Re: [PATCH] c++: outer args for level-lowered ttp [PR109651]

[Patrick Palka <ppalka@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 8758 bytes --]

On Thu, 27 Apr 2023, Patrick Palka wrote:

> On Thu, Apr 27, 2023 at 4:46 PM Patrick Palka <ppalka@redhat.com> wrote:
> >
> > Now that with r14-11-g2245459c85a3f4 made us coerce the template
> > arguments of a bound ttp again after level-lowering, this unfortunately
> > causes a crash from coerce_template_args_for_ttp in the below testcase.
> >
> > During the level-lowering substitution T=int into the bound ttp TT<int>
> > as part of substitution into the lambda signature, current_template_parms
> > is just U=U rather than the ideal TT=TT, U=U.  And because we don't
> > consistently set DECL_CONTEXT for level-lowered ttps (it's kind of a
> > chicken of the egg problem in this case), we attempt to use
> > current_template_parms to obtain the outer arguments during
> > coerce_template_args_for_ttp.  But the depth 1 of c_t_p
> > current_template_parms is less than the depth 2 of the level-lowered TT,
> > and we end up segfaulting from there.
> >
> > So for level-lowered ttps it seems we need to get the outer arguments a
> > different way -- namely, we can look at the trailing parms of its
> > DECL_TEMPLATE_PARMS.
> 
> Note this is not an ideal solution because TREE_CHAIN of
> DECL_TEMPLATE_PARMS in this case is just "2 <empty>, 1 U", so we're
> missing tparm information for the level that the ttp belongs to :/ So
> the only difference compared to using current_template_parms in this
> case is the extra empty level of args corresponding to the ttp's
> level.

And on the other hand, this issue seems specific to lambdas because
it's in tsubst_lambda_expr that we substitute the function type _before_
substituting and installing the template parameters, which is opposite
to the typical order that tsubst_template_decl does things in.  And
that's ultimately the reason the current_template_parms fallback in
coerce_template_args_for_ttp misbehaves in this testcase.

So the following seems to be a better fix.  With it, current_template_parms
is correctly 2 TT, 1 U during substitution the lambda's function type,
which makes coerce_template_args_for_ttp happy when level lowering
the bound ttp within the function type.

-- >8 --

Subject: [PATCH] c++: bound ttp in lambda function type [PR109651]

	PR c++/109651

gcc/cp/ChangeLog:

	* pt.cc (tsubst_template_decl): Add default argument to
	lambda_fntype parameter.  Add defaulted lambda_tparms parameter.
	Prefer to use lambda_tparms instead of substituting
	DECL_TEMPLATE_PARMS.
	(tsubst_decl) <case TEMPLATE_DECL>: Adjust tsubst_template_decl
	call.
	(tsubst_lambda_expr): For a generic lambda, substitute
	DECL_TEMPLATE_PARMS and update current_template_parms
	before substituting the function type.  Pass the substituted
	DECL_TEMPLATE_PARMS to tsubst_template_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-generic-ttp1.C: New test.
	* g++.dg/cpp2a/lambda-generic-ttp2.C: New test.
---
 gcc/cp/pt.cc                                  | 30 ++++++++++++++-----
 .../g++.dg/cpp2a/lambda-generic-ttp1.C        | 11 +++++++
 .../g++.dg/cpp2a/lambda-generic-ttp2.C        | 13 ++++++++
 3 files changed, 47 insertions(+), 7 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C
 create mode 100644 gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 678cb7930e3..43713d9ab72 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -14629,7 +14629,8 @@ tsubst_function_decl (tree t, tree args, tsubst_flags_t complain,
 
 static tree
 tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
-		      tree lambda_fntype)
+		      tree lambda_fntype = NULL_TREE,
+		      tree lambda_tparms = NULL_TREE)
 {
   /* We can get here when processing a member function template,
      member class template, or template template parameter.  */
@@ -14719,8 +14720,10 @@ tsubst_template_decl (tree t, tree args, tsubst_flags_t complain,
   auto tparm_guard = make_temp_override (current_template_parms);
   DECL_TEMPLATE_PARMS (r)
     = current_template_parms
-    = tsubst_template_parms (DECL_TEMPLATE_PARMS (t), args,
-			     complain);
+    = (lambda_tparms
+       ? lambda_tparms
+       : tsubst_template_parms (DECL_TEMPLATE_PARMS (t), args,
+				complain));
 
   bool class_p = false;
   tree inner = decl;
@@ -14888,7 +14891,7 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
   switch (TREE_CODE (t))
     {
     case TEMPLATE_DECL:
-      r = tsubst_template_decl (t, args, complain, /*lambda*/NULL_TREE);
+      r = tsubst_template_decl (t, args, complain);
       break;
 
     case FUNCTION_DECL:
@@ -20130,12 +20133,24 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 		  ? DECL_TI_TEMPLATE (oldfn)
 		  : NULL_TREE);
 
