[PATCH] Fix ICE on invalid variable template instantiation (PR c++/72759)

[Patrick Palka]

This patch fixes PR c++/72759.  The problem seems to be that when
instantiating a variable template, we fail to propagate error_mark_node
when its template arguments are erroneous, and we instead build a bogus
TEMPLATE_ID_EXPR which later confuses check_initializer().  Does this
look OK to commit after bootstrap + regtesting?

gcc/cp/ChangeLog:

        PR c++/72759
        * pt.c (tsubst_qualified_id): Return error_mark_node if
        template_args is error_mark_node.

gcc/testsuite/ChangeLog:

        PR c++/72759
        * g++.dg/cpp1y/pr72759.C: New test.
---
 gcc/cp/pt.c                          |  3 +++
 gcc/testsuite/g++.dg/cpp1y/pr72759.C | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/pr72759.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index a23a05a..6b70a65 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13907,6 +13907,9 @@ tsubst_qualified_id (tree qualified_id, tree args,
 
   if (is_template)
     {
+      if (template_args == error_mark_node)
+       return error_mark_node;
+
       if (variable_template_p (expr))
        expr = lookup_and_finish_template_variable (expr, template_args,
                                                    complain);
diff --git a/gcc/testsuite/g++.dg/cpp1y/pr72759.C 
b/gcc/testsuite/g++.dg/cpp1y/pr72759.C
new file mode 100644
index 0000000..4af6ea4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr72759.C
@@ -0,0 +1,18 @@
+// PR c++/72759
+// { dg-do compile { target c++14 } }
+
+template <typename> struct SpecPerType;
+class Specializer {
+  public:  template <bool> static void MbrFnTempl();
+  template <unsigned> struct A { static void InnerMemberFn(); };
+  void Trigger() { A<0>::InnerMemberFn; }
+};
+template <> struct SpecPerType<Specializer> {
+  using FnType = void *;
+  template <bool P>
+  static constexpr FnType SpecMbrFnPtr = Specializer::MbrFnTempl<P>;
+};
+template <unsigned X> void Specializer::A<X>::InnerMemberFn() {
+  using Spec = SpecPerType<Specializer>;
+  Spec ErrorSite = Spec::SpecMbrFnPtr<SpecMbrFnPtr>;  // { dg-error "not 
declared" }
+}
-- 
2.9.2.564.g4d4f0b7