Re: [PATCH] c++: Fix ICE in tsubst_default_argument [PR92010]

[Jason Merrill via Gcc-patches]

On 4/7/20 1:40 PM, Patrick Palka wrote:
On Mon, 6 Apr 2020, Jason Merrill wrote:
On 4/6/20 11:45 AM, Patrick Palka wrote:
On Wed, 1 Apr 2020, Jason Merrill wrote:
On 4/1/20 6:29 PM, Jason Merrill wrote:
On 3/31/20 3:50 PM, Patrick Palka wrote:
On Tue, 31 Mar 2020, Jason Merrill wrote:

On 3/30/20 6:46 PM, Patrick Palka wrote:
On Mon, 30 Mar 2020, Jason Merrill wrote:
On 3/30/20 3:58 PM, Patrick Palka wrote:
On Thu, 26 Mar 2020, Jason Merrill wrote:

On 3/22/20 9:21 PM, Patrick Palka wrote:
This patch relaxes an assertion in tsubst_default_argument
that
exposes
a
latent
bug in how we substitute an array type into a cv-qualified
wildcard
function
parameter type.  Concretely, the latent bug is that given
the
function
template

        template<typename T> void foo(const T t);

one would expect the type of foo<int[]> to be void(const
int*), but
we
(seemingly prematurely) strip function parameter types of
their
top-level
cv-qualifiers when building the function's TYPE_ARG_TYPES,
and
instead
end
up
obtaining void(int*) as the type of foo<int[]> after
substitution
and
decaying.

We still however correctly substitute into and decay the
formal
parameter
type,
obtaining const int* as the type of t after substitution.
But
this
then
leads
to us tripping over the assert in tsubst_default_argument
that
verifies
the
formal parameter type and the function type are
consistent.

Assuming it's too late at this stage to fix the
substitution
bug, we
can
still
relax the assertion like so.  Tested on
x86_64-pc-linux-gnu,
does
this
look
OK?

This is core issues 1001/1322, which have not been resolved.
Clang
does
the
substitution the way you suggest; EDG rejects the testcase
because the
two
substitutions produce different results.  I think it would
make
sense
to
follow the EDG behavior until this issue is actually
resolved.

Here is what I have so far towards that end.  When
substituting
into the
PARM_DECLs of a function decl, we now additionally check if
the
aforementioned Core issues are relevant and issue a (fatal)
diagnostic
if so.  This patch checks this in tsubst_decl <case PARM_DECL>
rather
than in tsubst_function_decl for efficiency reasons, so that
we
don't
have to perform another traversal over the DECL_ARGUMENTS /
TYPE_ARG_TYPES just to implement this check.

Hmm, this seems like writing more complicated code for a very
marginal
optimization; how many function templates have so many
parameters
that
walking
over them once to compare types will have any effect on compile
time?

Good point... though I just tried implementing this check in
tsubst_function_decl, and it seems it might be just as complicated
to
implement it there instead, at least if we want to handle function
parameter packs correctly.

If we were to implement this check in tsubst_function_decl, then
since
we have access to the instantiated function, it would presumably
suffice
to compare its substituted DECL_ARGUMENTS with its substituted
TYPE_ARG_TYPES to see if they're consistent.  Doing so would
certainly
catch the original testcase, i.e.

      template<typename T>
        void foo(const T);
      int main() { foo<int[]>(0); }

because the DECL_ARGUMENTS of foo<int[]> would be {const int*} and
its
TYPE_ARG_TYPES would be {int*}.  But apparently it doesn't catch
the
corresponding testcase that uses a function parameter pack, i.e.

      template<typename... Ts>
        void foo(const Ts...);
      int main() { foo<int[]>(0); }

because it turns out we don't strip top-level cv-qualifiers from
function parameter packs from TYPE_ARG_TYPES at declaration time,
as
we
do with regular function parameters.  So in this second testcase
both
DECL_ARGUMENTS and TYPE_ARG_TYPES of foo<int[]> would be {const
int*},
and yet we would (presumably) want to reject this instantiation
too.

So it seems comparing TYPE_ARG_TYPES and DECL_ARGUMENTS from
tsubst_function_decl would not suffice, and we would still need to
do
a
variant of the trick that's done in this patch, i.e. substitute
into
each dependent parameter type stripped of its top-level
cv-qualifiers,
to see if these cv-qualifiers make a material difference in the
resulting function type.  Or maybe there's yet another way to
detect
this?

