c++: remove Concepts TS code

In GCC 14 we deprecated Concepts TS and discussed removing the code
in GCC 15.  This patch removes Concepts TS code from the front end,
including support for template-introductions, as in:

  template<typename T>
  concept C = true;

  C{T} void foo (T); // write template<C T> void foo (T);

The biggest part of this patch is adjusting the testsuite.  We don't
want to lose coverage so I've converted most of -fconcepts-ts tests
to C++20.  That means they no longer have to be c++17_only.  Mostly
this meant turning "concept bool" into "concept" and turning function
concepts into C++20 concepts.  I've added missing "auto"s where
required, but "auto"s in template-argument-lists are not supported
anymore so I've removed some of the tests; some of them are still
present to verify we don't crash on such autos.  I've also added ()
around "requires" expressions.

I plan to add a porting_to.html entry with a few hints.

I've rebased and tested the patch after the recent r15-1103.

gcc/c-family/ChangeLog:

	* c-cppbuiltin.cc (c_cpp_builtins): Remove flag_concepts_ts code.
	* c-opts.cc (c_common_post_options): Likewise.
	* c.opt: Remove -fconcepts-ts.
	* c.opt.urls: Regenerate.

gcc/cp/ChangeLog:

	* constraint.cc (deduce_concept_introduction, get_deduced_wildcard,
	get_introduction_prototype, introduce_type_template_parameter,
	introduce_template_template_parameter,
	introduce_nontype_template_parameter,
	build_introduced_template_parameter, introduce_template_parameter,
	introduce_template_parameter_pack, introduce_template_parameter,
	introduce_template_parameters, process_introduction_parms,
	check_introduction_list, finish_template_introduction): Remove.
	(finish_shorthand_constraint): Remove a Concepts TS comment.
	* cp-tree.h (check_auto_in_tmpl_args, finish_template_introduction):
	Remove.
	* decl.cc (function_requirements_equivalent_p): Remove pre-C++20 code.
	(grokfndecl): Don't check flag_concepts_ts.
	(grokvardecl): Don't check that concept have type bool.
	* parser.cc (cp_parser_decl_specifier_seq): Don't check
	flag_concepts_ts.
	(cp_parser_introduction_list): Remove.
	(cp_parser_template_id): Remove dead code.
	(cp_parser_simple_type_specifier): Don't check flag_concepts_ts.
	(cp_parser_placeholder_type_specifier): Require require auto or
	decltype(auto) even pre-C++20.  Don't check flag_concepts_ts.
	(cp_parser_type_id_1): Don't check flag_concepts_ts.
	(cp_parser_template_type_arg): Likewise.
	(cp_parser_requires_clause_opt): Remove flag_concepts_ts code.
	(cp_parser_compound_requirement): Don't check flag_concepts_ts.
	(cp_parser_template_introduction): Remove.
	(cp_parser_template_declaration_after_export): Don't call
	cp_parser_template_introduction.
	* pt.cc (template_heads_equivalent_p): Remove pre-C++20 code.
	(find_parameter_pack_data): Remove type_pack_expansion_p.
	(find_parameter_packs_r): Remove flag_concepts_ts code.  Remove
	type_pack_expansion_p code.
	(uses_parameter_packs): Remove type_pack_expansion_p code.
	(make_pack_expansion): Likewise.
	(check_for_bare_parameter_packs): Likewise.
	(fixed_parameter_pack_p): Likewise.
	(tsubst_qualified_id): Remove dead code.
	(extract_autos_r): Remove.
	(extract_autos): Remove.
	(do_auto_deduction): Remove flag_concepts_ts code.
	(type_uses_auto): Likewise.
	(check_auto_in_tmpl_args): Remove.

gcc/ChangeLog:

	* doc/invoke.texi: Mention that -fconcepts-ts was removed.

libstdc++-v3/ChangeLog:

