Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/branches?

-- >8 --
Here we crash in decl_namespace_context because function types
(or pointer-to-data-member types) like void() have no TYPE_MAIN_DECL:

         decl = CP_DECL_CONTEXT (TYPE_MAIN_DECL (decl));

We got there from the constrain_class_visibility TYPE_FIELDS walk
which emits the -Wsubobject-linkage warning.  And this test has:

  namespace { struct P { }; }
  struct S { void (*f) (P *); };

I'm using OVERLOAD_TYPE_P, inspired by min_vis_r.

        PR c++/127672

gcc/cp/ChangeLog:

        * decl2.cc (constrain_class_visibility): Check OVERLOAD_TYPE_P
        before calling decl_anon_ns_mem_p.

gcc/testsuite/ChangeLog:

        * g++.dg/warn/Wsubobject-linkage-6.C: New test.
---
 gcc/cp/decl2.cc                                  |  1 +
 gcc/testsuite/g++.dg/warn/Wsubobject-linkage-6.C | 10 ++++++++++
 2 files changed, 11 insertions(+)
 create mode 100644 gcc/testsuite/g++.dg/warn/Wsubobject-linkage-6.C

diff --git a/gcc/cp/decl2.cc b/gcc/cp/decl2.cc
index 7fe567f98db..563fae5db0d 100644
--- a/gcc/cp/decl2.cc
+++ b/gcc/cp/decl2.cc
@@ -3598,6 +3598,7 @@ constrain_class_visibility (tree type)
                               type, t, nlt);
                  }
                else if (cxx_dialect > cxx98
+                        && OVERLOAD_TYPE_P (ftype)
                         && !decl_anon_ns_mem_p (ftype))
                  warning (OPT_Wsubobject_linkage, "\
 %qT has a field %q#D whose type has internal linkage",
diff --git a/gcc/testsuite/g++.dg/warn/Wsubobject-linkage-6.C 
b/gcc/testsuite/g++.dg/warn/Wsubobject-linkage-6.C
new file mode 100644
index 00000000000..ea77f14386f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wsubobject-linkage-6.C
@@ -0,0 +1,10 @@
+// PR c++/127672
+// { dg-do compile { target c++11 } }
+
+namespace { struct P { }; }
+
+#line 7 "foo.C"
+struct S { void (*f) (P *); };   // { dg-warning "uses the anonymous 
namespace" }
+struct S2 { void (P::*m) (); };          // { dg-warning "uses the anonymous 
namespace" }
+struct S3 { P (*g) (); };        // { dg-warning "uses the anonymous 
namespace" }
+struct S4 { int P::*d; };        // { dg-warning "uses the anonymous 
namespace" }

base-commit: f7094fe5ff882f0c206d51ebe93f33f0193ce636
-- 
2.55.0

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