I think let's go ahead with comparing TYPE_ARG_TYPES and
DECL_ARGUMENTS;
the
problem comes when they disagree.  If we're handling pack expansions
wrong,
that's a separate issue.

Hm, comparing TYPE_ARG_TYPES and DECL_ARGUMENTS for compatibility
seems
to be exposing a latent bug with how we handle lambdas that appear in
function parameter types.  Take g++.dg/cpp2a/lambda-uneval3.C for
example:

       template <class T> void spam(decltype([]{}) (*s)[sizeof(T)]) {}
       int main() { spam<char>(nullptr); }

According to tsubst_function_decl in current trunk, the type of the
function paremeter 's' of spam<char> according to its TYPE_ARG_TYPES
is
       struct ._anon_4[1] *
and according to its DECL_ARGUMENTS the type of 's' is
       struct ._anon_5[1] *

The disagreement happens because we call tsubst_lambda_expr twice
during
substitution and thereby generate two distinct lambda types, one when
substituting into the TYPE_ARG_TYPES and another when substituting
into
the DECL_ARGUMENTS.  I'm not sure how to work around this
bug/false-positive..

Oof.

I think probably the right answer is to rebuild TYPE_ARG_TYPES from
DECL_ARGUMENTS if they don't match.

...and treat that as a resolution of 1001/1322, so not giving an error.

Is something like this what you have in mind?  Bootstrap and testing in
progress.

Yes, thanks.

-- >8 --

Subject: [PATCH] c++: Rebuild function type when it disagrees with formal
   parameter types [PR92010]

gcc/cp/ChangeLog:

        Core issues 1001 and 1322
        PR c++/92010
        * pt.c (maybe_rebuild_function_type): New function.
        (tsubst_function_decl): Use it.

gcc/testsuite/ChangeLog:

        Core issues 1001 and 1322
        PR c++/92010
        * g++.dg/cpp2a/lambda-uneval11.c: New test.
        * g++.dg/template/array33.C: New test.
        * g++.dg/template/array34.C: New test.
        * g++.dg/template/defarg22.C: New test.
---
   gcc/cp/pt.c                                  | 55 +++++++++++++++++
   gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C | 10 ++++
   gcc/testsuite/g++.dg/template/array33.C      | 63 ++++++++++++++++++++
   gcc/testsuite/g++.dg/template/array34.C      | 63 ++++++++++++++++++++
   gcc/testsuite/g++.dg/template/defarg22.C     | 13 ++++
   5 files changed, 204 insertions(+)
   create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C
   create mode 100644 gcc/testsuite/g++.dg/template/array33.C
   create mode 100644 gcc/testsuite/g++.dg/template/array34.C
   create mode 100644 gcc/testsuite/g++.dg/template/defarg22.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 041ce35a31c..fc0df790c0f 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13475,6 +13475,59 @@ lookup_explicit_specifier (tree v)
     return *explicit_specifier_map->get (v);
   }
   +/* Check if the function type of DECL, a FUNCTION_DECL, agrees with the
type of
+   each of its formal parameters.  If there is a disagreement then rebuild
+   DECL's function type according to its formal parameter types, as part of
a
+   resolution for Core issues 1001/1322.  */
+
+static void
+maybe_rebuild_function_decl_type (tree decl)
+{
+  bool function_type_needs_rebuilding = false;
+  if (tree parm_list = FUNCTION_FIRST_USER_PARM (decl))
+    {
+      tree parm_type_list = FUNCTION_FIRST_USER_PARMTYPE (decl);
+      while (parm_type_list && parm_type_list != void_list_node)
+       {
+         tree parm_type = TREE_VALUE (parm_type_list);
+         tree formal_parm_type_unqual = strip_top_quals (TREE_TYPE
(parm_list));
+         if (!same_type_p (parm_type, formal_parm_type_unqual))
+           {
+             function_type_needs_rebuilding = true;
+             break;
+           }
+
+         parm_list = DECL_CHAIN (parm_list);
+         parm_type_list = TREE_CHAIN (parm_type_list);
+       }
+    }
+
+  if (!function_type_needs_rebuilding)
+    return;
+
+  const tree new_arg_types = copy_list (TYPE_ARG_TYPES (TREE_TYPE (decl)));
+
+  tree parm_list = FUNCTION_FIRST_USER_PARM (decl);
+  tree old_parm_type_list = FUNCTION_FIRST_USER_PARMTYPE (decl);
+  tree new_parm_type_list = skip_artificial_parms_for (decl,
new_arg_types);
+  while (old_parm_type_list && old_parm_type_list != void_list_node)
+    {
+      tree *new_parm_type = &TREE_VALUE (new_parm_type_list);
+      tree formal_parm_type_unqual = strip_top_quals (TREE_TYPE
(parm_list));
+      if (!same_type_p (*new_parm_type, formal_parm_type_unqual))
+       *new_parm_type = formal_parm_type_unqual;
+
+      if (TREE_CHAIN (old_parm_type_list) == void_list_node)
+       TREE_CHAIN (new_parm_type_list) = void_list_node;
+      parm_list = DECL_CHAIN (parm_list);
+      old_parm_type_list = TREE_CHAIN (old_parm_type_list);
+      new_parm_type_list = TREE_CHAIN (new_parm_type_list);
+    }