	* testsuite/std/ranges/access/101782.cc: Don't compile with
	-fconcepts-ts.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/auto3.C: Compile with -fconcepts.  Run in C++17 and
	up.  Add dg-error.
	* g++.dg/concepts/auto5.C: Likewise.
	* g++.dg/concepts/auto7.C: Compile with -fconcepts.  Add dg-error.
	* g++.dg/concepts/auto8a.C: Compile with -fconcepts.
	* g++.dg/concepts/class-deduction1.C: Compile with -fconcepts.  Run in
	C++17 and up.  Convert to C++20.
	* g++.dg/concepts/class5.C: Likewise.
	* g++.dg/concepts/class6.C: Likewise.
	* g++.dg/concepts/debug1.C: Likewise.
	* g++.dg/concepts/decl-diagnose.C: Compile with -fconcepts.  Run in
	C++17 and up.  Add dg-error.
	* g++.dg/concepts/deduction-constraint1.C: Compile with -fconcepts.
	Run in C++17 and up.  Convert to C++20.
	* g++.dg/concepts/diagnostic1.C: Likewise.
	* g++.dg/concepts/dr1430.C: Likewise.
	* g++.dg/concepts/equiv.C: Likewise.
	* g++.dg/concepts/equiv2.C: Likewise.
	* g++.dg/concepts/expression.C: Likewise.
	* g++.dg/concepts/expression2.C: Likewise.
	* g++.dg/concepts/expression3.C: Likewise.
	* g++.dg/concepts/fn-concept2.C: Compile with -fconcepts.  Run in
	C++17 and up.  Remove code.  Add dg-prune-output.
	* g++.dg/concepts/fn-concept3.C: Compile with -fconcepts.  Run in
	C++17 and up.  Convert to C++20.
	* g++.dg/concepts/fn1.C: Likewise.
	* g++.dg/concepts/fn10.C: Likewise.
	* g++.dg/concepts/fn2.C: Likewise.
	* g++.dg/concepts/fn3.C: Likewise.
	* g++.dg/concepts/fn4.C: Likewise.
	* g++.dg/concepts/fn5.C: Likewise.
	* g++.dg/concepts/fn6.C: Likewise.
	* g++.dg/concepts/fn7.C: Compile with -fconcepts.  Add dg-error.
	* g++.dg/concepts/fn8.C: Compile with -fconcepts.  Run in C++17 and up.
	Convert to C++20.
	* g++.dg/concepts/fn9.C: Likewise.
	* g++.dg/concepts/generic-fn-err.C: Likewise.
	* g++.dg/concepts/generic-fn.C: Likewise.
	* g++.dg/concepts/inherit-ctor1.C: Likewise.
	* g++.dg/concepts/inherit-ctor3.C: Likewise.
	* g++.dg/concepts/intro1.C: Likewise.
	* g++.dg/concepts/locations1.C: Compile with -fconcepts.  Run in C++17
	and up.  Add dg-prune-output.
	* g++.dg/concepts/partial-concept-id1.C: Compile with -fconcepts.
	Run in C++17 and up.  Convert to C++20.
	* g++.dg/concepts/partial-concept-id2.C: Likewise.
	* g++.dg/concepts/partial-spec5.C: Likewise.
	* g++.dg/concepts/placeholder2.C: Likewise.
	* g++.dg/concepts/placeholder3.C: Likewise.
	* g++.dg/concepts/placeholder4.C: Likewise.
	* g++.dg/concepts/placeholder5.C: Likewise.
	* g++.dg/concepts/placeholder6.C: Likewise.
	* g++.dg/concepts/pr65634.C: Likewise.
	* g++.dg/concepts/pr65636.C: Likewise.
	* g++.dg/concepts/pr65681.C: Likewise.
	* g++.dg/concepts/pr65848.C: Likewise.
	* g++.dg/concepts/pr67249.C: Likewise.
	* g++.dg/concepts/pr67595.C: Likewise.
	* g++.dg/concepts/pr68434.C: Likewise.
	* g++.dg/concepts/pr71127.C: Likewise.
	* g++.dg/concepts/pr71128.C: Compile with -fconcepts.  Run in C++17
	and up.  Add dg-error.
	* g++.dg/concepts/pr71131.C: Compile with -fconcepts.  Run in C++17
	and up.  Convert to C++20.
	* g++.dg/concepts/pr71385.C: Likewise.
	* g++.dg/concepts/pr85065.C: Likewise.
	* g++.dg/concepts/pr92804-2.C: Compile with -fconcepts.  Convert to
	C++20.
	* g++.dg/concepts/template-parm11.C: Compile with -fconcepts.  Run in
	C++17 and up.  Convert to C++20.
	* g++.dg/concepts/template-parm12.C: Likewise.
	* g++.dg/concepts/template-parm2.C: Likewise.
	* g++.dg/concepts/template-parm3.C: Likewise.
	* g++.dg/concepts/template-parm4.C: Likewise.
	* g++.dg/concepts/template-template-parm1.C: Likewise.
