an error rather than reattempting
> > the deduction at instantiation time.
> >
> > This patch makes do_class_deduction more tolerant about dependent
> > initializers, in a manner similar to what do_auto_deduction does: if
> > deduction from a dependent initializer fails, just return the original
> > placeholder unchanged.
>
> Why doesn't the type_dependent_expression_p check in do_auto_deduction catch
> this already?
That check applies only when context != adc_unify, but here we have
context == adc_unify since we're being called from
convert_template_argument.
And currently, when 'auto' deduction fails for a dependent initializer,
do_auto_deduction will just silently return the original placeholder:
int val = type_unification_real (tparms, targs, parms, &init, 1, 0,
DEDUCE_CALL,
NULL, /*explain_p=*/false);
if (val > 0)
{
if (processing_template_decl)
/* Try again at instantiation time. */
return type;
so I suppose this patch just makes do_class_deduction behave more
similarly to do_auto_deduction in this situation.
>
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, and also tested on
> > cmcstl2 and range-v3, and on all the testcases in PR93383, does this
> > look OK for trunk?
> >
> > gcc/cp/ChangeLog:
> >
> > PR c++/89565
> > PR c++/93383
> > PR c++/99200
> > * pt.c (do_class_deduction): If an argument has no type, don't
> > attempt deduction. If deduction fails and the initializer is
> > type-dependent, try again at instantiation time.
> >
> > gcc/testsuite/ChangeLog:
> >
> > PR c++/89565
> > PR c++/93383
> > PR c++/99200
> > * g++.dg/cpp2a/nontype-class39.C: Remove dg-ice.
> > * g++.dg/cpp2a/nontype-class44.C: New test.
> > * g++.dg/cpp2a/nontype-class45.C: New test.
> > ---
> > gcc/cp/pt.c | 11 +++++++
> > gcc/testsuite/g++.dg/cpp2a/nontype-class39.C | 1 -
> > gcc/testsuite/g++.dg/cpp2a/nontype-class44.C | 11 +++++++
> > gcc/testsuite/g++.dg/cpp2a/nontype-class45.C | 32 ++++++++++++++++++++
> > 4 files changed, 54 insertions(+), 1 deletion(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class44.C
> > create mode 100644 gcc/testsuite/g++.dg/cpp2a/nontype-class45.C
> >
> > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > index 0ce7fa359c1..612feac7976 100644
> > --- a/gcc/cp/pt.c
> > +++ b/gcc/cp/pt.c
> > @@ -29334,6 +29334,13 @@ do_class_deduction (tree ptype, tree tmpl, tree
> > init,
> > else
> > args = make_tree_vector_single (init);
> > + /* If an argument is missing its type, we can't possibly deduce from
> > this
> > + (type-dependent) initializer ahead of time. */
> > + if (processing_template_decl)
> > + for (tree arg : *args)
> > + if (!TREE_TYPE (arg))
> > + return ptype;
> > /* Do this now to avoid problems with erroneous args later on. */
> > args = resolve_args (args, complain);
> > if (args == NULL)
> > @@ -29419,6 +29426,10 @@ do_class_deduction (tree ptype, tree tmpl, tree
> > init,
> > if (call == error_mark_node)
> > {
> > + if (type_dependent_expression_p (init))
> > + /* Try again at instantiation time. */
> > + return ptype;
> > +
> > if (complain & tf_warning_or_error)
> > {
> > error ("class template argument deduction failed:");
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class39.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class39.C
> > index f5f79a71ec2..9b4da4f02ea 100644
> > --- a/gcc/testsuite/g++.dg/cpp2a/nontype-class39.C
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class39.C
> > @@ -1,6 +1,5 @@
> > // PR c++/89565
> > // { dg-do compile { target c++20 } }
> > -// { dg-ice "resolve_args" }
> > template <auto>
> > struct N{};
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class44.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class44.C
> > new file mode 100644
> > index 00000000000..d91e800424f
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class44.C
> > @@ -0,0 +1,11 @@
> > +// PR c++/93383
> > +// { dg-do compile { target c++20 } }
> > +
> > +template <int> struct A {};
> > +
> > +template <A a> struct B {
> > + void foo(B<+a>);
> > + void bar(B<a.x>);
> > + template <class T> using type = B<T{}>;
> > + template <class> static inline auto y = A{0}; // { dg-error "deduction|no
> > match" }
> > +};
> > diff --git a/gcc/testsuite/g++.dg/cpp2a/nontype-class45.C
> > b/gcc/testsuite/g++.dg/cpp2a/nontype-class45.C
> > new file mode 100644
> > index 00000000000..e7addf5f291
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/nontype-class45.C
> > @@ -0,0 +1,32 @@
> > +// PR c++/99200
> > +// { dg-do compile { target c++20 } }
> > +
> > +template <int N>
> > +struct A
> > +{
> > + constexpr A (const char (&s)[N]) { for (int i = 0; i < N; i++) v[i] =
> > s[i]; v[N] = 0; }
> > + char v[N + 1];
> > +};
> > +
> > +template <A s>
> > +struct B
> > +{
> > + constexpr operator const char *() { return s.v; }
> > +};
> > +
> > +template <typename T>
> > +const char *
> > +foo ()
> > +{
> > + return B<__PRETTY_FUNCTION__>{};
> > +}
> > +
> > +template <typename T>
> > +const char *
> > +bar ()
> > +{
> > + return B<__FUNCTION__>{};
> > +}
> > +
> > +auto a = foo <int> ();
> > +auto b = bar <double> ();
> >
>
>
