C++ PATCH to fix ICE in strip_typedefs with -Wpadded (PR c++/79900)

[Marek Polacek <polacek at redhat dot com>]

We crash on an assert in strip_typedefs, because we find ourselves in a
scenario where RESULT, the main variant of a struct, was modified in
finalize_record_size (its TYPE_ALIGN changed), but its variants (T in
strip_typedefs) weren't fixed-up yet; that will happen slightly later in
finalize_type_size.  So there's a discrepancy that confuses the code.

This patch implements what Jason suggested to me on IRC, i.e., skip the
attribute handling if RESULT is complete and T isn't.

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

2017-03-08  Marek Polacek  <polacek@redhat.com>

	PR c++/79900 - ICE in strip_typedefs
	* tree.c (strip_typedefs): Skip the attribute handling if T is
	a variant type which hasn't been updated yet.

	* g++.dg/warn/Wpadded-1.C: New test.

diff --git gcc/cp/tree.c gcc/cp/tree.c
index d3c63b8..b3a4a10 100644
--- gcc/cp/tree.c
+++ gcc/cp/tree.c
@@ -1548,29 +1548,40 @@ strip_typedefs (tree t, bool *remove_attributes)
 	result = TYPE_MAIN_VARIANT (t);
     }
   gcc_assert (!typedef_variant_p (result));
-  if (TYPE_USER_ALIGN (t) != TYPE_USER_ALIGN (result)
-      || TYPE_ALIGN (t) != TYPE_ALIGN (result))
+
+  if (COMPLETE_TYPE_P (result) && !COMPLETE_TYPE_P (t))
+  /* If RESULT is complete and T isn't, it's likely the case that T
+     is a variant of RESULT which hasn't been updated yet.  Skip the
+     attribute handling.  */;
+  else
     {
-      gcc_assert (TYPE_USER_ALIGN (t));
-      if (remove_attributes)
-	*remove_attributes = true;
-      else
+      if (TYPE_USER_ALIGN (t) != TYPE_USER_ALIGN (result)
+	  || TYPE_ALIGN (t) != TYPE_ALIGN (result))
 	{
-	  if (TYPE_ALIGN (t) == TYPE_ALIGN (result))
-	    result = build_variant_type_copy (result);
+	  gcc_assert (TYPE_USER_ALIGN (t));
+	  if (remove_attributes)
+	    *remove_attributes = true;
 	  else
-	    result = build_aligned_type (result, TYPE_ALIGN (t));
-	  TYPE_USER_ALIGN (result) = true;
+	    {
+	      if (TYPE_ALIGN (t) == TYPE_ALIGN (result))
+		result = build_variant_type_copy (result);
+	      else
+		result = build_aligned_type (result, TYPE_ALIGN (t));
+	      TYPE_USER_ALIGN (result) = true;
+	    }
+	}
+
+      if (TYPE_ATTRIBUTES (t))
+	{
+	  if (remove_attributes)
+	    result = apply_identity_attributes (result, TYPE_ATTRIBUTES (t),
+						remove_attributes);
+	  else
+	    result = cp_build_type_attribute_variant (result,
+						      TYPE_ATTRIBUTES (t));
 	}
     }
-  if (TYPE_ATTRIBUTES (t))
-    {
-      if (remove_attributes)
-	result = apply_identity_attributes (result, TYPE_ATTRIBUTES (t),
-					    remove_attributes);
-      else
-	result = cp_build_type_attribute_variant (result, TYPE_ATTRIBUTES (t));
-    }
+
   return cp_build_qualified_type (result, cp_type_quals (t));
 }
 
diff --git gcc/testsuite/g++.dg/warn/Wpadded-1.C gcc/testsuite/g++.dg/warn/Wpadded-1.C
index e69de29..b3f0581 100644
--- gcc/testsuite/g++.dg/warn/Wpadded-1.C
+++ gcc/testsuite/g++.dg/warn/Wpadded-1.C
@@ -0,0 +1,22 @@
+// PR c++/79900 - ICE in strip_typedefs
+// { dg-do compile }
+// { dg-options "-Wpadded" }
+
+template <class> struct A;
+template <typename> struct B { // { dg-warning "padding struct size to alignment boundary" }
+  long _M_off;
+  int _M_state;
+};
+template <> struct A<char> { typedef B<int> pos_type; };
+enum _Ios_Openmode {};
+struct C {
+  typedef _Ios_Openmode openmode;
+};
+template <typename, typename _Traits> struct D {
+  typedef typename _Traits::pos_type pos_type;
+  pos_type m_fn1(pos_type, C::openmode);
+};
+template class D<char, A<char> >;
+template <typename _CharT, typename _Traits>
+typename D<_CharT, _Traits>::pos_type D<_CharT, _Traits>::m_fn1(pos_type x,
+                                                                C::openmode) { return x; }

	Marek