diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 2896efdd7f2..566f4e38fac 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2016,14 +2016,6 @@ type_deducible_p (tree expr, tree type, tree placeholder, tree args,
      references are preserved in the result.  */
   expr = force_paren_expr_uneval (expr);
 
-  /* When args is NULL, we're evaluating a non-templated requires expression,
-     but even those are parsed under processing_template_decl == 1, and so the
-     placeholder 'auto' inside this return-type-requirement has level 2.  In
-     order to have all parms and arguments match up for satisfaction, we need
-     to pass an empty level of OUTER_TARGS in this case.  */
-  if (!args)
-    args = make_tree_vec (0);
-
   tree deduced_type = do_auto_deduction (type, expr, placeholder,
 					 info.complain, adc_requirement,
 					 /*outer_targs=*/args);
@@ -3064,14 +3056,6 @@ normalize_placeholder_type_constraints (tree t, bool diag)
      parameters for normalization.  */
   tree initial_parms = TREE_PURPOSE (ci);
 
-  if (!initial_parms && TEMPLATE_TYPE_LEVEL (t) == 2)
-    /* This is a return-type-requirement of a non-templated requires-expression,
-       which are parsed under processing_template_decl == 1 and empty
-       current_template_parms; hence the 'auto' has level 2 and initial_parms
-       is empty.  Fix up initial_parms to be consistent with the value of
-       processing_template_decl whence the 'auto' was created.  */
-    initial_parms = build_tree_list (size_int (1), make_tree_vec (0));
-
   /* The 'auto' itself is used as the first argument in its own constraints,
      and its level is one greater than its template depth.  So in order to
      capture all used template parameters, we need to add an extra level of
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index b1c3bc5ed1f..9e10a947a43 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -1887,6 +1887,8 @@ extern GTY(()) struct saved_scope *scope_chain;
    stored in the TREE_VALUE.  */
 
 #define current_template_parms scope_chain->template_parms
+#define current_template_depth \
+  (current_template_parms ? TMPL_PARMS_DEPTH (current_template_parms) : 0)
 
 #define processing_template_decl scope_chain->x_processing_template_decl
 #define processing_specialization scope_chain->x_processing_specialization
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index 56f80775ca0..7c2048c6acb 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -5567,7 +5567,7 @@ start_decl (const cp_declarator *declarator,
 
   if (TYPE_P (context) && COMPLETE_TYPE_P (complete_type (context)))
     {
-      bool this_tmpl = (processing_template_decl
+      bool this_tmpl = (current_template_depth
 			> template_class_depth (context));
       if (VAR_P (decl))
 	{
@@ -9878,7 +9878,7 @@ grokfndecl (tree ctype,
       tree ctx = friendp ? current_class_type : ctype;
       bool block_local = TREE_CODE (current_scope ()) == FUNCTION_DECL;
       bool memtmpl = (!block_local
-		      && (processing_template_decl
+		      && (current_template_depth
 			  > template_class_depth (ctx)));
       if (memtmpl)
 	{
@@ -10300,7 +10300,7 @@ grokfndecl (tree ctype,
   if (ctype != NULL_TREE && check)
     {
       tree old_decl = check_classfn (ctype, decl,
-				     (processing_template_decl
+				     (current_template_depth
 				      > template_class_depth (ctype))
 				     ? current_template_parms
 				     : NULL_TREE);
@@ -10576,7 +10576,7 @@ grokvardecl (tree type,
         }
     }
   else if (flag_concepts
-	   && processing_template_decl > template_class_depth (scope))
+	   && current_template_depth > template_class_depth (scope))
     {
       tree reqs = TEMPLATE_PARMS_CONSTRAINTS (current_template_parms);
       tree ci = build_constraints (reqs, NULL_TREE);
@@ -13975,7 +13975,7 @@ grokdeclarator (const cp_declarator *declarator,
 		  }
 
 		/* Set the constraints on the declaration.  */
-		bool memtmpl = (processing_template_decl
+		bool memtmpl = (current_template_depth
 				> template_class_depth (current_class_type));
 		if (memtmpl)
 		  {
diff --git a/gcc/cp/friend.c b/gcc/cp/friend.c
index 4f6288414d9..e970fdce0cc 100644
--- a/gcc/cp/friend.c
+++ b/gcc/cp/friend.c
@@ -261,7 +261,7 @@ make_friend_class (tree type, tree friend_type, bool complain)
      The friend is a template friend iff FRIEND_DEPTH is nonzero.  */
 
   int class_template_depth = template_class_depth (type);
-  int friend_depth = processing_template_decl - class_template_depth;
+  int friend_depth = current_template_depth - class_template_depth;
 
   if (! MAYBE_CLASS_TYPE_P (friend_type)
       && TREE_CODE (friend_type) != TEMPLATE_TEMPLATE_PARM)
@@ -517,7 +517,7 @@ do_friend (tree ctype, tree declarator, tree decl,
 	 3. TEMPLATE_MEMBER_P is true (for `W').  */
 
       int class_template_depth = template_class_depth (current_class_type);
-      int friend_depth = processing_template_decl - class_template_depth;
+      int friend_depth = current_template_depth - class_template_depth;
       /* We will figure this out later.  */
       bool template_member_p = false;
 
