Re: [PATCH 1/2] c++: factor out TYPENAME_TYPE substitution

[Patrick Palka <ppalka@redhat.com>]

On Tue, 14 Feb 2023, Jason Merrill wrote:

> On 2/13/23 09:23, Patrick Palka wrote:
> > [N.B. this is a corrected version of
> > https://gcc.gnu.org/pipermail/gcc-patches/2022-November/607443.html ]
> > 
> > This patch factors out the TYPENAME_TYPE case of tsubst into a separate
> > function tsubst_typename_type.  It also factors out the two tsubst flags
> > controlling TYPENAME_TYPE substitution, tf_keep_type_decl and tf_tst_ok,
> > into distinct boolean parameters of this new function (and of
> > make_typename_type).  Consequently, callers which used to pass tf_tst_ok
> > to tsubst now instead must directly call tsubst_typename_type when
> > appropriate.
> 
> Hmm, I don't love how that turns 4 lines into 8 more complex lines in each
> caller.  And the previous approach of saying "a CTAD placeholder is OK" seem
> like better abstraction than repeating the specific TYPENAME_TYPE handling in
> each place.

Ah yeah, I see what you mean.  I was thinking since tf_tst_ok is
specific to TYPENAME_TYPE handling and isn't propagated (i.e. it only
affects top-level TYPENAME_TYPEs), it seemed cleaner to encode the flag
as a bool parameter "template_ok" of tsubst_typename_type instead of as
a global tsubst_flag that gets propagated freely.

> 
> > In a subsequent patch we'll add another flag to
> > tsubst_typename_type controlling whether we want to ignore non-types
> > during the qualified lookup.

As mentioned above, the second patch in this series would just add
another flag "type_only" alongside "template_ok", since this flag will
also only affects top-level TYPENAME_TYPEs and doesn't need to propagate
like tsubst_flags.

Except, it turns it, this new flag _does_ need to propagate, namely when
expanding a variadic using:

  using typename Ts::type::m...; // from typename25a.C below

Here we have a USING_DECL whose USING_DECL_SCOPE is a
TYPE_PACK_EXPANSION over TYPENAME_TYPE.  In order to correctly
substitute this TYPENAME_TYPE, the USING_DECL case of tsubst_decl needs
to pass an appropriate tsubst_flag to tsubst_pack_expansion to be
propagated to tsubst (to be propagated to make_typename_type).

So in light of this case it seems adding a new tsubst_flag is the
way to go, which means we can avoid this refactoring patch entirely.

Like so?  Bootstrapped and regtested on x86_64-pc-linux-gnu.

-- >8 --

Subject: [PATCH] c++: TYPENAME_TYPE lookup ignoring non-types [PR107773]

Currently when resolving a TYPENAME_TYPE for 'typename T::m' via
make_typename_type, we consider only type bindings of 'm' and ignore
non-type ones.  But [temp.res.general]/3 says, in a note, "the usual
qualified name lookup ([basic.lookup.qual]) applies even in the presence
of 'typename'", and qualified name lookup doesn't discriminate between
type and non-type bindings.  So when resolving such a TYPENAME_TYPE
we want the lookup to consider all bindings.

An exception is when we have a TYPENAME_TYPE corresponding to the
qualifying scope of the :: scope resolution operator, such as
'T::type' in 'typename T::type::m'.  In that case, [basic.lookup.qual]/1
applies, and lookup for such a TYPENAME_TYPE must ignore non-type bindings.
So in order to correctly handle all cases, make_typename_type needs an
additional flag controlling whether lookup should ignore non-types or not.

To that end this patch adds a new tsubst flag tf_qualifying_scope to
communicate to make_typename_type whether we want to ignore non-type
bindings during the lookup (by default we don't want to ignore them).
In contexts where we do want to ignore non-types (when substituting
into the scope of TYPENAME_TYPE, SCOPE_REF or USING_DECL) we simply
pass tf_qualifying_scope to the relevant tsubst / tsubst_copy call.
This flag is intended to apply only to top-level TYPENAME_TYPEs so
we must be careful to clear the flag to avoid propagating it during
substitution of sub-trees.

