[gcc r13-980] c++: find_template_parameters and PARM_DECLs [PR105797]

[gcc r13-980] c++: find_template_parameters and PARM_DECLs [PR105797]

[Patrick Palka]

https://gcc.gnu.org/g:df4f95dbd4764ffc1e8270e5b3c0fd71b6724562

commit r13-980-gdf4f95dbd4764ffc1e8270e5b3c0fd71b6724562
Author: Patrick Palka <ppalka@redhat.com>
Date:   Fri Jun 3 09:08:41 2022 -0400

    c++: find_template_parameters and PARM_DECLs [PR105797]
    
    As explained in r11-4959-gde6f64f9556ae3, the atom cache assumes two
    equivalent expressions (according to cp_tree_equal) must use the same
    template parameters (according to find_template_parameters).  This
    assumption turned out to not hold for TARGET_EXPR, which was addressed
    by that commit.
    
    But this assumption apparently doesn't hold for PARM_DECL either:
    find_template_parameters walks its DECL_CONTEXT but cp_tree_equal by
    default doesn't consider DECL_CONTEXT unless comparing_specializations
    is set.  Thus in the first testcase below, the atomic constraints of #1
    and #2 are equivalent according to cp_tree_equal, but according to
    find_template_parameters the former uses T and the latter uses both T
    and U (surprisingly).
    
    We could fix this assumption violation by setting comparing_specializations
    in the atom_hasher, which would make cp_tree_equal return false for the
    two atoms, but that seems overly pessimistic here.  Ideally the atoms
    should continue being considered equivalent and we instead fix
    find_template_paremeters to return just T for #2's atom.
    
    To that end this patch makes for_each_template_parm_r stop walking the
    DECL_CONTEXT of a PARM_DECL.  This should be safe to do because
    tsubst_copy / tsubst_decl only substitutes the TREE_TYPE of a PARM_DECL
    and doesn't bother substituting the DECL_CONTEXT, thus the only relevant
    template parameters are those used in its type.  any_template_parm_r is
    currently responsible for walking its TREE_TYPE, but I suppose it now makes
    sense for for_each_template_parm_r to do so instead.
    
    In passing this patch also makes for_each_template_parm_r stop walking
    the DECL_CONTEXT of a VAR_/FUNCTION_DECL since doing so after walking
    DECL_TI_ARGS is redundant, I think.
    
    I experimented with not walking DECL_CONTEXT for CONST_DECL, but the
    second testcase below demonstrates it's necessary to walk it.
    
            PR c++/105797
    
    gcc/cp/ChangeLog:
    
            * pt.cc (for_each_template_parm_r) <case FUNCTION_DECL, VAR_DECL>:
            Don't walk DECL_CONTEXT.
            <case PARM_DECL>: Likewise.  Walk TREE_TYPE.
            <case CONST_DECL>: Simplify.
            (any_template_parm_r) <case PARM_DECL>: Don't walk TREE_TYPE.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp2a/concepts-decltype4.C: New test.
            * g++.dg/cpp2a/concepts-memfun3.C: New test.

Diff:
---
 gcc/cp/pt.cc                                    | 10 +++++-----
 gcc/testsuite/g++.dg/cpp2a/concepts-decltype4.C | 16 ++++++++++++++++
 gcc/testsuite/g++.dg/cpp2a/concepts-memfun3.C   | 12 ++++++++++++
 3 files changed, 33 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 836861e1039..d0c4c9889c9 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -10561,11 +10561,14 @@ for_each_template_parm_r (tree *tp, int *walk_subtrees, void *d)
     case VAR_DECL:
       if (DECL_LANG_SPECIFIC (t) && DECL_TEMPLATE_INFO (t))
 	WALK_SUBTREE (DECL_TI_ARGS (t));
-      /* Fall through.  */
+      break;
 
     case PARM_DECL:
+      WALK_SUBTREE (TREE_TYPE (t));
+      break;
+
     case CONST_DECL:
-      if (TREE_CODE (t) == CONST_DECL && DECL_TEMPLATE_PARM_P (t))
+      if (DECL_TEMPLATE_PARM_P (t))
 	WALK_SUBTREE (DECL_INITIAL (t));
       if (DECL_CONTEXT (t)
 	  && pfd->include_nondeduced_p)
@@ -10824,9 +10827,6 @@ any_template_parm_r (tree t, void *data)
       break;
 
     case TEMPLATE_PARM_INDEX:
-    case PARM_DECL:
-      /* A parameter or constraint variable may also depend on a template
-	 parameter without explicitly naming it.  */
       WALK_SUBTREE (TREE_TYPE (t));
       break;
 
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-decltype4.C b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype4.C
new file mode 100644
index 00000000000..6683d224cf8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-decltype4.C
@@ -0,0 +1,16 @@
+// PR c++/105797
+// { dg-do compile { target c++20 } }
+
+template<class T>
+concept C = requires { T(); };
+
+template<class T>
+void f(T v) requires C<decltype(v)>; // #1
+
+template<class T, class U>
+void f(T v) requires C<decltype(v)>; // #2
+
+int main() {
+  f<int, int>(0);
+  f<int>(0);
+}
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-memfun3.C b/gcc/testsuite/g++.dg/cpp2a/concepts-memfun3.C
new file mode 100644
index 00000000000..beb30d0e350
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-memfun3.C
@@ -0,0 +1,12 @@
+// { dg-do compile { target c++20 } }
+
+template<int I>
+struct A {
+  enum E { e = I };
+  static void f() requires (e != 0);
+};
+
+int main() {
+  A<1>::f();
+  A<0>::f(); // { dg-error "no match" }
+}