[pushed] c++: recursive instantiation diagnostic [PR120204]

[Jason Merrill]

Tested x86_64-pc-linux-gnu, applying to trunk.

-- 8< --

Here tsubst_baselink was returning error_mark_node silently despite
tf_error; we need to actually give an error.

        PR c++/120204

gcc/cp/ChangeLog:

        * pt.cc (tsubst_baselink): Always error if lookup fails.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp1y/constexpr-recursion3.C: New test.
---
 gcc/cp/pt.cc                                     | 16 ++++++++++++----
 .../g++.dg/cpp1y/constexpr-recursion3.C          | 15 +++++++++++++++
 2 files changed, 27 insertions(+), 4 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 09f74a2814b..0d64a1cfb12 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -17477,10 +17477,18 @@ tsubst_baselink (tree baselink, tree object_type,
 
       if (!baselink)
        {
-         if ((complain & tf_error)
-             && constructor_name_p (name, qualifying_scope))
-           error ("cannot call constructor %<%T::%D%> directly",
-                  qualifying_scope, name);
+         if (complain & tf_error)
+           {
+             if (constructor_name_p (name, qualifying_scope))
+               error ("cannot call constructor %<%T::%D%> directly",
+                      qualifying_scope, name);
+             else
+               /* Lookup succeeded at parse time, but failed during
+                  instantiation; must be because we're trying to refer to it
+                  while forming its declaration (c++/120204).  */
+               error ("declaration of %<%T::%D%> depends on itself",
+                      qualifying_scope, name);
+           }
          return error_mark_node;
        }
 
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C 
b/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C
new file mode 100644
index 00000000000..cadbdf08c97
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-recursion3.C
@@ -0,0 +1,15 @@
+// PR c++/120204
+// { dg-do compile { target c++14 } }
+
+template<class T, int t>
+struct array{};
+
+template <typename... TArgs> struct ILEArglist {
+  using Sizes = array<int, sizeof...(TArgs)>;
+  static constexpr int size() {        // { dg-bogus "not usable" }
+    Sizes &offsets_c = offsets;        // { dg-error "depends on itself" }
+    return 0;
+  }
+  array<char, size()> offsets(); // { dg-error "constant expression" }
+};
+auto arglist = ILEArglist<>();

base-commit: d50e08095b57131e6f1a80b45959087e233376e8
-- 
2.49.0