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.

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;
 

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);
+}

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" }
+}