c++: can't eval PTRMEM_CST in incomplete class [PR107574]

Here we're attempting to evaluate a PTRMEM_CST in a class that hasn't
been completed yet, but that doesn't work:

        /* We can't lower this until the class is complete.  */
        if (!COMPLETE_TYPE_P (DECL_CONTEXT (member)))
          return cst;

and then this unlowered PTRMEM_CST is used as EXPR in

    tree op1 = build_nop (ptrdiff_type_node, expr);

and we crash in a subsequent cp_fold_convert which gets type=ptrdiff_type_node,
expr=PTRMEM_CST and does

  else if (TREE_CODE (expr) == PTRMEM_CST
           && same_type_p (TYPE_PTRMEM_CLASS_TYPE (type),
                           PTRMEM_CST_CLASS (expr)))

where TYPE_PTRMEM_CLASS_TYPE (type) is going to crash since the type
is ptrdiff_type_node.  We could just add a TYPE_PTRMEM_P check before
accessing TYPE_PTRMEM_CLASS_TYPE but I think it's nicer to explain why
we couldn't evaluate the expression.

	PR c++/107574

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_constant_expression): Emit an error when
	a PTRMEM_CST cannot be evaluated.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/ptrmem-cst1.C: New test.

gcc/cp/constexpr.cc

@@ -7662,7 +7662,18 @@ cxx_eval_constant_expression (const constexpr_ctx *ctx, tree t,
 	  }
 
 	if (TREE_CODE (op) == PTRMEM_CST && !TYPE_PTRMEM_P (type))
-	  op = cplus_expand_constant (op);
+	  {
+	    op = cplus_expand_constant (op);
+	    if (TREE_CODE (op) == PTRMEM_CST)
+	      {
+		if (!ctx->quiet)
+		  error_at (loc, "%qE is not a constant expression when the "
+			    "class %qT is still incomplete", op,
+			    PTRMEM_CST_CLASS (op));
+		*non_constant_p = true;
+		return t;
+	      }
+	  }
 
 	if (TREE_CODE (op) == PTRMEM_CST && tcode == NOP_EXPR)
 	  {

gcc/testsuite/g++.dg/cpp0x/ptrmem-cst1.C

@@ -0,0 +1,11 @@
+// PR c++/107574
+// { dg-do compile { target c++11 } }
+
+struct A { int i; };
+struct B:A { int j; };
+struct C:B {
+  int k;
+  static_assert((int B::*) &C::k, ""); // { dg-error "non-constant|still incomplete" }
+};
+
+static_assert((int B::*) &C::k, "");