[C++ Patch/RFC] PR 83796 ("[6/7/8 Regression] Abstract classes allowed to be instantiated when initialised as default parameter to function or constructor")

[Paolo Carlini]

Hi,

I'm looking into this rather mild regression, which should be relatively 
easy to fix. In short, Jason's fix for c++/54325 moved an 
abstract_virtuals_error_sfinae check from build_aggr_init_expr to 
build_cplus_new therefore the testcase in this new bug isn't rejected 
anymore because a special conditional for value-initialization from { } 
in convert_like_real simply calls build_value_init and quickly returns, 
thus build_cplus_new isn't involved. Thus I'm working on the best way to 
add back the check. The below, which also uses cp_unevaluated_operand, 
appears to work. Likewise something similar inside build_value_init 
itself, which however seems too generic to me (build_value_init is 
called in many other cases). I'm also not sure about 
cp_unevaluated_operand, whether we need something more precise.

Thanks! Paolo.

//////////////////////

Index: cp/call.c
===================================================================
--- cp/call.c   (revision 257013)
+++ cp/call.c   (working copy)
@@ -6765,6 +6765,9 @@ convert_like_real (conversion *convs, tree expr, t
            && TYPE_HAS_DEFAULT_CONSTRUCTOR (totype))
          {
            bool direct = CONSTRUCTOR_IS_DIRECT_INIT (expr);
+           if (cp_unevaluated_operand
+               && abstract_virtuals_error_sfinae (NULL_TREE, totype, complain))
+             return error_mark_node;
            expr = build_value_init (totype, complain);
            expr = get_target_expr_sfinae (expr, complain);
            if (expr != error_mark_node)
Index: testsuite/g++.dg/cpp0x/abstract-default1.C
===================================================================
--- testsuite/g++.dg/cpp0x/abstract-default1.C  (nonexistent)
+++ testsuite/g++.dg/cpp0x/abstract-default1.C  (working copy)
@@ -0,0 +1,26 @@
+// PR c++/83796
+// { dg-do compile { target c++11 } }
+
+struct MyAbstractClass
+{
+  virtual int foo() const = 0;
+};
+
+struct TestClass
+{
+  TestClass(const MyAbstractClass& m = {})  // { dg-error "abstract type" }
+  : value_(m.foo()) {}
+
+  int value_;
+};
+
+int TestFunction(const MyAbstractClass& m = {})  // { dg-error "abstract type" 
}
+{
+  return m.foo();
+}
+
+int main()
+{
+  TestClass testInstance;
+  TestFunction();
+}