My r10-7007 patch tweaked tsubst not to reduce the template level of
template parameters when tf_partial. That caused infinite looping in
is_specialization_of: we ended up with a class template specialization
whose TREE_TYPE (CLASSTYPE_TI_TEMPLATE (t)) == t, so the second for
loop in is_specialization_of never finished.
There's a lot going on in this test, but essentially: the template fn
here has two template parameters, we call it with one explicitly
provided, the other one has to be deduced. So we'll find ourselves
in fn_type_unification which uses tf_partial when tsubsting the
*explicit* template arguments into the function type. That leads to
tsubstituting the return type, C<T>. C is a member template; its
most general template is
template<class U> template<class V> struct B<U>::C
we figure out (tsubst_template_args) that the template argument list
is <int, int>. They come from different levels, one comes from B<int>,
the other one from fn<int>.
So now we lookup_template_class to see if we have C<int, int>. We
do the
/* This is a full instantiation of a member template. Find
the partial instantiation of which this is an instance. */
TREE_VEC_LENGTH (arglist)--;
// arglist is now <int>, not <int, int>
found = tsubst (gen_tmpl, arglist, complain, NULL_TREE);
TREE_VEC_LENGTH (arglist)++;
magic which is looking for the partial instantiation, in this case,
that would be template<class V> struct B<int>::C. Note we're still
in a tf_partial context! So the tsubst_template_args in the tsubst
(which tries to substitute <int> into <U, V>) returns <int, V>, but
V's template level hasn't been reduced! After tsubst_template_args,
tsubst_template_decl looks to see if we already have this specialization:
// t = template_decl C
// full_args = <int, V>
spec = retrieve_specialization (t, full_args, hash);
but doesn't find the one we created a while ago, when processing
B<int> b; in the test, because V's levels don't match. Whereupon
tsubst_template_decl creates a new TEMPLATE_DECL, one that leads to
the infinite looping problem.
I think let's clear tf_partial when looking for an existing partial
instantiation.
It also occurred to me that I should be able to trigger a similar
problem with 'auto', since r10-7007 removed an is_auto check. And lo,
I constructed deduce10.C which exhibits the same issue with pre-r10-7007
compilers. This patch fixes that problem as well. I'm ecstatic.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/10? Also
built cmcstl2.
gcc/cp/ChangeLog:
PR c++/95888
* pt.c (lookup_template_class_1): Clear tf_partial when looking for
the partial instantiation.
gcc/testsuite/ChangeLog:
PR c++/95888
* g++.dg/template/deduce10.C: New test.
* g++.dg/template/deduce9.C: New test.
---
gcc/cp/pt.c | 10 +++++++++-
gcc/testsuite/g++.dg/template/deduce10.C | 23 +++++++++++++++++++++++
gcc/testsuite/g++.dg/template/deduce9.C | 23 +++++++++++++++++++++++
3 files changed, 55 insertions(+), 1 deletion(-)
create mode 100644 gcc/testsuite/g++.dg/template/deduce10.C
create mode 100644 gcc/testsuite/g++.dg/template/deduce9.C
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index f73deb3aee3..5b8a08a30e0 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -10137,7 +10137,15 @@ lookup_template_class_1 (tree d1, tree arglist, tree
in_decl, tree context,
/* Temporarily reduce by one the number of levels in the ARGLIST
so as to avoid comparing the last set of arguments. */
TREE_VEC_LENGTH (arglist)--;
- found = tsubst (gen_tmpl, arglist, complain, NULL_TREE);
+ /* Clear tf_partial, which can be set because we might be at the
+ beginning of template argument deduction when any explicitly
+ specified template arguments are substituted into the function
+ type. If we don't clear tf_partial, we won't find the partial
+ instantiation that might have been created outside tf_partial
+ context, because the levels of template parameters wouldn't
+ match, because in a tf_partial context, tsubst doesn't reduce
+ TEMPLATE_PARM_LEVEL. */
+ found = tsubst (gen_tmpl, arglist, complain & ~tf_partial, NULL_TREE);
TREE_VEC_LENGTH (arglist)++;
/* FOUND is either a proper class type, or an alias
template specialization. In the later case, it's a
diff --git a/gcc/testsuite/g++.dg/template/deduce10.C
b/gcc/testsuite/g++.dg/template/deduce10.C
new file mode 100644
index 00000000000..165ff195728
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce10.C
@@ -0,0 +1,23 @@
+// PR c++/95888
+// { dg-do compile { target c++17 } }
+
+template <typename T> class A {
+ A(int, int);
+ template <typename> friend class A;
+ friend T;
+};
+
+template<typename U> struct B {
+ template<auto V> struct C {
+ A<B> begin() { return {1, 0}; }
+ };
+ template<auto Z, int *P = nullptr>
+ C<Z> fn();
+};
+
+int
+main ()
+{
+ B<int> b;
+ b.fn<1>().begin();
+}
diff --git a/gcc/testsuite/g++.dg/template/deduce9.C
b/gcc/testsuite/g++.dg/template/deduce9.C
new file mode 100644
index 00000000000..5f55a84ed0a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/deduce9.C
@@ -0,0 +1,23 @@
+// PR c++/95888
+// { dg-do compile { target c++11 } }
+
+template <typename T> class A {
+ A(int, int);
+ template <typename> friend class A;
+ friend T;
+};
+
+template<typename U> struct B {
+ template<typename V> struct C {
+ A<B> begin() { return {1, 0}; }
+ };
+ template<typename Z, int *P = nullptr>
+ C<Z> fn();
+};
+
+int
+main ()
+{
+ B<int> b;
+ b.fn<int>().begin();
+}
base-commit: 20f28986a8d3cad3c848d1e7da48f4bea7637298
--
2.29.2
