[PATCH] c++: deduction guides and ttp rewriting [PR102479]

[Patrick Palka via Gcc-patches]

The problem here is ultimately that rewrite_tparm_list when rewriting a
TEMPLATE_TEMPLATE_PARM introduces a tree cycle in the rewritten
ttp that structural_comptypes can't cope with.  In particular the
DECL_TEMPLATE_PARMS of a ttp's TEMPLATE_DECL normally captures an empty
parameter list at its own level (and so the TEMPLATE_DECL doesn't appear
in its own DECL_TEMPLATE_PARMS), but rewrite_tparm_list ends up giving
it a complete parameter list.  In the new testcase below, this causes
infinite recursion from structural_comptypes when comparing Tmpl
with Tmpl (here both 'Tmpl's are rewritten).

This patch fixes this by making rewrite_template_parm give a rewritten
template template parm an empty parameter list at its own level, thereby
avoiding the tree cycle.  Testing the alias CTAD case revealed that
we're not setting current_template_parms in alias_ctad_tweaks, which
this patch also fixes.  Also, the change to use TMPL_ARGS_LEVEL instead
of TREE_VEC_ELT is needed because alias_ctad_tweaks passes only a single
level of targs to rewrite_tparm_list.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?

PR c++/102479

gcc/cp/ChangeLog:

* pt.c (rewrite_template_parm): Use TMPL_ARGS_LEVEL instead of
TREE_VEC_ELT directly to properly handle one-level tsubst_args.
Avoid a tree cycle when assigning the DECL_TEMPLATE_PARMS for a
rewritten ttp.
(alias_ctad_tweaks): Set current_template_parms accordingly.

gcc/testsuite/ChangeLog:

* g++.dg/cpp1z/class-deduction12.C: Also test alias CTAD in the
same way.
* g++.dg/cpp1z/class-deduction99.C: New test.
---
 gcc/cp/pt.c   | 20 +--
 .../g++.dg/cpp1z/class-deduction12.C  |  6 
 .../g++.dg/cpp1z/class-deduction99.C  | 35 +++
 3 files changed, 58 insertions(+), 3 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction99.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 6bd6ceb29be..cba0f5c8279 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28754,7 +28754,7 @@ rewrite_template_parm (tree olddecl, unsigned index, 
unsigned level,
  const int depth = TMPL_ARGS_DEPTH (tsubst_args);
  tree ttargs = make_tree_vec (depth + 1);
  for (int i = 0; i < depth; ++i)
-   TREE_VEC_ELT (ttargs, i) = TREE_VEC_ELT (tsubst_args, i);
+   TREE_VEC_ELT (ttargs, i) = TMPL_ARGS_LEVEL (tsubst_args, i + 1);
  TREE_VEC_ELT (ttargs, depth)
= template_parms_level_to_args (ttparms);
  // Substitute ttargs into ttparms to fix references to
@@ -28767,8 +28767,17 @@ rewrite_template_parm (tree olddecl, unsigned index, 
unsigned level,
  ttparms = tsubst_template_parms_level (ttparms, ttargs,
 complain);
  // Finally, tack the adjusted parms onto tparms.
- ttparms = tree_cons (size_int (depth), ttparms,
-  current_template_parms);
+ ttparms = tree_cons (size_int (level + 1), ttparms,
+  copy_node (current_template_parms));
+ // As with all template template parms, the parameter list captured
+ // by this template template parm that corresponds to its own level
+ // should be empty.  This avoids infinite recursion when structurally
+ // comparing two such rewritten template template parms (102479).
+ gcc_assert (!TREE_VEC_LENGTH
+ (TREE_VALUE (TREE_CHAIN (DECL_TEMPLATE_PARMS 
(olddecl);
+ gcc_assert (TMPL_PARMS_DEPTH (TREE_CHAIN (ttparms)) == level);
+ TREE_VALUE (TREE_CHAIN (ttparms)) = make_tree_vec (0);
+ // All done.
  DECL_TEMPLATE_PARMS (newdecl) = ttparms;
}
 }
@@ -29266,6 +29275,11 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
  ++ndlen;
  tree gtparms = make_tree_vec (natparms + ndlen);
 
+ /* Set current_template_parms as in build_deduction_guide.  */
+ auto ctp = make_temp_override (current_template_parms);
+ current_template_parms = copy_node (DECL_TEMPLATE_PARMS (tmpl));
+ TREE_VALUE (current_template_parms) = gtparms;
+
  /* First copy over the parms of A.  */
  for (j = 0; j < natparms; ++j)
TREE_VEC_ELT (gtparms, j) = TREE_VEC_ELT (atparms, j);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
index a31cc1526db..f0d7ea0e16b 100644
--- a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
@@ -15,3 +15,9 @@ A a(&i,2,B<42>());
 template  class same;
 template  class same {};
 same> s;
+
+#if __cpp_deduction_guides >= 201907
+template  using C = A;
+
+same())), A> t;
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C
new file mode 100644
index 00

Re: [PATCH] c++: deduction guides and ttp rewriting [PR102479]

[Jason Merrill via Gcc-patches]

On 9/27/21 10:44, Patrick Palka wrote:

The problem here is ultimately that rewrite_tparm_list when rewriting a
TEMPLATE_TEMPLATE_PARM introduces a tree cycle in the rewritten
ttp that structural_comptypes can't cope with.  In particular the
DECL_TEMPLATE_PARMS of a ttp's TEMPLATE_DECL normally captures an empty
parameter list at its own level (and so the TEMPLATE_DECL doesn't appear
in its own DECL_TEMPLATE_PARMS), but rewrite_tparm_list ends up giving
it a complete parameter list.  In the new testcase below, this causes
infinite recursion from structural_comptypes when comparing Tmpl
with Tmpl (here both 'Tmpl's are rewritten).