The usual pattern for this sort of thing is to use a tree* to track the end of
the new list, which should also avoid making a garbage copy of void_list_node.
e.g. from tsubst_attribute:

       tree list = NULL_TREE;
       tree *q = &list;
       for (int i = 0; i < len; ++i)
         {
           tree elt = TREE_VEC_ELT (pack, i);
           *q = build_tree_list (purp, elt);
           q = &TREE_CHAIN (*q);
         }

Ah so that's the right way do it :) Patch updated to make use of this
pattern.

This version of the patch is more complete.  It builds the new
FUNCTION_TYPE and METHOD_TYPE the same way that tsubst_function_type
does, by splitting out and reusing the relevant parts of
tsubst_function_type into a separate subroutine that is responsible for
propagating TYPE_ATTRIBUTES, TYPE_RAISES_EXCEPTION, ref-qualifiers, etc.

I wonder if for consistency and correctness we might have to update
other callers of tsubst_function_type/tsubst to make sure this
function-type-rebuilding based on parameter types is done in these
callers too.  For example, there is is_specialization_of_friend which
calls tsubst_function_type on the type of a function decl, and
fn_type_unification and determine_specialization which also call tsubst
on the type of a function decl (and pass the tf_fndecl_type flag).

If so, maybe we could instead leverage the tf_fndecl_type flag and the
'in_decl' tsubst parameter to change tsubst_arg_types to immediately
build the function type according to the parameter types of in_decl
(which would presumably be the FUNCTION_DECL)?  That way, we would just
have to update the above potentially problematic callers to pass
tf_fndecl_type and set in_decl appropriately when calling tsubst and
would only have to build the function type once.

Patch partially tested on unbootstrapped x86_64-pc-linux-gnu, and
bootstrap/regtest is in progress.

-- >8 --

Subject: [PATCH] c++: Rebuild function type when it disagrees with formal
  parameter types [PR92010]

OK, thanks.  Note that we're trying to keep the length of the git 
subject line (not the email subject line that adds [PATCH] and such) 
under 50 chars for the sake of things like git log --oneline.  Going 
over isn't a terrible thing, but please keep that in mind.

gcc/cp/ChangeLog:

        Core issues 1001 and 1322
        PR c++/92010
        * pt.c (rebuild_function_or_method_type): Split function out from ...
        (tsubst_function_type): ... here.
        (maybe_rebuild_function_type): New function.
        (tsubst_function_decl): Use it.

gcc/testsuite/ChangeLog:

        Core issues 1001 and 1322
        PR c++/92010
        * g++.dg/cpp2a/lambda-uneval11.c: New test.
        * g++.dg/template/array33.C: New test.
        * g++.dg/template/array34.C: New test.
        * g++.dg/template/defarg22.C: New test.
---
  gcc/cp/pt.c                                  | 151 ++++++++++++++-----
  gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C |  10 ++
  gcc/testsuite/g++.dg/template/array33.C      |  63 ++++++++
  gcc/testsuite/g++.dg/template/array34.C      |  63 ++++++++
  gcc/testsuite/g++.dg/template/defarg22.C     |  13 ++
  5 files changed, 263 insertions(+), 37 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C
  create mode 100644 gcc/testsuite/g++.dg/template/array33.C
  create mode 100644 gcc/testsuite/g++.dg/template/array34.C
  create mode 100644 gcc/testsuite/g++.dg/template/defarg22.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 6122227c22f..256a937eace 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -13475,6 +13475,116 @@ lookup_explicit_specifier (tree v)
    return *explicit_specifier_map->get (v);
  }
  