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index de464afdb54..aedfab209ba 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -29779,14 +29779,9 @@ static tree
 cp_parser_requires_clause_expression (cp_parser *parser, bool lambda_p)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  temp_override<int> ovr (processing_template_decl);
-  if (!processing_template_decl)
-    /* Adjust processing_template_decl so that we always obtain template
-       trees here.  We don't do the usual ++processing_template_decl
-       because that would skew the template parameter depth of a lambda
-       within if we're already inside a template.  */
-    processing_template_decl = 1;
+  ++processing_template_decl;
   cp_expr expr = cp_parser_constraint_logical_or_expression (parser, lambda_p);
+  --processing_template_decl;
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
   return expr;
@@ -29805,12 +29800,10 @@ static tree
 cp_parser_constraint_expression (cp_parser *parser)
 {
   processing_constraint_expression_sentinel parsing_constraint;
-  temp_override<int> ovr (processing_template_decl);
-  if (!processing_template_decl)
-    /* As in cp_parser_requires_clause_expression.  */
-    processing_template_decl = 1;
+  ++processing_template_decl;
   cp_expr expr = cp_parser_binary_expression (parser, false, true,
 					      PREC_NOT_OPERATOR, NULL);
+  --processing_template_decl;
   if (check_for_bare_parameter_packs (expr))
     expr = error_mark_node;
   expr.maybe_add_location_wrapper ();
@@ -29924,11 +29917,9 @@ cp_parser_requires_expression (cp_parser *parser)
       parms = NULL_TREE;
 
     /* Parse the requirement body. */
-    temp_override<int> ovr (processing_template_decl);
-    if (!processing_template_decl)
-      /* As in cp_parser_requires_clause_expression.  */
-      processing_template_decl = 1;
+    ++processing_template_decl;
     reqs = cp_parser_requirement_body (parser);
+    --processing_template_decl;
     if (reqs == error_mark_node)
       return error_mark_node;
   }
@@ -48091,6 +48082,10 @@ synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
   gcc_assert(!proto || TREE_CODE (proto) == TYPE_DECL);
   synth_tmpl_parm = finish_template_type_parm (class_type_node, synth_id);
 
+  if (become_template)
+    current_template_parms = tree_cons (size_int (current_template_depth + 1),
+					NULL_TREE, current_template_parms);
+
   /* Attach the constraint to the parm before processing.  */
   tree node = build_tree_list (NULL_TREE, synth_tmpl_parm);
   TREE_TYPE (node) = constr;
@@ -48130,8 +48125,7 @@ synthesize_implicit_template_parm  (cp_parser *parser, tree constr)
 
       tree new_parms = make_tree_vec (1);
       TREE_VEC_ELT (new_parms, 0) = parser->implicit_template_parms;
-      current_template_parms = tree_cons (size_int (processing_template_decl),
-					  new_parms, current_template_parms);
+      TREE_VALUE (current_template_parms) = new_parms;
     }
   else
     {
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 9834baf34db..85294f72ba6 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -448,7 +448,7 @@ inline_needs_template_parms (tree decl, bool nsdmi)
     return false;
 
   return (TMPL_PARMS_DEPTH (DECL_TEMPLATE_PARMS (most_general_template (decl)))
-	  > (processing_template_decl + DECL_TEMPLATE_SPECIALIZATION (decl)));
+	  > (current_template_depth + DECL_TEMPLATE_SPECIALIZATION (decl)));
 }
 
 /* Subroutine of maybe_begin_member_template_processing.
@@ -467,7 +467,7 @@ push_inline_template_parms_recursive (tree parmlist, int levels)
 
   ++processing_template_decl;
   current_template_parms
-    = tree_cons (size_int (processing_template_decl),
+    = tree_cons (size_int (current_template_depth + 1),
 		 parms, current_template_parms);
   TEMPLATE_PARMS_FOR_INLINE (current_template_parms) = 1;
 
