[C++ Patch] PR 84540 ("[6/7/8 Regression] ICE with alignas in variadic template")

[Paolo Carlini]

Hi,

this regression is an ICE on valid, and seems rather easy to analyze: 
both apply_late_template_attributes and tsubst_attributes don't appear 
to cope correctly with tsubst_attribute returning NULL_TREE. That, in 
turn, seems normal for an empty pack - as in the testcases - given the 
structure of the last part of the latter, handling PACK_EXPANSION_P 
(val). I'm finishing testing on x86_64-linux (in libstdc++-v3, so far so 
good).

Thanks, Paolo.

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

/cp
2018-02-26  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/84540
        * pt.c (tsubst_attributes): Handle correctly tsubst_attribute
        returning NULL_TREE.
        (apply_late_template_attributes): Likewise.

/testsuite
2018-02-26  Paolo Carlini  <paolo.carl...@oracle.com>

        PR c++/84540
        * g++.dg/cpp0x/alignas14.C: New.
        * g++.dg/cpp0x/alignas15.C: Likewise.
Index: cp/pt.c
===================================================================
--- cp/pt.c     (revision 257989)
+++ cp/pt.c     (working copy)
@@ -10245,9 +10245,8 @@ tsubst_attributes (tree attributes, tree args,
            if (subst != t)
              {
                *p = subst;
-               do
+               while (*p)
                  p = &TREE_CHAIN (*p);
-               while (*p);
                *p = TREE_CHAIN (t);
                continue;
              }
@@ -10314,9 +10313,8 @@ apply_late_template_attributes (tree *decl_p, tree
              *p = TREE_CHAIN (t);
              TREE_CHAIN (t) = NULL_TREE;
              *q = tsubst_attribute (t, decl_p, args, complain, in_decl);
-             do
+             while (*q)
                q = &TREE_CHAIN (*q);
-             while (*q);
            }
          else
            p = &TREE_CHAIN (t);
Index: testsuite/g++.dg/cpp0x/alignas14.C
===================================================================
--- testsuite/g++.dg/cpp0x/alignas14.C  (nonexistent)
+++ testsuite/g++.dg/cpp0x/alignas14.C  (working copy)
@@ -0,0 +1,6 @@
+// PR c++/84540
+// { dg-do compile { target c++11 } }
+
+template<typename... T> struct alignas(alignof(T)...) A {};
+
+A<> a;
Index: testsuite/g++.dg/cpp0x/alignas15.C
===================================================================
--- testsuite/g++.dg/cpp0x/alignas15.C  (nonexistent)
+++ testsuite/g++.dg/cpp0x/alignas15.C  (working copy)
@@ -0,0 +1,6 @@
+// PR c++/84540
+// { dg-do compile { target c++11 } }
+
+template<typename... T> struct A { enum alignas(alignof(T)...) E {}; };
+
+A<> a;