	PR c++/107773

gcc/cp/ChangeLog:

	* cp-tree.h (enum tsubst_flags): New flag tf_qualifying_scope.
	* decl.cc (make_typename_type): Use lookup_member instead of
	lookup_field.  If tf_qualifying_scope is set, pass want_type=true
	instead of =false to lookup_member.  Generalize format specifier
	in diagnostic to handle both type and non-type bindings.
	* pt.cc (tsubst_aggr_type_1): Clear tf_qualifying_scope.  Tidy
	the function.
	(tsubst_decl) <case USING_DECL>: Set tf_qualifying_scope when
	substituting USING_DECL_SCOPE.
	(tsubst): Clear tf_qualifying_scope right away and remember if
	it was set.  Do the same for tf_tst_ok sooner.
	<case TYPENAME_TYPE>: Set tf_qualifying_scope when substituting
	TYPE_CONTEXT.  Pass tf_qualifying_scope to make_typename_type
	if it was set.
	(tsubst_qualified_id): Set tf_qualifying_scope when substituting
	the scope.
	(tsubst_copy): Clear tf_qualifying_scope and remember if it was
	set.
	<case SCOPE_REF>: Set tf_qualifying_scope when substituting the
	scope.
	<case *_TYPE>: Pass tf_qualifying_scope to tsubst if it was set.
	* search.cc (lookup_member): Document default argument.

gcc/testsuite/ChangeLog:

	* g++.dg/template/typename24.C: New test.
	* g++.dg/template/typename25.C: New test.
	* g++.dg/template/typename25a.C: New test.
	* g++.dg/template/typename26.C: New test.
---
 gcc/cp/cp-tree.h                            |  3 ++
 gcc/cp/decl.cc                              |  9 ++--
 gcc/cp/pt.cc                                | 58 ++++++++++++---------
 gcc/cp/search.cc                            |  2 +-
 gcc/testsuite/g++.dg/template/typename24.C  | 18 +++++++
 gcc/testsuite/g++.dg/template/typename25.C  | 33 ++++++++++++
 gcc/testsuite/g++.dg/template/typename25a.C | 37 +++++++++++++
 gcc/testsuite/g++.dg/template/typename26.C  | 20 +++++++
 8 files changed, 150 insertions(+), 30 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/template/typename24.C
 create mode 100644 gcc/testsuite/g++.dg/template/typename25.C
 create mode 100644 gcc/testsuite/g++.dg/template/typename25a.C
 create mode 100644 gcc/testsuite/g++.dg/template/typename26.C

diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index be8775ed0f8..891fcf521a8 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -5597,6 +5597,9 @@ enum tsubst_flags {
   tf_tst_ok = 1 << 12,		 /* Allow a typename-specifier to name
 				    a template (C++17 or later).  */
   tf_dguide = 1 << 13,		/* Building a deduction guide from a ctor.  */
+  tf_qualifying_scope = 1 << 14, /* Substituting the LHS of the :: operator.
+				    Controls TYPENAME_TYPE resolution from
+				    make_typename_type.  */
   /* Convenient substitution flags combinations.  */
   tf_warning_or_error = tf_warning | tf_error
 };
diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index d606b31d7a7..2f6412d04e6 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -4305,9 +4305,10 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
      member of the current instantiation or a non-dependent base;
      lookup will stop when we hit a dependent base.  */
   if (!dependent_scope_p (context))
-    /* We should only set WANT_TYPE when we're a nested typename type.
-       Then we can give better diagnostics if we find a non-type.  */
-    t = lookup_field (context, name, 2, /*want_type=*/true);
+    {
+      bool want_type = (complain & tf_qualifying_scope);
+      t = lookup_member (context, name, /*protect=*/2, want_type, complain);
+    }
   else
     t = NULL_TREE;
 