@@ -523,7 +523,7 @@ maybe_begin_member_template_processing (tree decl)
   if (inline_needs_template_parms (decl, nsdmi))
     {
       parms = DECL_TEMPLATE_PARMS (most_general_template (decl));
-      levels = TMPL_PARMS_DEPTH (parms) - processing_template_decl;
+      levels = TMPL_PARMS_DEPTH (parms) - current_template_depth;
 
       if (DECL_TEMPLATE_SPECIALIZATION (decl))
 	{
@@ -716,7 +716,7 @@ begin_template_parm_list (void)
 
   /* Add a dummy parameter level while we process the parameter list.  */
   current_template_parms
-    = tree_cons (size_int (processing_template_decl),
+    = tree_cons (size_int (current_template_depth + 1),
 		 make_tree_vec (0),
 		 current_template_parms);
 }
@@ -4613,8 +4613,8 @@ process_template_parm (tree list, location_t parm_loc, tree parm,
       TREE_CONSTANT (decl) = 1;
       TREE_READONLY (decl) = 1;
       DECL_INITIAL (parm) = DECL_INITIAL (decl)
-	= build_template_parm_index (idx, processing_template_decl,
-				     processing_template_decl,
+	= build_template_parm_index (idx, current_template_depth,
+				     current_template_depth,
 				     decl, TREE_TYPE (parm));
 
       TEMPLATE_PARM_PARAMETER_PACK (DECL_INITIAL (parm))
@@ -4655,8 +4655,8 @@ process_template_parm (tree list, location_t parm_loc, tree parm,
       TYPE_STUB_DECL (t) = decl;
       parm = decl;
       TEMPLATE_TYPE_PARM_INDEX (t)
-	= build_template_parm_index (idx, processing_template_decl,
-				     processing_template_decl,
+	= build_template_parm_index (idx, current_template_depth,
+				     current_template_depth,
 				     decl, TREE_TYPE (parm));
       TEMPLATE_TYPE_PARAMETER_PACK (t) = is_parameter_pack;
       if (TREE_CODE (t) == TEMPLATE_TEMPLATE_PARM)
@@ -4705,7 +4705,7 @@ end_template_parm_list (tree parms)
   current_template_parms = TREE_CHAIN (current_template_parms);
 
   current_template_parms
-    = tree_cons (size_int (processing_template_decl),
+    = tree_cons (size_int (current_template_depth + 1),
 		 saved_parmlist, current_template_parms);
 
   for (unsigned ix = 0; parms; ix++)
@@ -6157,7 +6157,7 @@ add_inherited_template_parms (tree fn, tree inherited)
     = INNERMOST_TEMPLATE_PARMS (DECL_TEMPLATE_PARMS (inherited));
   inner_parms = copy_node (inner_parms);
   tree parms
-    = tree_cons (size_int (processing_template_decl + 1),
+    = tree_cons (size_int (current_template_depth + 1),
 		 inner_parms, current_template_parms);
   tree tmpl = build_template_decl (fn, parms, /*member*/true);
   tree args = template_parms_to_args (parms);
@@ -12165,13 +12165,13 @@ instantiate_class_template_1 (tree type)
 	      /* Build new CLASSTYPE_FRIEND_CLASSES.  */
 
 	      tree friend_type = t;
-	      bool adjust_processing_template_decl = false;
+	      bool adjust_template_depth = false;
 
 	      if (TREE_CODE (friend_type) == TEMPLATE_DECL)
 		{
 		  /* template <class T> friend class C;  */
 		  friend_type = tsubst_friend_class (friend_type, args);
-		  adjust_processing_template_decl = true;
+		  adjust_template_depth = true;
 		}
 	      else if (TREE_CODE (friend_type) == UNBOUND_CLASS_TEMPLATE)
 		{
@@ -12180,7 +12180,7 @@ instantiate_class_template_1 (tree type)
 					tf_warning_or_error, NULL_TREE);
 		  if (TREE_CODE (friend_type) == TEMPLATE_DECL)
 		    friend_type = TREE_TYPE (friend_type);
-		  adjust_processing_template_decl = true;
+		  adjust_template_depth = true;
 		}
 	      else if (TREE_CODE (friend_type) == TYPENAME_TYPE
 		       || TREE_CODE (friend_type) == TEMPLATE_TYPE_PARM)
@@ -12200,7 +12200,7 @@ instantiate_class_template_1 (tree type)
 		  friend_type = tsubst (friend_type, args,
 					tf_warning_or_error, NULL_TREE);
 		  if (dependent_type_p (friend_type))
-		    adjust_processing_template_decl = true;
+		    adjust_template_depth = true;
 		  --processing_template_decl;
 		}
 	      else if (uses_template_parms (friend_type))
@@ -12218,19 +12218,23 @@ instantiate_class_template_1 (tree type)
 
