[gcc r16-7096] c++: Error diagnostics for pointer-to-member type [PR38612]

[Jason Merrill]

https://gcc.gnu.org/g:6e9cf03a7602055b534a0bb1f0219d9d35104c2e

commit r16-7096-g6e9cf03a7602055b534a0bb1f0219d9d35104c2e
Author: Alex Yesmanchyk <aliaksandr.yesmanchyk@gmail.com>
Date:   Tue Jan 27 21:07:07 2026 -0500

    c++: Error diagnostics for pointer-to-member type [PR38612]
    
    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 Base::*' 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 Base::*' 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.yesmanchyk@gmail.com>
    Co-authored-by: Jason Merrill <jason@redhat.com>

Diff:
---
 gcc/cp/typeck2.cc                           | 12 ++++++++++--
 gcc/testsuite/g++.dg/diagnostic/ptrtomem4.C | 24 ++++++++++++++++++++++++
 2 files changed, 34 insertions(+), 2 deletions(-)

diff --git a/gcc/cp/typeck2.cc b/gcc/cp/typeck2.cc
index 1e5e3b24a233..f18c5a7499e8 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 000000000000..564c28b4300d
--- /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'" }
+}