@@ -4359,7 +4360,7 @@ make_typename_type (tree context, tree name, enum tag_types tag_type,
       else
 	{
 	  if (complain & tf_error)
-	    error ("%<typename %T::%D%> names %q#T, which is not a type",
+	    error ("%<typename %T::%D%> names %q#D, which is not a type",
 		   context, name, t);
 	  return error_mark_node;
 	}
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index e89dbf47097..d11d540ab44 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -13919,8 +13919,7 @@ tsubst_aggr_type_1 (tree t,
 {
   if (TYPE_TEMPLATE_INFO (t) && uses_template_parms (t))
     {
-      tree argvec;
-      tree r;
+      complain &= ~tf_qualifying_scope;
 
       /* Figure out what arguments are appropriate for the
 	 type we are trying to find.  For example, given:
@@ -13931,18 +13930,14 @@ tsubst_aggr_type_1 (tree t,
 	 and supposing that we are instantiating f<int, double>,
 	 then our ARGS will be {int, double}, but, when looking up
 	 S we only want {double}.  */
-      argvec = tsubst_template_args (TYPE_TI_ARGS (t), args,
-				     complain, in_decl);
+      tree argvec = tsubst_template_args (TYPE_TI_ARGS (t), args,
+					  complain, in_decl);
       if (argvec == error_mark_node)
-	r = error_mark_node;
-      else
-	{
-	  r = lookup_template_class (t, argvec, in_decl, NULL_TREE,
-				     entering_scope, complain);
-	  r = cp_build_qualified_type (r, cp_type_quals (t), complain);
-	}
+	return error_mark_node;
 
-      return r;
+      tree r = lookup_template_class (t, argvec, in_decl, NULL_TREE,
+				      entering_scope, complain);
+      return cp_build_qualified_type (r, cp_type_quals (t), complain);
     }
   else
     /* This is not a template type, so there's nothing to do.  */
@@ -15003,11 +14998,15 @@ tsubst_decl (tree t, tree args, tsubst_flags_t complain)
 	  tree scope = USING_DECL_SCOPE (t);
 	  if (PACK_EXPANSION_P (scope))
 	    {
-	      scope = tsubst_pack_expansion (scope, args, complain, in_decl);
+	      scope = tsubst_pack_expansion (scope, args,
+					     complain | tf_qualifying_scope,
+					     in_decl);
 	      variadic_p = true;
 	    }
 	  else
-	    scope = tsubst_copy (scope, args, complain, in_decl);
+	    scope = tsubst_copy (scope, args,
+				 complain | tf_qualifying_scope,
+				 in_decl);
 
 	  tree name = DECL_NAME (t);
 	  if (IDENTIFIER_CONV_OP_P (name)
@@ -15821,6 +15820,12 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
       || TREE_CODE (t) == TRANSLATION_UNIT_DECL)
     return t;
 
+  tsubst_flags_t tst_ok_flag = (complain & tf_tst_ok);
+  complain &= ~tf_tst_ok;
+
+  tsubst_flags_t qualifying_scope_flag = (complain & tf_qualifying_scope);
+  complain &= ~tf_qualifying_scope;
+
   if (DECL_P (t))
     return tsubst_decl (t, args, complain);
 
@@ -15889,9 +15894,6 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
   bool fndecl_type = (complain & tf_fndecl_type);
   complain &= ~tf_fndecl_type;
 