	* g++.dg/concepts/var-concept1.C: Likewise.
	* g++.dg/concepts/var-concept2.C: Likewise.
	* g++.dg/concepts/var-concept3.C: Likewise.
	* g++.dg/concepts/var-concept4.C: Likewise.
	* g++.dg/concepts/var-concept5.C: Likewise.
	* g++.dg/concepts/var-concept6.C: Likewise.
	* g++.dg/concepts/var-concept7.C: Likewise.
	* g++.dg/concepts/var-templ1.C: Run in C++17 and up.
	* g++.dg/concepts/var-templ2.C: Compile with -fconcepts.  Run in C++17
	and up.  Convert to C++20.
	* g++.dg/concepts/var-templ3.C: Likewise.
	* g++.dg/concepts/variadic1.C: Likewise.
	* g++.dg/concepts/variadic2.C: Likewise.
	* g++.dg/concepts/variadic3.C: Likewise.
	* g++.dg/concepts/variadic4.C: Likewise.
	* g++.dg/cpp2a/concepts-pr65575.C: Likewise.
	* g++.dg/cpp2a/concepts-pr66091.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67148.C: Compile with -fconcepts.  Convert
	to C++20.
	* g++.dg/cpp2a/concepts-pr67225-1.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67225-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67225-3.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67225-4.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67225-5.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67319.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67427.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67654.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67658.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67684.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67697.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67719.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67774.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67825.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67860.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67862.C: Likewise.
	* g++.dg/cpp2a/concepts-pr67969.C: Likewise.
	* g++.dg/cpp2a/concepts-pr68093-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr68372.C: Likewise.
	* g++.dg/cpp2a/concepts-pr68812.C: Likewise.
	* g++.dg/cpp2a/concepts-pr69235.C: Likewise.
	* g++.dg/cpp2a/concepts-pr78752-2.C: Likewise.
	* g++.dg/cpp2a/concepts-pr78752.C: Likewise.
	* g++.dg/cpp2a/concepts-pr79759.C: Likewise.
	* g++.dg/cpp2a/concepts-pr80746.C: Likewise.
	* g++.dg/cpp2a/concepts-pr80773.C: Likewise.
	* g++.dg/cpp2a/concepts-pr82507.C: Likewise.
	* g++.dg/cpp2a/concepts-pr82740.C: Likewise.
	* g++.dg/cpp2a/concepts-pr84980.C: Compile with -fconcepts.  Run in
	C++17 and up.  Convert to C++20.
	* g++.dg/cpp2a/concepts-pr85265.C: Likewise.
	* g++.dg/cpp2a/concepts-pr85808.C: Compile with -fconcepts.  Convert
	to C++20.
	* g++.dg/cpp2a/concepts-pr86269.C: Likewise.
	* g++.dg/cpp2a/concepts-pr87441.C: Likewise.
	* g++.dg/cpp2a/concepts-requires5.C: Compile with -fconcepts.
	Adjust dg-error.  Add same_as.
	* g++.dg/cpp2a/nontype-class50a.C: Compile with -fconcepts.
	* g++.dg/concepts/auto1.C: Removed.
	* g++.dg/concepts/auto4.C: Removed.
	* g++.dg/concepts/auto6.C: Removed.
	* g++.dg/concepts/fn-concept1.C: Removed.
	* g++.dg/concepts/intro2.C: Removed.
	* g++.dg/concepts/intro3.C: Removed.
	* g++.dg/concepts/intro4.C: Removed.
	* g++.dg/concepts/intro5.C: Removed.
	* g++.dg/concepts/intro6.C: Removed.
	* g++.dg/concepts/intro7.C: Removed.
	* g++.dg/cpp2a/concepts-ts1.C: Removed.
	* g++.dg/cpp2a/concepts-ts2.C: Removed.
	* g++.dg/cpp2a/concepts-ts3.C: Removed.
	* g++.dg/cpp2a/concepts-ts4.C: Removed.
	* g++.dg/cpp2a/concepts-ts5.C: Removed.
	* g++.dg/cpp2a/concepts-ts6.C: Removed.

