[C++ Patch] PR 81054 ("[7/8 Regression] ICE with volatile variable in constexpr function") [Take 2]

[Paolo Carlini]

Hi again,

thus I figured out what was badly wrong in my first try: I misread 
ensure_literal_type_for_constexpr_object and missed that it can return 
NULL_TREE without emitting an hard error. Thus my first try even caused 
miscompilations :( Anyway, when DECL_DECLARED_CONSTEXPR_P is true we are 
safe and indeed we want to clear it as matter of error recovery. Then, 
in this safe case the only change in the below is returning early, thus 
avoiding any internal inconsistencies later and also the redundant / 
misleading diagnostic which I already mentioned. Testing on x86_64-linux 
is still in progress - in libstdc++ - but I separately checked that all 
the regressions are gone.

Thanks! Paolo.

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


/cp
2018-01-16  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/81054
        * decl.c (cp_finish_decl): Early return when
        ensure_literal_type_for_constexpr_object returns NULL_TREE 
        and DECL_DECLARED_CONSTEXPR_P is true.

/testsuite
2018-01-16  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/81054
        * g++.dg/cpp0x/constexpr-ice19.C: New.
Index: cp/decl.c
===================================================================
--- cp/decl.c   (revision 256753)
+++ cp/decl.c   (working copy)
@@ -6810,8 +6810,12 @@ cp_finish_decl (tree decl, tree init, bool init_co
       cp_apply_type_quals_to_decl (cp_type_quals (type), decl);
     }
 
-  if (!ensure_literal_type_for_constexpr_object (decl))
-    DECL_DECLARED_CONSTEXPR_P (decl) = 0;
+  if (!ensure_literal_type_for_constexpr_object (decl)
+      && DECL_DECLARED_CONSTEXPR_P (decl))
+    {
+      DECL_DECLARED_CONSTEXPR_P (decl) = 0;
+      return;
+    }
 
   if (VAR_P (decl)
       && DECL_CLASS_SCOPE_P (decl)
Index: testsuite/g++.dg/cpp0x/constexpr-ice19.C
===================================================================
--- testsuite/g++.dg/cpp0x/constexpr-ice19.C    (nonexistent)
+++ testsuite/g++.dg/cpp0x/constexpr-ice19.C    (working copy)
@@ -0,0 +1,13 @@
+// PR c++/81054
+// { dg-do compile { target c++11 } }
+
+struct A
+{
+  volatile int i;
+  constexpr A() : i() {}
+};
+
+struct B
+{
+  static constexpr A a {};  // { dg-error "not literal" }
+};