On Tue, 5 Oct 2021, Jason Merrill wrote:
> On 10/5/21 15:17, Patrick Palka wrote:
> > On Mon, 4 Oct 2021, Patrick Palka wrote:
> >
> > > When passing a function template as the argument to a function NTTP
> > > inside a template, we resolve it to the right specialization ahead of
> > > time via resolve_address_of_overloaded_function, though the call to
> > > mark_used within defers odr-using it until instantiation time (as usual).
> > > But at instantiation time we end up never calling mark_used on the
> > > specialization.
> > >
> > > This patch fixes this by adding a call to mark_used in
> > > convert_nontype_argument_function.
> > >
> > > PR c++/53164
> > >
> > > gcc/cp/ChangeLog:
> > >
> > > * pt.c (convert_nontype_argument_function): Call mark_used.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > * g++.dg/template/non-dependent16.C: New test.
> > > ---
> > > gcc/cp/pt.c | 3 +++
> > > gcc/testsuite/g++.dg/template/non-dependent16.C | 16 ++++++++++++++++
> > > 2 files changed, 19 insertions(+)
> > > create mode 100644 gcc/testsuite/g++.dg/template/non-dependent16.C
> > >
> > > diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
> > > index f950f4a21b7..5e819c9598c 100644
> > > --- a/gcc/cp/pt.c
> > > +++ b/gcc/cp/pt.c
> > > @@ -6668,6 +6668,9 @@ convert_nontype_argument_function (tree type, tree
> > > expr,
> > > return NULL_TREE;
> > > }
> > > + if (!mark_used (fn_no_ptr, complain) && !(complain & tf_error))
> > > + return NULL_TREE;
> > > +
> > > linkage = decl_linkage (fn_no_ptr);
> > > if (cxx_dialect >= cxx11 ? linkage == lk_none : linkage !=
> > > lk_external)
> > > {
> > > diff --git a/gcc/testsuite/g++.dg/template/non-dependent16.C
> > > b/gcc/testsuite/g++.dg/template/non-dependent16.C
> > > new file mode 100644
> > > index 00000000000..b7dca8f6752
> > > --- /dev/null
> > > +++ b/gcc/testsuite/g++.dg/template/non-dependent16.C
> > > @@ -0,0 +1,16 @@
> > > +// PR c++/53164
> > > +
> > > +template<class T>
> > > +void f(T) {
> > > + T::fail; // { dg-error "not a member" }
> > > +}
> > > +
> > > +template<void(int)>
> > > +struct A { };
> > > +
> > > +template<int>
> > > +void g() {
> > > + A<f> a;
> > > +}
> >
> > I should mention that the original testcase in the PR was slightly
> > different than this one in that it also performed a call to the NTTP,
> > e.g.
> >
> > template<void p(int)>
> > struct A {
> > static void h() {
> > p(0);
> > }
> > };
> >
> > template<int>
> > void g() {
> > A<f>::h();
> > }
> >
> > templated void g<0>();
> >
> > and not even the call was enough to odr-use f, apparently because the
> > CALL_EXPR case of tsubst_expr calls mark_used on the callee only when
> > it's a FUNCTION_DECL, but in this case after substitution it's an
> > ADDR_EXPR of a FUNCTION_DECL. Fixing this by looking through the ADDR_EXPR
> > worked, but IIUC the call isn't necessary for f to be odr-used, simply
> > using f as a template argument should be sufficient, so it seems the
> > above is better fix.
>
> I agree that pedantically the use happens when substituting into the use of
> A<f>, but convert_nontype_argument_function seems like a weird place to
> implement that; it's only called again during instantiation of A<f>, when we
> instantiate the injected-class-name. If A<f> isn't instantiated, e.g. if 'a'
> is a pointer to A<f>, we again don't instantiate f<int>.
I see, makes sense.. I'm not sure where else we can mark the use, then.
Since we resolve the OVERLOAD f to the FUNCTION_DECL f<int> ahead of
time (during which mark_used doesn't actually instantiate f<int> because
we're inside a template), at instantiation time the type A<f> is already
non-dependent so tsubst_aggr_type avoids doing the work that would end
up calling convert_nontype_argument_function.
>
> I see that clang doesn't reject your testcase, either, but MSVC and icc do
> (even with 'a' a pointer): https://godbolt.org/z/MGE6TcMch
FWIW although Clang doesn't reject 'A<f> a;', it does reject
'using type = A<f>;' weirdly enough: https://godbolt.org/z/T9qEn6bWW
Shall we just go with the other more specific approach, that makes sure
the CALL_EXPR case of tsubst_expr calls mark_used when the callee is an
ADDR_EXPR? Something like (bootstrapped and regtested):
-- >8 --
PR c++/53164
gcc/cp/ChangeLog:
* pt.c (tsubst_copy_and_build) <case CALL_EXPR>: Look through
ADDR_EXPR after substituting into the callee.
gcc/testsuite/ChangeLog:
* g++.dg/template/fn-ptr3.C: New test.
---
gcc/cp/pt.c | 4 ++++
gcc/testsuite/g++.dg/template/fn-ptr3.C | 20 ++++++++++++++++++++
2 files changed, 24 insertions(+)
create mode 100644 gcc/testsuite/g++.dg/template/fn-ptr3.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 19e03369ffa..5af3a6472f8 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -20310,6 +20310,10 @@ tsubst_copy_and_build (tree t,
if (BASELINK_P (function))
qualified_p = true;
+
+ if (TREE_CODE (function) == ADDR_EXPR
+ && TREE_CODE (TREE_OPERAND (function, 0)) == FUNCTION_DECL)
+ function = TREE_OPERAND (function, 0);
}
nargs = call_expr_nargs (t);
diff --git a/gcc/testsuite/g++.dg/template/fn-ptr3.C
b/gcc/testsuite/g++.dg/template/fn-ptr3.C
new file mode 100644
index 00000000000..4aa193bd961
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/fn-ptr3.C
@@ -0,0 +1,20 @@
+// PR c++/53164
+
+template<class T>
+void f(T) {
+ T::fail; // { dg-error "not a member" }
+}
+
+template<void p(int)>
+struct A {
+ static void h() {
+ p(0);
+ }
+};
+
+template<int>
+void g() {
+ A<f>::h();
+}
+
+template void g<0>();
--
2.33.0.664.g0785eb7698