1 files changed, 34 insertions, 226 deletions

diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc
index 31ae9c2..6bf3f52 100644
--- a/gcc/cp/parser.cc
+++ b/gcc/cp/parser.cc
@@ -16624,16 +16624,15 @@ cp_parser_decl_specifier_seq (cp_parser* parser,
 
           /* Warn for concept as a decl-specifier. We'll rewrite these as
              concept declarations later.  */
-          if (!flag_concepts_ts)
-            {
-	      cp_token *next = cp_lexer_peek_token (parser->lexer);
-	      if (next->keyword == RID_BOOL)
-		permerror (next->location, "the %<bool%> keyword is not "
-			   "allowed in a C++20 concept definition");
-	      else
-		error_at (token->location, "C++20 concept definition syntax "
-			  "is %<concept <name> = <expr>%>");
-            }
+	  {
+	    cp_token *next = cp_lexer_peek_token (parser->lexer);
+	    if (next->keyword == RID_BOOL)
+	      permerror (next->location, "the %<bool%> keyword is not "
+			 "allowed in a C++20 concept definition");
+	    else
+	      error_at (token->location, "C++20 concept definition syntax "
+			"is %<concept <name> = <expr>%>");
+	  }
 
 	  /* In C++20 a concept definition is just 'concept name = expr;'
 	     Support that syntax as a TS extension by pretending we've seen
@@ -18431,61 +18430,6 @@ cp_parser_template_parameter_list (cp_parser* parser)
   return end_template_parm_list (parameter_list);
 }
 
-/* Parse a introduction-list.
-
-   introduction-list:
-     introduced-parameter
-     introduction-list , introduced-parameter
-
-   introduced-parameter:
-     ...[opt] identifier
-
-   Returns a TREE_VEC of WILDCARD_DECLs.  If the parameter is a pack
-   then the introduced parm will have WILDCARD_PACK_P set.  In addition, the
-   WILDCARD_DECL will also have DECL_NAME set and token location in
-   DECL_SOURCE_LOCATION.  */
-
-static tree
-cp_parser_introduction_list (cp_parser *parser)
-{
-  vec<tree, va_gc> *introduction_vec = make_tree_vector ();
-
-  while (true)
-    {
-      bool is_pack = cp_lexer_next_token_is (parser->lexer, CPP_ELLIPSIS);
-      if (is_pack)
-	cp_lexer_consume_token (parser->lexer);
-
-      tree identifier = cp_parser_identifier (parser);
-      if (identifier == error_mark_node)
-	break;
-
-      /* Build placeholder. */
-      tree parm = build_nt (WILDCARD_DECL);
-      DECL_SOURCE_LOCATION (parm)
-	= cp_lexer_peek_token (parser->lexer)->location;
-      DECL_NAME (parm) = identifier;
-      WILDCARD_PACK_P (parm) = is_pack;
-      vec_safe_push (introduction_vec, parm);
-
-      /* If the next token is not a `,', we're done.  */
-      if (cp_lexer_next_token_is_not (parser->lexer, CPP_COMMA))
-	break;
-      /* Otherwise, consume the `,' token.  */
-      cp_lexer_consume_token (parser->lexer);
-    }
-
-  /* Convert the vec into a TREE_VEC.  */
-  tree introduction_list = make_tree_vec (introduction_vec->length ());
-  unsigned int n;
-  tree parm;
-  FOR_EACH_VEC_ELT (*introduction_vec, n, parm)
-    TREE_VEC_ELT (introduction_list, n) = parm;
-
-  release_tree_vector (introduction_vec);
-  return introduction_list;
-}
-
 /* Given a declarator, get the declarator-id part, or NULL_TREE if this
    is an abstract declarator. */
 
