In discussion of jakub's patch for C++20 pointer-interconvertibility, it
came up that we allow anonymous structs to have bases, but don't do anything
usable with them.  Let's reject it.

The comment change is something I noticed while looking for the right place
to diagnose this: finish_struct_anon does not actually check for anything
invalid, so it shouldn't claim to.

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

gcc/cp/ChangeLog:

        * class.c (finish_struct_anon): Improve comment.
        * decl.c (fixup_anonymous_aggr): Reject anonymous struct
        with bases.

gcc/testsuite/ChangeLog:

        * g++.dg/ext/anon-struct8.C: New test.
---
 gcc/cp/class.c                          | 3 +--
 gcc/cp/decl.c                           | 3 +++
 gcc/testsuite/g++.dg/ext/anon-struct8.C | 9 +++++++++
 3 files changed, 13 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/ext/anon-struct8.C

diff --git a/gcc/cp/class.c b/gcc/cp/class.c
index 14db06692dc..6f31700c06c 100644
--- a/gcc/cp/class.c
+++ b/gcc/cp/class.c
@@ -3072,8 +3072,7 @@ finish_struct_anon_r (tree field)
     }
 }
 
-/* Check for things that are invalid.  There are probably plenty of other
-   things we should check for also.  */
+/* Fix up any anonymous union/struct members of T.  */
 
 static void
 finish_struct_anon (tree t)
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 01d64a16125..71308a06c63 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -5084,6 +5084,9 @@ fixup_anonymous_aggr (tree t)
     {
       tree field, type;
 
+      if (BINFO_N_BASE_BINFOS (TYPE_BINFO (t)))
+       error_at (location_of (t), "anonymous struct with base classes");
+
       for (field = TYPE_FIELDS (t); field; field = DECL_CHAIN (field))
        if (TREE_CODE (field) == FIELD_DECL)
          {
diff --git a/gcc/testsuite/g++.dg/ext/anon-struct8.C 
b/gcc/testsuite/g++.dg/ext/anon-struct8.C
new file mode 100644
index 00000000000..f4e3f11b678
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ext/anon-struct8.C
@@ -0,0 +1,9 @@
+// { dg-options "" }
+
+struct A { };
+struct B {
+  struct: A { int i; };                // { dg-error "anonymous struct with 
base" }
+};
+union U {
+  struct: A { int i; };                // { dg-error "anonymous struct with 
base" }
+};

base-commit: 0ba2003cf306aa98b6ec91c9d849ab9bafcf17c2
-- 
2.27.0

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