+/* Given T, a FUNCTION_TYPE or METHOD_TYPE, construct and return a corresponding
+   FUNCTION_TYPE or METHOD_TYPE whose return type is RETURN_TYPE, argument 
types
+   are ARG_TYPES, and exception specification is RAISES, and otherwise is
+   identical to T.  */
+
+static tree
+rebuild_function_or_method_type (tree t, tree return_type, tree arg_types,
+                                tree raises, tsubst_flags_t complain)
+{
+  gcc_assert (FUNC_OR_METHOD_TYPE_P (t));
+
+  tree new_type;
+  if (TREE_CODE (t) == FUNCTION_TYPE)
+    {
+      new_type = build_function_type (return_type, arg_types);
+      new_type = apply_memfn_quals (new_type, type_memfn_quals (t));
+    }
+  else
+    {
+      tree r = TREE_TYPE (TREE_VALUE (arg_types));
+      /* Don't pick up extra function qualifiers from the basetype.  */
+      r = cp_build_qualified_type_real (r, type_memfn_quals (t), complain);
+      if (! MAYBE_CLASS_TYPE_P (r))
+       {
+         /* [temp.deduct]
+
+            Type deduction may fail for any of the following
+            reasons:
+
+            -- Attempting to create "pointer to member of T" when T
+            is not a class type.  */
+         if (complain & tf_error)
+           error ("creating pointer to member function of non-class type %qT",
+                  r);
+         return error_mark_node;
+       }
+
+      new_type = build_method_type_directly (r, return_type,
+                                            TREE_CHAIN (arg_types));
+    }
+  new_type = cp_build_type_attribute_variant (new_type, TYPE_ATTRIBUTES (t));
+
+  cp_ref_qualifier rqual = type_memfn_rqual (t);
+  bool late_return_type_p = TYPE_HAS_LATE_RETURN_TYPE (t);
+  return build_cp_fntype_variant (new_type, rqual, raises, late_return_type_p);
+}
+
+/* Check if the function type of DECL, a FUNCTION_DECL, agrees with the type of
+   each of its formal parameters.  If there is a disagreement then rebuild
+   DECL's function type according to its formal parameter types, as part of a
+   resolution for Core issues 1001/1322.  */
+
+static void
+maybe_rebuild_function_decl_type (tree decl)
+{
+  bool function_type_needs_rebuilding = false;
+  if (tree parm_list = FUNCTION_FIRST_USER_PARM (decl))
+    {
+      tree parm_type_list = FUNCTION_FIRST_USER_PARMTYPE (decl);
+      while (parm_type_list && parm_type_list != void_list_node)
+       {
+         tree parm_type = TREE_VALUE (parm_type_list);
+         tree formal_parm_type_unqual = strip_top_quals (TREE_TYPE 
(parm_list));
+         if (!same_type_p (parm_type, formal_parm_type_unqual))
+           {
+             function_type_needs_rebuilding = true;
+             break;
+           }
+
+         parm_list = DECL_CHAIN (parm_list);
+         parm_type_list = TREE_CHAIN (parm_type_list);
+       }
+    }
+
+  if (!function_type_needs_rebuilding)
+    return;
+
+  const tree fntype = TREE_TYPE (decl);
+  tree parm_list = DECL_ARGUMENTS (decl);
+  tree old_parm_type_list = TYPE_ARG_TYPES (fntype);
+  tree new_parm_type_list = NULL_TREE;
+  tree *q = &new_parm_type_list;
+  for (int skip = num_artificial_parms_for (decl); skip > 0; skip--)
+    {
+      *q = copy_node (old_parm_type_list);
+      parm_list = DECL_CHAIN (parm_list);
+      old_parm_type_list = TREE_CHAIN (old_parm_type_list);
+      q = &TREE_CHAIN (*q);
+    }
+  while (old_parm_type_list && old_parm_type_list != void_list_node)
+    {
+      *q = copy_node (old_parm_type_list);
+      tree *new_parm_type = &TREE_VALUE (*q);
+      tree formal_parm_type_unqual = strip_top_quals (TREE_TYPE (parm_list));
+      if (!same_type_p (*new_parm_type, formal_parm_type_unqual))
+       *new_parm_type = formal_parm_type_unqual;
+
+      parm_list = DECL_CHAIN (parm_list);
+      old_parm_type_list = TREE_CHAIN (old_parm_type_list);
+      q = &TREE_CHAIN (*q);
+    }
+  if (old_parm_type_list == void_list_node)
+    *q = void_list_node;
+
+  TREE_TYPE (decl)
+    = rebuild_function_or_method_type (fntype,
+                                      TREE_TYPE (fntype), new_parm_type_list,
+                                      TYPE_RAISES_EXCEPTIONS (fntype), 
tf_none);
+}
+
  /* Subroutine of tsubst_decl for the case when T is a FUNCTION_DECL.  */
  
