[PATCH v2] c++: Error diagnostics for pointer-to-member type [PR38612]

[Alex Yesmanchyk]

Consider the following ptrtomem4.C file.

Now for test1 and test3 it produces the following errors:

ptrtomem4.C: In function 'int test1(int Base::*, X*)':
ptrtomem4.C:8:21: error: pointer to member type 'int' incompatible with 
incomplete object type 'X'
ptrtomem4.C: In function 'int test3(int Base::*, Y*)':
ptrtomem4.C:22:21: error: pointer to member type 'int' incompatible with object 
type 'Y' because 'Y' is not derived from 'Base'

        PR c++/38612

gcc/cp/ChangeLog:

        * typeck2.cc (build_m_component_ref): Improve class mismatch
        diagnostic.

gcc/testsuite/ChangeLog:

        * g++.dg/diagnostic/ptrtomem4.C: New test.

Signed-off-by: Alex Yesmanchyk <aliaksandr.yesmanc...@gmail.com>
Co-authored-by: Jason Merrill <ja...@redhat.com>
---
 gcc/cp/typeck2.cc                           | 12 +++++++++--
 gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C | 24 +++++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1e5e3b24a23..f18c5a7499e 100644
--- a/gcc/cp/typeck2.cc
+++ b/gcc/cp/typeck2.cc
@@ -2459,8 +2459,16 @@ build_m_component_ref (tree datum, tree component, 
tsubst_flags_t complain)
        {
        mismatch:
          if (complain & tf_error)
-           error ("pointer to member type %qT incompatible with object "
-                  "type %qT", type, objtype);
+           {
+             if (COMPLETE_TYPE_P (objtype))
+               error ("pointer to member type %qT incompatible "
+                      "with object type %qT because %qT is not "
+                      "derived from %qT", ptrmem_type, objtype,
+                      objtype, ctype);
+             else
+               error ("pointer to member type %qT incompatible with "
+                      "incomplete object type %qT", ptrmem_type, objtype);
+           }
          return error_mark_node;
        }
       else if (binfo == error_mark_node)
diff --git a/gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C 
b/gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C
new file mode 100644
index 00000000000..564c28b4300
--- /dev/null
+++ b/gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C
@@ -0,0 +1,24 @@
+// PR c++/38612
+
+struct Base {};
+struct X;    // X derived from Base later but incomplete here
+struct Y {}; // Y not derived from Base
+
+int test1(int Base::* p2m, X* object)
+{
+    return object->*p2m; // { dg-error "int Base::.' incompatible with 
incomplete object type 'X'" }
+}
+
+struct X : Base
+{
+};
+
+int test2(int Base::* p2m, X* object)
+{
+    return object->*p2m;        // OK
+}
+
+int test3(int Base::* p2m, Y* object)
+{
+    return object->*p2m;        // { dg-error "int Base::.' incompatible with 
object type 'Y' because 'Y' is not derived from 'Base'" }
+}
-- 
2.52.0