[gcc r13-6377] c++: variable template and targ deduction [PR108550]

[Marek Polacek <mpolacek@gcc.gnu.org>]

https://gcc.gnu.org/g:d918c3a221434521f90cc9b63d5d87f5129e9231

commit r13-6377-gd918c3a221434521f90cc9b63d5d87f5129e9231
Author: Marek Polacek <polacek@redhat.com>
Date:   Tue Feb 21 19:13:59 2023 -0500

    c++: variable template and targ deduction [PR108550]
    
    In this test, we get a bogus error because we failed to deduce the auto in
    constexpr auto is_pointer_v = is_pointer<Tp>::value;
    to bool.  Then ensure_literal_type_for_constexpr_object thinks the object
    isn't literal and an error is reported.
    
    This is another case of the interaction between tf_partial and 'auto',
    where the auto was not reduced so the deduction failed.  In more detail:
    we have
    
      Wrap1<int>()
    
    in the code and we need to perform OR -> fn_type_unification.  The targ
    list is incomplete, so we do
          tsubst_flags_t ecomplain = complain | tf_partial | tf_fndecl_type;
          fntype = tsubst (TREE_TYPE (fn), explicit_targs, ecomplain, NULL_TREE);
    where TREE_TYPE (fn) is struct integral_constant <T402> (void).  Then
    we substitute the return type, which results in tsubsting is_pointer_v<int>.
    is_pointer_v is a variable template with a placeholder type:
    
      template <class Tp>
      constexpr auto is_pointer_v = is_pointer<Tp>::value;
    
    so we find ourselves in lookup_and_finish_template_variable.  tf_partial is
    still set, so finish_template_variable -> instantiate_template -> tsubst
    won't reduce the level of auto.  But then we do mark_used which eventually
    calls do_auto_deduction which clears tf_partial, because we want to replace
    the auto now.  But we hadn't reduced auto's level so this fails.  And
    since we're not in an immediate context, we emit a hard error.
    
    I suppose that when we reach lookup_and_finish_template_variable it's
    probably time to clear tf_partial.  (I added an assert and our testsuite
    doesn't have a test whereby we get to lookup_and_finish_template_variable
    while tf_partial is still active.)
    
            PR c++/108550
    
    gcc/cp/ChangeLog:
    
            * pt.cc (lookup_and_finish_template_variable): Clear tf_partial.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/var-templ70.C: New test.
            * g++.dg/cpp1y/var-templ71.C: New test.
            * g++.dg/cpp1y/var-templ72.C: New test.

Diff:
---
 gcc/cp/pt.cc                             |  6 ++++++
 gcc/testsuite/g++.dg/cpp1y/var-templ70.C | 25 +++++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1y/var-templ71.C | 26 ++++++++++++++++++++++++++
 gcc/testsuite/g++.dg/cpp1y/var-templ72.C | 27 +++++++++++++++++++++++++++
 4 files changed, 84 insertions(+)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 1a071e95004..f636bac5413 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10355,6 +10355,12 @@ lookup_and_finish_template_variable (tree templ, tree targs,
   if (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (templ)) == 1
       && !any_dependent_template_arguments_p (targs))
     {
+      /* We may be called while doing a partial substitution, but the
+	 type of the variable template may be auto, in which case we
+	 will call do_auto_deduction in mark_used (which clears tf_partial)
+	 and the auto must be properly reduced at that time for the
+	 deduction to work.  */
+      complain &= ~tf_partial;
       var = finish_template_variable (var, complain);
       mark_used (var);
     }
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ70.C b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
new file mode 100644
index 00000000000..1d35c38c7cc
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ70.C
@@ -0,0 +1,25 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct is_pointer
+{
+  static constexpr bool value = false;
+};
+
+template<class T, T T1>
+struct integral_constant
+{
+  static constexpr T value = T1;
+};
+
+
+template <class Tp>
+constexpr auto is_pointer_v = is_pointer<Tp>::value;
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<int>> Wrap1();
+
+int main() {
+  static_assert(!decltype(Wrap1<int>())::value, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ71.C b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
new file mode 100644
index 00000000000..e0cf55230d9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ71.C
@@ -0,0 +1,26 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T, T T1>
+struct integral_constant
+{
+  static constexpr T value = T1;
+};
+
+template <typename T>
+struct S {
+  template <typename U, typename V>
+  static constexpr void foo(V) { }
+
+  constexpr bool bar () const { foo<int>(10); return false; }
+};
+
+template <class Tp>
+constexpr auto is_pointer_v = S<Tp>{}.bar();
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<int>> Wrap1();
+
+int main() {
+  static_assert(!decltype(Wrap1<int>())::value, "");
+}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ72.C b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
new file mode 100644
index 00000000000..7426bad4a6c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ72.C
@@ -0,0 +1,27 @@
+// PR c++/108550
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct is_pointer
+{
+  static constexpr bool value = false;
+};
+
+template<class T, T T1>
+struct integral_constant
+{
+  static constexpr T value = T1;
+};
+
+template<typename T>
+using PTR_P = is_pointer<T>;
+
+template <class Tp>
+constexpr auto is_pointer_v = PTR_P<Tp>::value;
+
+template <class Tp, int = 0>
+integral_constant<bool, is_pointer_v<Tp>> Wrap1();
+
+int main() {
+  static_assert(!decltype(Wrap1<int>())::value, "");
+}