C++/c-common PATCH to conversion from scoped enum to bool (part of 48450)

[Jason Merrill]

PR 48450 has to do with SFINAE bugs, but one of them turns out to be a 
different sort of bug: we were failing to convert from a non-constant 
value of scoped enum type to bool, because that conversion was doing a 
!= 0, which requires an implicit conversion to int.  So now we 
explicitly convert to the underlying integral type first, and prevent 
c-common from helpfully removing it again.

Tested x86_64-pc-linux-gnu, applying to trunk.
commit 87823d7d9dab304517871340e7b3aba6b73afe16
Author: Jason Merrill <ja...@redhat.com>
Date:   Tue Apr 5 15:02:14 2011 -0400
        PR c++/48450
        * c-family/c-common.c (c_common_truthvalue_conversion): Don't ignore
        conversion from C++0x scoped enum.
        * cp/cvt.c (ocp_convert): Handle converting scoped enum to bool.

diff --git a/gcc/c-family/c-common.c b/gcc/c-family/c-common.c
index 1252b18..e0acfea 100644
--- a/gcc/c-family/c-common.c
+++ b/gcc/c-family/c-common.c
@@ -3939,16 +3939,25 @@ c_common_truthvalue_conversion (location_t location, 
tree expr)
        }
 
     CASE_CONVERT:
-      /* Don't cancel the effect of a CONVERT_EXPR from a REFERENCE_TYPE,
-        since that affects how `default_conversion' will behave.  */
-      if (TREE_CODE (TREE_TYPE (expr)) == REFERENCE_TYPE
-         || TREE_CODE (TREE_TYPE (TREE_OPERAND (expr, 0))) == REFERENCE_TYPE)
-       break;
-      /* If this is widening the argument, we can ignore it.  */
-      if (TYPE_PRECISION (TREE_TYPE (expr))
-         >= TYPE_PRECISION (TREE_TYPE (TREE_OPERAND (expr, 0))))
-       return c_common_truthvalue_conversion (location,
-                                              TREE_OPERAND (expr, 0));
+      {
+       tree totype = TREE_TYPE (expr);
+       tree fromtype = TREE_TYPE (TREE_OPERAND (expr, 0));
+
+       /* Don't cancel the effect of a CONVERT_EXPR from a REFERENCE_TYPE,
+          since that affects how `default_conversion' will behave.  */
+       if (TREE_CODE (totype) == REFERENCE_TYPE
+           || TREE_CODE (fromtype) == REFERENCE_TYPE)
+         break;
+       /* Don't strip a conversion from C++0x scoped enum, since they
+          don't implicitly convert to other types.  */
+       if (TREE_CODE (fromtype) == ENUMERAL_TYPE
+           && ENUM_IS_SCOPED (fromtype))
+         break;
+       /* If this isn't narrowing the argument, we can ignore it.  */
+       if (TYPE_PRECISION (totype) >= TYPE_PRECISION (fromtype))
+         return c_common_truthvalue_conversion (location,
+                                                TREE_OPERAND (expr, 0));
+      }
       break;
 
     case MODIFY_EXPR:
diff --git a/gcc/cp/cvt.c b/gcc/cp/cvt.c
index 8ab0001..290b926 100644
--- a/gcc/cp/cvt.c
+++ b/gcc/cp/cvt.c
@@ -727,7 +727,13 @@ ocp_convert (tree type, tree expr, int convtype, int flags)
          return error_mark_node;
        }
       if (code == BOOLEAN_TYPE)
-       return cp_truthvalue_conversion (e);
+       {
+         /* We can't implicitly convert a scoped enum to bool, so convert
+            to the underlying type first.  */
+         if (SCOPED_ENUM_P (intype) && (convtype & CONV_STATIC))
+           e = convert (ENUM_UNDERLYING_TYPE (intype), e);
+         return cp_truthvalue_conversion (e);
+       }
 
       converted = fold_if_not_in_template (convert_to_integer (type, e));
 
diff --git a/gcc/testsuite/g++.dg/cpp0x/enum9.C 
b/gcc/testsuite/g++.dg/cpp0x/enum9.C
new file mode 100644
index 0000000..10e510b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/enum9.C
@@ -0,0 +1,5 @@
+// { dg-options -std=c++0x }
+
+enum class E { };
+E f();
+bool b2 = static_cast<bool>(f());