  static tree
@@ -13665,6 +13775,8 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t 
complain,
    DECL_ARGUMENTS (r) = parms;
    DECL_RESULT (r) = NULL_TREE;
  
+  maybe_rebuild_function_decl_type (r);
+
    TREE_STATIC (r) = 0;
    TREE_PUBLIC (r) = TREE_PUBLIC (t);
    DECL_EXTERNAL (r) = 1;
@@ -14694,7 +14806,6 @@ tsubst_function_type (tree t,
  {
    tree return_type;
    tree arg_types = NULL_TREE;
-  tree fntype;
  
    /* The TYPE_CONTEXT is not used for function/method types.  */
    gcc_assert (TYPE_CONTEXT (t) == NULL_TREE);
@@ -14765,42 +14876,8 @@ tsubst_function_type (tree t,
      }
  
    /* Construct a new type node and return it.  */
-  if (TREE_CODE (t) == FUNCTION_TYPE)
-    {
-      fntype = build_function_type (return_type, arg_types);
-      fntype = apply_memfn_quals (fntype, type_memfn_quals (t));
-    }
-  else
-    {
-      tree r = TREE_TYPE (TREE_VALUE (arg_types));
-      /* Don't pick up extra function qualifiers from the basetype.  */
-      r = cp_build_qualified_type_real (r, type_memfn_quals (t), complain);
-      if (! MAYBE_CLASS_TYPE_P (r))
-       {
-         /* [temp.deduct]
-
-            Type deduction may fail for any of the following
-            reasons:
-
-            -- Attempting to create "pointer to member of T" when T
-            is not a class type.  */
-         if (complain & tf_error)
-           error ("creating pointer to member function of non-class type %qT",
-                     r);
-         return error_mark_node;
-       }
-
-      fntype = build_method_type_directly (r, return_type,
-                                          TREE_CHAIN (arg_types));
-    }
-  fntype = cp_build_type_attribute_variant (fntype, TYPE_ATTRIBUTES (t));
-
-  /* See comment above.  */
-  tree raises = NULL_TREE;
-  cp_ref_qualifier rqual = type_memfn_rqual (t);
-  fntype = build_cp_fntype_variant (fntype, rqual, raises, late_return_type_p);
-
-  return fntype;
+  return rebuild_function_or_method_type (t, return_type, arg_types,
+                                         /*raises=*/NULL_TREE, complain);
  }
  
