C++ PATCH for c++/89024 - ICE with incomplete enum type

[Marek Polacek]

On Fri, Jan 25, 2019 at 12:14:07PM -0500, Jason Merrill wrote:
> On 1/25/19 12:09 PM, Marek Polacek wrote:
> > On Fri, Jan 25, 2019 at 10:55:55AM -0600, Tim Song wrote:
> > > On Thu, Jan 24, 2019 at 4:14 PM Jason Merrill <jason@redhat.com> wrote:
> > > > 
> > > > On 1/24/19 2:16 PM, Marek Polacek wrote:
> > > > > This test ICEs since r159006 which added
> > > > > 
> > > > >      type = ENUM_UNDERLYING_TYPE (type);
> > > > > 
> > > > > to type_promotes_to.  In this test ENUM_UNDERLYING_TYPE is null because we
> > > > > haven't yet parsed '}' of the enum and the underlying type isn't fixed, and
> > > > > so checking TYPE_UNSIGNED crashed.
> > > > > 
> > > > > I've added some checks to the test to see if the types seem to be OK; clang++
> > > > > agrees.
> > > > > 
> > > > > Bootstrapped/regtested on x86_64-linux, ok for trunk/8/7?
> > > > > 
> > > > > 2019-01-24  Marek Polacek  <polacek@redhat.com>
> > > > > 
> > > > >        PR c++/89024 - ICE with incomplete enum type.
> > > > >        * cvt.c (type_promotes_to): Check if prom is non-null.
> > > > 
> > > > 9.6/6: An enumeration whose underlying type is not fixed is an
> > > > incomplete type from its point of declaration to immediately after the
> > > > closing } of its enum-specifier, at which point it becomes a complete type.
> > > > 
> > > > So the conversion is ill-formed.
> > > > 
> > > > Jason
> > > 
> > > But the conversion in the example (in
> > > decltype(__test_aux<_To1>(declval<_From1>())))
> > > is in a SFINAE context, so shouldn't it gracefully fall back to the
> > > `(...)` overload?
> > 
> > I think so, and clang++ and icc also compile the testcase fine (and we used to
> > too, before r159006).
> 
> Absolutely, the conversion being ill-formed means substitution fails, and we
> reject that candidate.  I meant that we shouldn't get as far as
> type_promotes_to for an incomplete type.

Makes sense.  So here's another attempt:

Bootstrapped/regtested on x86_64-linux, ok for trunk?

2019-01-25  Marek Polacek  <polacek@redhat.com>

	PR c++/89024 - ICE with incomplete enum type.
	* call.c (standard_conversion): When converting an
	ARITHMETIC_TYPE_P to an incomplete type, return NULL.
	
	* g++.dg/cpp0x/enum37.C: New test.

diff --git gcc/cp/call.c gcc/cp/call.c
index 515a9420032..c74d1b4ebdf 100644
--- gcc/cp/call.c
+++ gcc/cp/call.c
@@ -1412,6 +1412,13 @@ standard_conversion (tree to, tree from, tree expr, bool c_cast_p,
 	     || (fcode == REAL_TYPE && !(flags & LOOKUP_NO_NON_INTEGRAL)))
           || SCOPED_ENUM_P (from))
 	return NULL;
+
+      /* If we're parsing an enum with no fixed underlying type, we're
+	 dealing with an incomplete type, which renders the conversion
+	 ill-formed.  */
+      if (!COMPLETE_TYPE_P (from))
+	return NULL;
+
       conv = build_conv (ck_std, to, conv);
 
       /* Give this a better rank if it's a promotion.  */
diff --git gcc/testsuite/g++.dg/cpp0x/enum37.C gcc/testsuite/g++.dg/cpp0x/enum37.C
new file mode 100644
index 00000000000..6aa3d4015d7
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp0x/enum37.C
@@ -0,0 +1,24 @@
+// PR c++/89024
+// { dg-do compile { target c++11 } }
+
+template <class T, class U> struct same;
+template <class T> struct same<T,T> {};
+
+template<class T> T&& declval();
+
+template<typename _To1>
+void __test_aux(_To1);
+
+template<typename _From1, typename _To1,
+        typename = decltype(__test_aux<_To1>(declval<_From1>()))>
+char __test(int);
+
+template<typename, typename>
+int __test(...);
+
+enum E {
+    x = decltype(__test<E, int>(0))(0)
+};
+
+same<E,decltype(x)> s;
+same<unsigned int,__underlying_type(E)> s2;