+  tree tparms = NULL_TREE;
+  if (oldtmpl)
+    tparms = tsubst_template_parms (DECL_TEMPLATE_PARMS (oldtmpl), args, complain);
+
   tree fntype = static_fn_type (oldfn);
+
+  tree saved_ctp = current_template_parms;
   if (oldtmpl)
-    ++processing_template_decl;
+    {
+      ++processing_template_decl;
+      current_template_parms = tparms;
+    }
   fntype = tsubst (fntype, args, complain, in_decl);
   if (oldtmpl)
-    --processing_template_decl;
+    {
+      current_template_parms = saved_ctp;
+      --processing_template_decl;
+    }
 
   if (fntype == error_mark_node)
     r = error_mark_node;
@@ -20151,7 +20166,8 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 				 type_memfn_quals (fntype),
 				 type_memfn_rqual (fntype));
       tree inst = (oldtmpl
-		   ? tsubst_template_decl (oldtmpl, args, complain, fntype)
+		   ? tsubst_template_decl (oldtmpl, args, complain,
+					   fntype, tparms)
 		   : tsubst_function_decl (oldfn, args, complain, fntype));
       if (inst == error_mark_node)
 	{
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C
new file mode 100644
index 00000000000..876e5b6232a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp1.C
@@ -0,0 +1,11 @@
+// PR c++/109651
+// { dg-do compile { target c++20 } }
+
+template<class T>
+void f() {
+  []<class U>() {
+    []<template<class> class TT>(TT<int>) { };
+  };
+}
+
+template void f<int>();
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C
new file mode 100644
index 00000000000..dc2029d7227
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-generic-ttp2.C
@@ -0,0 +1,13 @@
+// PR c++/109651
+// { dg-do compile { target c++20 } }
+
+template<class T>
+auto f() {
+  return []<class U, template<class V, U, V> class TT>(U, TT<int, 1, 2>) { };
+}
+
+template<class T, int N, int M> struct A { };
+
+int main() {
+  f<int>()(0, A<int, 1, 2>{});
+}
-- 
2.40.1.423.g2807bd2c10


> 
> 
> 
> >
> > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > trunk/13?
> >
> >         PR c++/109651
> >
> > gcc/cp/ChangeLog:
> >
> >         * pt.cc (coerce_template_args_for_ttp): For level-lowered
> >         ttps with DECL_CONTEXT not set, obtain the outer template
> >         arguments via its DECL_TEMPLATE_PARMS.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * g++.dg/template/ttp37.C: New test.
> > ---
> >  gcc/cp/pt.cc                          |  5 +++++
> >  gcc/testsuite/g++.dg/template/ttp37.C | 11 +++++++++++
> >  2 files changed, 16 insertions(+)
> >  create mode 100644 gcc/testsuite/g++.dg/template/ttp37.C
> >
> > diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
> > index 678cb7930e3..bbde61061f6 100644
> > --- a/gcc/cp/pt.cc
> > +++ b/gcc/cp/pt.cc
> > @@ -7872,6 +7872,11 @@ coerce_template_args_for_ttp (tree templ, tree arglist,
> >    tree outer = DECL_CONTEXT (templ);
> >    if (outer)
> >      outer = generic_targs_for (outer);
> > +  else if (TEMPLATE_TYPE_LEVEL (TREE_TYPE (templ))
> > +          != TEMPLATE_TYPE_ORIG_LEVEL (TREE_TYPE (templ)))
> > +    /* This is a level-lowered template template parameter, for which
> > +       we don't consistently set DECL_CONTEXT (FIXME).  */
> > +    outer = template_parms_to_args (TREE_CHAIN (DECL_TEMPLATE_PARMS (templ)));
> >    else if (current_template_parms)
> >      {
> >        /* This is an argument of the current template, so we haven't set
> > diff --git a/gcc/testsuite/g++.dg/template/ttp37.C b/gcc/testsuite/g++.dg/template/ttp37.C
> > new file mode 100644
> > index 00000000000..876e5b6232a
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/template/ttp37.C
> > @@ -0,0 +1,11 @@
> > +// PR c++/109651
> > +// { dg-do compile { target c++20 } }
> > +
> > +template<class T>
> > +void f() {
> > +  []<class U>() {
> > +    []<template<class> class TT>(TT<int>) { };
> > +  };
> > +}
> > +
> > +template void f<int>();
> > --
> > 2.40.1.423.g2807bd2c10
> >
>