  /* FNTYPE is a FUNCTION_TYPE or METHOD_TYPE.  Substitute the template
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C 
b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C
new file mode 100644
index 00000000000..a04262494c7
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval11.C
@@ -0,0 +1,10 @@
+// PR c++/92010
+// { dg-do compile { target c++2a } }
+
+template <class T> void spam(decltype([]{}) (*s)[sizeof(T)] = nullptr)
+{ }
+
+void foo()
+{
+  spam<int>();
+}
diff --git a/gcc/testsuite/g++.dg/template/array33.C 
b/gcc/testsuite/g++.dg/template/array33.C
new file mode 100644
index 00000000000..0aa587351b4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/array33.C
@@ -0,0 +1,63 @@
+// Verify that top-level cv-qualifiers on parameter types are considered
+// when determining the function type of an instantiated function template.
+// This resolves a part of Core issues 1001/1322.
+// { dg-do compile }
+// { dg-additional-options "-Wno-volatile" }
+
+template<typename T>
+void foo0(T t = 0);
+
+template<typename T>
+void foo1(const T = 0);
+
+template<typename T>
+void foo2(volatile T t = 0);
+
+template<typename T>
+void foo3(const volatile T t = 0);
+
+#if __cplusplus >= 201103L
+#define SA(X) static_assert(X,#X)
+SA(__is_same(decltype(foo0<char[]>), void(char*)));
+SA(__is_same(decltype(foo0<const char[]>), void(const char*)));
+SA(__is_same(decltype(foo0<volatile char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo0<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo1<char[]>), void(const char*)));
+SA(__is_same(decltype(foo1<const char[]>), void(const char*)));
+SA(__is_same(decltype(foo1<volatile char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo1<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo2<char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo2<const char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo2<volatile char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo2<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo3<char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<const char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<volatile char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<const volatile char[]>), void(const volatile 
char*)));
+#endif
+
+int main()
+{
+  foo0<char[]>();
+  foo0<const char[]>();
+  foo0<volatile char[]>();
+  foo0<const volatile char[]>();
+
+  foo1<char[]>();
+  foo1<const char[]>();
+  foo1<volatile char[]>();
+  foo1<const volatile char[]>();
+
+  foo2<char[]>();
+  foo2<const char[]>();
+  foo2<volatile char[]>();
+  foo2<const volatile char[]>();
+
+  foo3<char[]>();
+  foo3<const char[]>();
+  foo3<volatile char[]>();
+  foo3<const volatile char[]>();
+}
diff --git a/gcc/testsuite/g++.dg/template/array34.C 
b/gcc/testsuite/g++.dg/template/array34.C
new file mode 100644
index 00000000000..38c06401974
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/array34.C
@@ -0,0 +1,63 @@
+// Verify that top-level cv-qualifiers on parameter types are considered
+// when determining the function type of an instantiated function template.
+// This resolves a part of Core issues 1001/1322.
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-Wno-volatile" }
+
+template<typename... Ts>
+void foo0(Ts... t);
+
+template<typename... Ts>
+void foo1(const Ts... t);
+
+template<typename... Ts>
+void foo2(volatile Ts... t);
+
+template<typename... Ts>
+void foo3(const volatile Ts... t);
+
+#if __cplusplus >= 201103L
+#define SA(X) static_assert(X,#X)
+SA(__is_same(decltype(foo0<char[]>), void(char*)));
+SA(__is_same(decltype(foo0<const char[]>), void(const char*)));
+SA(__is_same(decltype(foo0<volatile char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo0<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo1<char[]>), void(const char*)));
+SA(__is_same(decltype(foo1<const char[]>), void(const char*)));
+SA(__is_same(decltype(foo1<volatile char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo1<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo2<char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo2<const char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo2<volatile char[]>), void(volatile char*)));
+SA(__is_same(decltype(foo2<const volatile char[]>), void(const volatile 
char*)));
+
+SA(__is_same(decltype(foo3<char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<const char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<volatile char[]>), void(const volatile char*)));
+SA(__is_same(decltype(foo3<const volatile char[]>), void(const volatile 
char*)));
+#endif
+
+int main()
+{
+  foo0<char[]>(0);
+  foo0<const char[]>(0);
+  foo0<volatile char[]>(0);
+  foo0<const volatile char[]>(0);
+
+  foo1<char[]>(0);
+  foo1<const char[]>(0);
+  foo1<volatile char[]>(0);
+  foo1<const volatile char[]>(0);
+
+  foo2<char[]>(0);
+  foo2<const char[]>(0);
+  foo2<volatile char[]>(0);
+  foo2<const volatile char[]>(0);
+
+  foo3<char[]>(0);
+  foo3<const char[]>(0);
+  foo3<volatile char[]>(0);
+  foo3<const volatile char[]>(0);
+}
diff --git a/gcc/testsuite/g++.dg/template/defarg22.C 
b/gcc/testsuite/g++.dg/template/defarg22.C
new file mode 100644
index 00000000000..599061cedb0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/defarg22.C
@@ -0,0 +1,13 @@
+// PR c++/92010
+// { dg-do compile { target c++11 } }
+
+template <typename T = char[3]>
+void foo(const T t = "; ")
+{
+}
+
+int main()
+{
+  foo ();
+}
+