This patch fixes this by making rewrite_template_parm give a rewritten
template template parm an empty parameter list at its own level, thereby
avoiding the tree cycle.  Testing the alias CTAD case revealed that
we're not setting current_template_parms in alias_ctad_tweaks, which
this patch also fixes.  Also, the change to use TMPL_ARGS_LEVEL instead
of TREE_VEC_ELT is needed because alias_ctad_tweaks passes only a single
level of targs to rewrite_tparm_list.

Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
trunk?


OK.


PR c++/102479

gcc/cp/ChangeLog:

* pt.c (rewrite_template_parm): Use TMPL_ARGS_LEVEL instead of
TREE_VEC_ELT directly to properly handle one-level tsubst_args.
Avoid a tree cycle when assigning the DECL_TEMPLATE_PARMS for a
rewritten ttp.
(alias_ctad_tweaks): Set current_template_parms accordingly.

gcc/testsuite/ChangeLog:

* g++.dg/cpp1z/class-deduction12.C: Also test alias CTAD in the
same way.
* g++.dg/cpp1z/class-deduction99.C: New test.
---
  gcc/cp/pt.c   | 20 +--
  .../g++.dg/cpp1z/class-deduction12.C  |  6 
  .../g++.dg/cpp1z/class-deduction99.C  | 35 +++
  3 files changed, 58 insertions(+), 3 deletions(-)
  create mode 100644 gcc/testsuite/g++.dg/cpp1z/class-deduction99.C

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 6bd6ceb29be..cba0f5c8279 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -28754,7 +28754,7 @@ rewrite_template_parm (tree olddecl, unsigned index, 
unsigned level,
  const int depth = TMPL_ARGS_DEPTH (tsubst_args);
  tree ttargs = make_tree_vec (depth + 1);
  for (int i = 0; i < depth; ++i)
-   TREE_VEC_ELT (ttargs, i) = TREE_VEC_ELT (tsubst_args, i);
+   TREE_VEC_ELT (ttargs, i) = TMPL_ARGS_LEVEL (tsubst_args, i + 1);
  TREE_VEC_ELT (ttargs, depth)
= template_parms_level_to_args (ttparms);
  // Substitute ttargs into ttparms to fix references to
@@ -28767,8 +28767,17 @@ rewrite_template_parm (tree olddecl, unsigned index, 
unsigned level,
  ttparms = tsubst_template_parms_level (ttparms, ttargs,
 complain);
  // Finally, tack the adjusted parms onto tparms.
- ttparms = tree_cons (size_int (depth), ttparms,
-  current_template_parms);
+ ttparms = tree_cons (size_int (level + 1), ttparms,
+  copy_node (current_template_parms));
+ // As with all template template parms, the parameter list captured
+ // by this template template parm that corresponds to its own level
+ // should be empty.  This avoids infinite recursion when structurally
+ // comparing two such rewritten template template parms (102479).
+ gcc_assert (!TREE_VEC_LENGTH
+ (TREE_VALUE (TREE_CHAIN (DECL_TEMPLATE_PARMS 
(olddecl);
+ gcc_assert (TMPL_PARMS_DEPTH (TREE_CHAIN (ttparms)) == level);
+ TREE_VALUE (TREE_CHAIN (ttparms)) = make_tree_vec (0);
+ // All done.
  DECL_TEMPLATE_PARMS (newdecl) = ttparms;
}
  }
@@ -29266,6 +29275,11 @@ alias_ctad_tweaks (tree tmpl, tree uguides)
  ++ndlen;
  tree gtparms = make_tree_vec (natparms + ndlen);
  
+	  /* Set current_template_parms as in build_deduction_guide.  */

+ auto ctp = make_temp_override (current_template_parms);
+ current_template_parms = copy_node (DECL_TEMPLATE_PARMS (tmpl));
+ TREE_VALUE (current_template_parms) = gtparms;
+
  /* First copy over the parms of A.  */
  for (j = 0; j < natparms; ++j)
TREE_VEC_ELT (gtparms, j) = TREE_VEC_ELT (atparms, j);
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C 
b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
index a31cc1526db..f0d7ea0e16b 100644
--- a/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
+++ b/gcc/testsuite/g++.dg/cpp1z/class-deduction12.C
@@ -15,3 +15,9 @@ A a(&i,2,B<42>());
  template  class same;
  template  class same {};
  same> s;
+
+#if __cpp_deduction_guides >= 201907
+template  using C = A;
+
+same())), A> t;
+#endif
diff --git a/gcc/testsuite/g++.dg/cpp1z/class-deduction99.C 
b/gcc/testsuite/g++.dg/cpp1z/