@@ -19218,16 +19162,8 @@ cp_parser_template_id (cp_parser *parser,
   location_t combined_loc
     = make_location (token->location, token->location, parser->lexer);
 
-  /* Check for concepts autos where they don't belong.  We could
-     identify types in some cases of identifier TEMPL, looking ahead
-     for a CPP_SCOPE, but that would buy us nothing: we accept auto in
-     types.  We reject them in functions, but if what we have is an
-     identifier, even with none_type we can't conclude it's NOT a
-     type, we have to wait for template substitution.  */
-  if (flag_concepts && check_auto_in_tmpl_args (templ, arguments))
-    template_id = error_mark_node;
   /* Build a representation of the specialization.  */
-  else if (identifier_p (templ))
+  if (identifier_p (templ))
     template_id = build_min_nt_loc (combined_loc,
 				    TEMPLATE_ID_EXPR,
 				    templ, arguments);
@@ -20549,10 +20485,9 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 			 "only available with "
 			 "%<-std=c++14%> or %<-std=gnu++14%>");
 	    }
-	  else if (!flag_concepts_ts && parser->in_template_argument_list_p)
-	    pedwarn (token->location, 0,
-		     "use of %<auto%> in template argument "
-		     "only available with %<-fconcepts-ts%>");
+	  else if (parser->in_template_argument_list_p)
+	    error_at (token->location,
+		     "use of %<auto%> in template argument");
 	  else if (!flag_concepts)
 	    pedwarn (token->location, 0,
 		     "use of %<auto%> in parameter declaration "
@@ -20895,10 +20830,7 @@ cp_parser_simple_type_specifier (cp_parser* parser,
 
   TENTATIVE is true if the type-specifier parsing is tentative; in that case,
   don't give an error if TMPL isn't a valid type-constraint, as the template-id
-  might actually be a concept-check,
-
-  Note that the Concepts TS allows the auto or decltype(auto) to be
-  omitted in a constrained-type-specifier.  */
+  might actually be a concept-check.  */
 
 static tree
 cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
@@ -20929,38 +20861,29 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
   if (con == error_mark_node)
     return error_mark_node;
 
-  /* As per the standard, require auto or decltype(auto), except in some
-     cases (template parameter lists, -fconcepts-ts enabled).  */
+  /* As per the standard, require auto or decltype(auto).  */
   cp_token *placeholder = NULL, *close_paren = NULL;
-  if (cxx_dialect >= cxx20)
+  if (cp_lexer_next_token_is_keyword (parser->lexer, RID_AUTO))
+    placeholder = cp_lexer_consume_token (parser->lexer);
+  else if (cp_lexer_next_token_is_keyword (parser->lexer, RID_DECLTYPE))
     {
-      if (cp_lexer_next_token_is_keyword (parser->lexer, RID_AUTO))
-	placeholder = cp_lexer_consume_token (parser->lexer);
-      else if (cp_lexer_next_token_is_keyword (parser->lexer, RID_DECLTYPE))
-	{
-	  placeholder = cp_lexer_consume_token (parser->lexer);
-	  matching_parens parens;
-	  parens.require_open (parser);
-	  cp_parser_require_keyword (parser, RID_AUTO, RT_AUTO);
-	  close_paren = parens.require_close (parser);
-	}
+      placeholder = cp_lexer_consume_token (parser->lexer);
+      matching_parens parens;
+      parens.require_open (parser);
+      cp_parser_require_keyword (parser, RID_AUTO, RT_AUTO);
+      close_paren = parens.require_close (parser);
     }
 
