c++: Recursive unification with packs and explicit targs [PR94628]

This PR seems to be similar to PR c++/43382, except that the recursive call to
the variadic function with trailing return type in this testcase is additionally
given some explicit template arguments.

In the first testcase below, when resolving the recursive call to 'select',
fn_type_unification first substitutes in the call's explicit template arguments
before doing unification, and so during this substitution the template argument
pack for Args is incomplete.

Since the pack is incomplete, the substitution of 'args...' in the trailing
return type decltype(f(args...)) is handled by the unsubstituted_packs case of
tsubst_pack_expansion.  But the handling of this case happens _before_ we reset
local_specializations, and so the substitution ends up reusing the old binding
for 'args' from local_specializations rather than building a new one.

This patch fixes this issue by setting up local_specializations sooner in
tsubst_pack_expansion, before the handling of the unsubstituted_packs case.
It also adds a new policy to local_specialization_stack so that we could use the
class here to conditionally replace local_specializations.

gcc/cp/ChangeLog:

	PR c++/94628
	* cp-tree.h (lss_policy::lss_nop): New enumerator.
	* pt.c (local_specialization_stack::local_specialization_stack): Handle
	an lss_nop policy.
	(local_specialization_stack::~local_specialization_stack): Likewise.
	(tsubst_pack_expansion): Use a local_specialization_stack instead of
	manually saving and restoring local_specializations.  Conditionally
	replace local_specializations sooner, before the handling of the
	unsubstituted_packs case.

gcc/testsuite/ChangeLog:

	PR c++/94628
	* g++.dg/cpp0x/variadic179.C: New test.
	* g++.dg/cpp0x/variadic180.C: New test.

gcc/cp/ChangeLog

@@ -1,3 +1,15 @@
+2020-04-20  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94628
+	* cp-tree.h (lss_policy::lss_nop): New enumerator.
+	* pt.c (local_specialization_stack::local_specialization_stack): Handle
+	an lss_nop policy.
+	(local_specialization_stack::~local_specialization_stack): Likewise.
+	(tsubst_pack_expansion): Use a local_specialization_stack instead of
+	manually saving and restoring local_specializations.  Conditionally
+	replace local_specializations sooner, before the handling of the
+	unsubstituted_packs case.
+
 2020-04-20  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94505 - bogus -Wparentheses warning with fold-expression.

gcc/cp/cp-tree.h

@@ -5421,7 +5421,7 @@ enum unification_kind_t {
 // An RAII class used to create a new pointer map for local
 // specializations. When the stack goes out of scope, the
 // previous pointer map is restored.
-enum lss_policy { lss_blank, lss_copy };
+enum lss_policy { lss_blank, lss_copy, lss_nop };
 class local_specialization_stack
 {
 public:

gcc/cp/pt.c

@@ -83,7 +83,9 @@ static tree cur_stmt_expr;
 local_specialization_stack::local_specialization_stack (lss_policy policy)
   : saved (local_specializations)
 {
-  if (policy == lss_blank || !saved)
+  if (policy == lss_nop)
+    ;
+  else if (policy == lss_blank || !saved)
     local_specializations = new hash_map<tree, tree>;
   else
     local_specializations = new hash_map<tree, tree>(*saved);
@@ -91,8 +93,11 @@ local_specialization_stack::local_specialization_stack (lss_policy policy)
 
 local_specialization_stack::~local_specialization_stack ()
 {
-  delete local_specializations;
-  local_specializations = saved;
+  if (local_specializations != saved)
+    {
+      delete local_specializations;
+      local_specializations = saved;
+    }
 }
 
 /* True if we've recursed into fn_type_unification too many times.  */
@@ -12718,7 +12723,6 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
   bool unsubstituted_fn_pack = false;
   int i, len = -1;
   tree result;
-  hash_map<tree, tree> *saved_local_specializations = NULL;
   bool need_local_specializations = false;
   int levels;
 
@@ -12917,7 +12921,15 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 	= build_extra_args (pattern, args, complain);
       return t;
     }
-  else if (unsubstituted_packs)
+
+  /* If NEED_LOCAL_SPECIALIZATIONS then we're in a late-specified return
+     type, so create our own local specializations map; the current map is
+     either NULL or (in the case of recursive unification) might have
+     bindings that we don't want to use or alter.  */
+  local_specialization_stack lss (need_local_specializations
+				  ? lss_blank : lss_nop);
+
+  if (unsubstituted_packs)
     {
       /* There were no real arguments, we're just replacing a parameter
 	 pack with another version of itself. Substitute into the
@@ -12934,16 +12946,6 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
 
   gcc_assert (len >= 0);
 
-  if (need_local_specializations)
-    {
-      /* We're in a late-specified return type, so create our own local
-	 specializations map; the current map is either NULL or (in the
-	 case of recursive unification) might have bindings that we don't
-	 want to use or alter.  */
-      saved_local_specializations = local_specializations;
-      local_specializations = new hash_map<tree, tree>;
-    }
-
   /* For each argument in each argument pack, substitute into the
      pattern.  */
   result = make_tree_vec (len);
@@ -12990,12 +12992,6 @@ tsubst_pack_expansion (tree t, tree args, tsubst_flags_t complain,
         }
     }
 
-  if (need_local_specializations)
-    {
-      delete local_specializations;
-      local_specializations = saved_local_specializations;
-    }
-
   /* If the dependent pack arguments were such that we end up with only a
      single pack expansion again, there's no need to keep it in a TREE_VEC.  */
   if (len == 1 && TREE_CODE (result) == TREE_VEC

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2020-04-20  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94628
+	* g++.dg/cpp0x/variadic179.C: New test.
+	* g++.dg/cpp0x/variadic180.C: New test.
+
 2020-04-20  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/94505 - bogus -Wparentheses warning with fold-expression.

gcc/testsuite/g++.dg/cpp0x/variadic179.C

@@ -0,0 +1,16 @@
+// PR c++/94628
+// { dg-do compile { target c++11 } }
+
+int f(int, int);
+int f(int);
+
+template<class...Args>
+auto select(Args... args) -> decltype(f(args...))
+{
+  if (sizeof...(Args) > 1)
+    return select<char>(7);
+  else
+    return 0;
+}
+
+int a = select(0, 1);

gcc/testsuite/g++.dg/cpp0x/variadic180.C

@@ -0,0 +1,25 @@
+// PR c++/94628
+// A variant of variadic101.C where the recursive call to deref
+// has its first template argument explicitly provided.
+// { dg-do compile { target c++11 } }
+
+template<class T>
+struct Container
+{ T f() const; };
+
+template<class T>
+T deref(const T& t)
+{ return t; }
+
+
+template <class T, class... Args>
+auto
+deref(const T& u, int r, Args... args)
+-> decltype(deref(u.f(), args...))
+{ return deref<decltype(u.f())>(u.f(), args...); }
+
+int main(void)
+{
+    Container<Container<int>> v;
+    deref(v,1,2);
+}