[PATCH] c++: coercing variable template from current inst [PR110580]

[Patrick Palka via Gcc-patches]

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for trunk?

-- >8 --

Here during ahead of time coercion of the variable template-id v1<int>,
since we pass only the innermost arguments to coerce_template_parms (and
outer arguments are still dependent at this point), substitution of the
default template argument V=U prematurely lowers U from level 2 to level 1.
Thus we incorrectly resolve v1<int> to v1<int, T> (effectively) instead
of to v1<int, int>.

Coercion of a class/alias template-id on the other hand is always done
using the full set of arguments relative to the most general template,
so ahead of time coercion there does the right thing.  I suppose we
should do the same for variable template-ids.

        PR c++/110580

gcc/cp/ChangeLog:

        * pt.cc (lookup_template_variable): Pass all arguments to
        coerce_template_parms, and use the innermost parameters from
        the most general template.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1y/var-templ83.C: New test.
---
 gcc/cp/pt.cc                             |  4 +++-
 gcc/testsuite/g++.dg/cpp1y/var-templ83.C | 16 ++++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ83.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 076f788281e..fa15b75b9c5 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10345,7 +10345,9 @@ lookup_template_variable (tree templ, tree arglist, 
tsubst_flags_t complain)
   if (flag_concepts && variable_concept_p (templ))
     return build_concept_check (templ, arglist, tf_none);
 
-  tree parms = DECL_INNERMOST_TEMPLATE_PARMS (templ);
+  tree gen_templ = most_general_template (templ);
+  tree parms = DECL_INNERMOST_TEMPLATE_PARMS (gen_templ);
+  arglist = add_outermost_template_args (templ, arglist);
   arglist = coerce_template_parms (parms, arglist, templ, complain);
   if (arglist == error_mark_node)
     return error_mark_node;
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ83.C 
b/gcc/testsuite/g++.dg/cpp1y/var-templ83.C
new file mode 100644
index 00000000000..f5268f258d7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ83.C
@@ -0,0 +1,16 @@
+// PR c++/110580
+// { dg-do compile { target c++14 } }
+
+template<class T>
+struct A {
+  template<typename U, typename V = U>
+  static constexpr bool v1 = __is_same(U, V);
+
+  template<typename U, typename V = T>
+  static constexpr bool v2 = !__is_same(U, V);
+
+  static_assert(v1<int>, "");
+  static_assert(v2<int>, "");
+};
+
+template struct A<char>;
-- 
2.41.0.327.gaa9166bcc0