 		 We don't have to do anything in these cases.  */
 
-	      if (adjust_processing_template_decl)
-		/* Trick make_friend_class into realizing that the friend
-		   we're adding is a template, not an ordinary class.  It's
-		   important that we use make_friend_class since it will
-		   perform some error-checking and output cross-reference
-		   information.  */
-		++processing_template_decl;
+	      if (adjust_template_depth)
+		{
+		  /* Trick make_friend_class into realizing that the friend
+		     we're adding is a template, not an ordinary class.  It's
+		     important that we use make_friend_class since it will
+		     perform some error-checking and output cross-reference
+		     information.  */
+		  gcc_assert (!current_template_parms);
+		  current_template_parms
+		    = build_tree_list (size_int (1), make_tree_vec (0));
+		}
 
 	      if (friend_type != error_mark_node)
 		make_friend_class (type, friend_type, /*complain=*/false);
 
-	      if (adjust_processing_template_decl)
-		--processing_template_decl;
+	      if (adjust_template_depth)
+		current_template_parms = TREE_CHAIN (current_template_parms);
 	    }
 	  else
 	    {
@@ -28417,7 +28421,7 @@ make_auto_1 (tree name, bool set_canonical)
   TYPE_NAME (au) = build_decl (input_location, TYPE_DECL, name, au);
   TYPE_STUB_DECL (au) = TYPE_NAME (au);
   TEMPLATE_TYPE_PARM_INDEX (au) = build_template_parm_index
-    (0, processing_template_decl + 1, processing_template_decl + 1,
+    (0, current_template_depth + 1, current_template_depth + 1,
      TYPE_NAME (au), NULL_TREE);
   if (set_canonical)
     TYPE_CANONICAL (au) = canonical_type_parameter (au);
@@ -30070,7 +30074,7 @@ splice_late_return_type (tree type, tree late_return_type)
     }
 
   if (tree auto_node = find_type_usage (type, is_auto))
-    if (TEMPLATE_TYPE_LEVEL (auto_node) <= processing_template_decl)
+    if (TEMPLATE_TYPE_LEVEL (auto_node) <= current_template_depth)
       {
 	/* In an abbreviated function template we didn't know we were dealing
 	   with a function template when we saw the auto return type, so rebuild
diff --git a/gcc/cp/semantics.c b/gcc/cp/semantics.c
index cdf63c15e21..d8b20ff5b1b 100644
--- a/gcc/cp/semantics.c
+++ b/gcc/cp/semantics.c
@@ -3629,7 +3629,7 @@ fixup_template_type (tree type)
   // the scope we're trying to enter.
   tree parms = current_template_parms;
   int depth = template_class_depth (type);
-  for (int n = processing_template_decl; n > depth && parms; --n)
+  for (int n = current_template_depth; n > depth && parms; --n)
     parms = TREE_CHAIN (parms);
   if (!parms)
     return type;
diff --git a/gcc/testsuite/g++.dg/concepts/diagnostic18.C b/gcc/testsuite/g++.dg/concepts/diagnostic18.C
index 79f371b8092..4ac706f8e2d 100644
--- a/gcc/testsuite/g++.dg/concepts/diagnostic18.C
+++ b/gcc/testsuite/g++.dg/concepts/diagnostic18.C
@@ -4,4 +4,4 @@
 void foo(auto&& arg) requires({}); // { dg-error "statement-expressions are not allowed|braced-groups" }
 
 template <auto = 0> requires ([]{}()); // { dg-error "expected unqualified-id" }
-auto f() requires ([]{}());
+auto f() requires ([]{}()); // { dg-error "constraints on a non-templated" }
diff --git a/gcc/testsuite/g++.dg/cpp23/auto-fncast10.C b/gcc/testsuite/g++.dg/cpp23/auto-fncast10.C
new file mode 100644
index 00000000000..669dda14035
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/auto-fncast10.C
@@ -0,0 +1,19 @@
+// PR c++/103408
+// { dg-do compile { target c++23 } }
+
+static_assert(requires { auto(0); });
+static_assert(requires { auto{0}; });
+
+static_assert(requires { auto(auto(0)); });
+static_assert(requires { auto{auto{0}}; });
+
+static_assert(requires { auto(auto(auto(0))); });
+static_assert(requires { auto{auto{auto{0}}}; });
+
+static_assert(requires { requires auto(true); });
+static_assert(requires { requires auto(auto(true)); });
+
+static_assert(!requires { requires auto(false); });
+static_assert(!requires { requires auto(auto(false)); });
+
+auto f() requires (auto(false)); // { dg-error "constraints on non-templated" }
