c++: Fix up build_m_component_ref [PR113599]

The following testcase reduced from GDB is miscompiled starting with
r14-5503 PR112427 change.
The problem is in the build_m_component_ref hunk, which changed
-      datum = fold_build_pointer_plus (fold_convert (ptype, datum), component);
+      datum = cp_convert (ptype, datum, complain);
+      if (!processing_template_decl)
+       datum = build2 (POINTER_PLUS_EXPR, ptype,
+                       datum, convert_to_ptrofftype (component));
+      datum = cp_fully_fold (datum);
Component is e, (sizetype) e is 16, offset of c inside of C.
ptype is A *, pointer to type of C::c and datum is &d.
Now, previously the above created ((A *) &d) p+ (sizetype) e which is correct,
but in the new code cp_convert sees that C has A as base class and
instead of returning (A *) &d, it returns &d.D.2800 where D.2800 is
the FIELD_DECL for the A base at offset 8 into C.
So, instead of computing ((A *) &d) p+ (sizetype) e it computes
&d.D.2800 p+ (sizetype) e, which is ((A *) &d) p+ 24.

The following patch fixes it by using convert instead of cp_convert which
eventually calls build_nop (ptype, datum).

2024-01-26  Jakub Jelinek  <jakub@redhat.com>

	PR c++/113599
	* typeck2.cc (build_m_component_ref): Use convert instead of
	cp_convert for pointer conversion.

	* g++.dg/expr/ptrmem11.C: New test.

Author: jakubjelinek

gcc/cp/typeck2.cc

@@ -2378,7 +2378,7 @@ build_m_component_ref (tree datum, tree component, tsubst_flags_t complain)
       /* Build an expression for "object + offset" where offset is the
 	 value stored in the pointer-to-data-member.  */
       ptype = build_pointer_type (type);
-      datum = cp_convert (ptype, datum, complain);
+      datum = convert (ptype, datum);
       if (!processing_template_decl)
 	datum = build2 (POINTER_PLUS_EXPR, ptype,
 			datum, convert_to_ptrofftype (component));

gcc/testsuite/g++.dg/expr/ptrmem11.C

@@ -0,0 +1,17 @@
+// PR c++/113599
+// { dg-do run }
+
+struct A { void *a; };
+struct B { void *b; };
+struct C : public B, public A { A c; };
+static C d;
+
+int
+main ()
+{
+  A C::*e = &C::c;
+  A *f = &(d.*e);
+  A *g = &d.c;
+  if (f != g)
+    __builtin_abort ();
+}