-  bool tst_ok = (complain & tf_tst_ok);
-  complain &= ~tf_tst_ok;
-
   if (type
       && code != TYPENAME_TYPE
       && code != TEMPLATE_TYPE_PARM
@@ -16428,7 +16430,9 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	tree ctx = TYPE_CONTEXT (t);
 	if (TREE_CODE (ctx) == TYPE_PACK_EXPANSION)
 	  {
-	    ctx = tsubst_pack_expansion (ctx, args, complain, in_decl);
+	    ctx = tsubst_pack_expansion (ctx, args,
+					 complain | tf_qualifying_scope,
+					 in_decl);
 	    if (ctx == error_mark_node
 		|| TREE_VEC_LENGTH (ctx) > 1)
 	      return error_mark_node;
@@ -16442,8 +16446,9 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	    ctx = TREE_VEC_ELT (ctx, 0);
 	  }
 	else
-	  ctx = tsubst_aggr_type (ctx, args, complain, in_decl,
-				  /*entering_scope=*/1);
+	  ctx = tsubst_aggr_type (ctx, args,
+				  complain | tf_qualifying_scope,
+				  in_decl, /*entering_scope=*/1);
 	if (ctx == error_mark_node)
 	  return error_mark_node;
 
@@ -16473,8 +16478,7 @@ tsubst (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	  }
 
 	tsubst_flags_t tcomplain = complain | tf_keep_type_decl;
-	if (tst_ok)
-	  tcomplain |= tf_tst_ok;
+	tcomplain |= tst_ok_flag | qualifying_scope_flag;
 	f = make_typename_type (ctx, f, typename_type, tcomplain);
 	if (f == error_mark_node)
 	  return f;
@@ -16879,7 +16883,7 @@ tsubst_qualified_id (tree qualified_id, tree args,
   scope = TREE_OPERAND (qualified_id, 0);
   if (args)
     {
-      scope = tsubst (scope, args, complain, in_decl);
+      scope = tsubst (scope, args, complain | tf_qualifying_scope, in_decl);
       expr = tsubst_copy (name, args, complain, in_decl);
     }
   else
@@ -17125,6 +17129,9 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
   if (t == NULL_TREE || t == error_mark_node || args == NULL_TREE)
     return t;
 
+  tsubst_flags_t qualifying_scope_flag = (complain & tf_qualifying_scope);
+  complain &= ~tf_qualifying_scope;
+
   if (tree d = maybe_dependent_member_ref (t, args, complain, in_decl))
     return d;
 
@@ -17598,7 +17605,8 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
     case SCOPE_REF:
       {
-	tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args, complain, in_decl);
+	tree op0 = tsubst_copy (TREE_OPERAND (t, 0), args,
+				complain | tf_qualifying_scope, in_decl);
 	tree op1 = tsubst_copy (TREE_OPERAND (t, 1), args, complain, in_decl);
 	return build_qualified_name (/*type=*/NULL_TREE, op0, op1,
 				     QUALIFIED_NAME_IS_TEMPLATE (t));
@@ -17713,7 +17721,7 @@ tsubst_copy (tree t, tree args, tsubst_flags_t complain, tree in_decl)
     case TYPEOF_TYPE:
     case DECLTYPE_TYPE:
     case TYPE_DECL:
-      return tsubst (t, args, complain, in_decl);
+      return tsubst (t, args, complain | qualifying_scope_flag, in_decl);
 
     case USING_DECL:
       t = DECL_NAME (t);
diff --git a/gcc/cp/search.cc b/gcc/cp/search.cc
index f3f19cafec6..e472a97679d 100644
--- a/gcc/cp/search.cc
+++ b/gcc/cp/search.cc
@@ -1109,7 +1109,7 @@ build_baselink (tree binfo, tree access_binfo, tree functions, tree optype)
 
 tree
 lookup_member (tree xbasetype, tree name, int protect, bool want_type,
-	       tsubst_flags_t complain, access_failure_info *afi)
+	       tsubst_flags_t complain, access_failure_info *afi /* = NULL */)
 {
   tree rval, rval_binfo = NULL_TREE;
   tree type = NULL_TREE, basetype_path = NULL_TREE;
diff --git a/gcc/testsuite/g++.dg/template/typename24.C b/gcc/testsuite/g++.dg/template/typename24.C
new file mode 100644
index 00000000000..8b2b3718442
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/typename24.C
@@ -0,0 +1,18 @@
+// PR c++/107773
+// Verify lookup for a non-neste TYPENAME_TYPE correctly considers
+// non-types.
+
+struct a {
+  typedef void get;
+};
+
+struct b : a {
+  int get(int i) const;
+};
+
+template<class T>
+void f() {
+  typedef typename T::get type; // { dg-error "'int b::get\\(int\\) const', which is not a type" }
+}
+
+template void f<b>();
diff --git a/gcc/testsuite/g++.dg/template/typename25.C b/gcc/testsuite/g++.dg/template/typename25.C
new file mode 100644
index 00000000000..04e48e11724
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/typename25.C
@@ -0,0 +1,33 @@
+// PR c++/107773
+// Verify lookup for TYPENAME_TYPE appearing to the left of the ::
+// scope resolution operator correctly ignores non-types.
+
+struct a {
+  typedef void type;
+};
+
+struct c {
+  struct b : a {
+    typedef b self;
+    static int m;
+  };
+  int b;
+};
+
+template<class T>
+void f() {
+  // A TYPENAME_TYPE whose TYPE_CONTEXT is a nested TYPENAME_TYPE.
+  typedef typename T::b::type type;
+  // A SCOPE_REF whose first operand is a TYPENAME_TYPE.
+  int m = T::b::m;
+}
+
+template void f<c>();
+
+template<class T>
+struct d : T::b::self {
+  // A USING_DECL whose USING_DECL_SCOPE is a TYPENAME_TYPE.
+  using typename T::b::type;
+};
+
+template struct d<c>;
diff --git a/gcc/testsuite/g++.dg/template/typename25a.C b/gcc/testsuite/g++.dg/template/typename25a.C
new file mode 100644
index 00000000000..ecb34aada34
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/typename25a.C
@@ -0,0 +1,37 @@
+// PR c++/107773
+// A variadic version of typename25.C
+// { dg-do compile { target c++11 } }
+
+struct a {
+  typedef void type;
+};
+
+struct c {
+  struct b : a {
+    typedef b self;
+    static int m;
+  };
+  int b;
+};
+
+template<class...> void sink(...);
+
+template<class... Ts>
+void f() {
+  // A TYPENAME_TYPE whose TYPE_CONTEXT is a nested TYPENAME_TYPE.
+  sink<typename Ts::b::type...>();
+  // A SCOPE_REF whose first operand is a TYPENAME_TYPE.
+  sink(Ts::b::m...);
+}
+
+template void f<c>();
+
+template<class... Ts>
+struct d : Ts::b::self... {
+#if __cpp_variadic_using
+  // A USING_DECL whose USING_DECL_SCOPE is a TYPENAME_TYPE.
+  using typename Ts::b::type...;
+#endif
+};
+
+template struct d<c>;
diff --git a/gcc/testsuite/g++.dg/template/typename26.C b/gcc/testsuite/g++.dg/template/typename26.C
new file mode 100644
index 00000000000..4e6b764a97b
--- /dev/null
+++ b/gcc/testsuite/g++.dg/template/typename26.C
@@ -0,0 +1,20 @@
+// Example 4 from [temp.res.general]/3.
+
+struct A {
+  struct X { };
+  int X;
+};
+struct B {
+  struct X { };
+};
+template<class T> void f(T t) {
+  typename T::X x; // { dg-error "'int A::X', which is not a type" }
+}
+void foo() {
+  A a;
+  B b;
+  f(b); // OK, T::X refers to B::X
+  // { dg-bogus "" "" { target *-*-* } .-1 }
+  f(a); // error: T::X refers to the data member A::X not the struct A::X
+  // { dg-message "required from here" "" { target *-*-* } .-1 }
+}
-- 
2.39.2.422.gc867e4fa18