   /* A type constraint constrains a contextually determined type or type
-     parameter pack. However, the Concepts TS does allow concepts
-     to introduce non-type and template template parameters.  */
+     parameter pack.  */
   if (TREE_CODE (proto) != TYPE_DECL)
     {
-      if (!flag_concepts_ts
-	  || !processing_template_parmlist)
+      if (!tentative)
 	{
-	  if (!tentative)
-	    {
-	      error_at (loc, "%qE does not constrain a type", DECL_NAME (con));
-	      inform (DECL_SOURCE_LOCATION (con), "concept defined here");
-	    }
-	  return error_mark_node;
+	  error_at (loc, "%qE does not constrain a type", DECL_NAME (con));
+	  inform (DECL_SOURCE_LOCATION (con), "concept defined here");
 	}
+      return error_mark_node;
     }
 
   /* In a template parameter list, a type-parameter can be introduced
@@ -20978,9 +20901,7 @@ cp_parser_placeholder_type_specifier (cp_parser *parser, location_t loc,
     }
 
   /* Diagnose issues placeholder issues.  */
-  if (!flag_concepts_ts
-      && !parser->in_result_type_constraint_p
-      && !placeholder)
+  if (!parser->in_result_type_constraint_p && !placeholder)
     {
       if (tentative)
 	/* Perhaps it's a concept-check expression (c++/91073).  */
@@ -25245,14 +25166,11 @@ cp_parser_type_id_1 (cp_parser *parser, cp_parser_flags flags,
     abstract_declarator = NULL;
 
   bool auto_typeid_ok = false;
-  /* The concepts TS allows 'auto' as a type-id.  */
-  if (flag_concepts_ts)
-    auto_typeid_ok = !parser->in_type_id_in_expr_p;
   /* DR 625 prohibits use of auto as a template-argument.  We allow 'auto'
      outside the template-argument-list context here only for the sake of
      diagnostic: grokdeclarator then can emit a better error message for
      e.g. using T = auto.  */
-  else if (flag_concepts)
+  if (flag_concepts)
     auto_typeid_ok = (!parser->in_type_id_in_expr_p
 		      && !parser->in_template_argument_list_p);
 
@@ -25326,7 +25244,7 @@ cp_parser_template_type_arg (cp_parser *parser)
   parser->type_definition_forbidden_message = saved_message;
   /* cp_parser_type_id_1 checks for auto, but only for
      ->auto_is_implicit_function_template_parm_p.  */
-  if (cxx_dialect >= cxx14 && !flag_concepts_ts && type_uses_auto (r))
+  if (cxx_dialect >= cxx14 && type_uses_auto (r))
     {
       error ("invalid use of %<auto%> in template argument");
       r = error_mark_node;
@@ -31618,8 +31536,7 @@ cp_parser_constraint_logical_or_expression (cp_parser *parser, bool lambda_p)
   return lhs;
 }
 
-/* Parse the expression after a requires-clause. This has a different grammar
-    than that in the concepts TS.  */
+/* Parse the expression after a requires-clause.  */
 
 static tree
 cp_parser_requires_clause_expression (cp_parser *parser, bool lambda_p)
@@ -31704,10 +31621,7 @@ cp_parser_requires_clause_opt (cp_parser *parser, bool lambda_p)
   else
     cp_lexer_consume_token (parser->lexer);
 
-  if (!flag_concepts_ts)
-    return cp_parser_requires_clause_expression (parser, lambda_p);
-  else
-    return cp_parser_constraint_expression (parser);
+  return cp_parser_requires_clause_expression (parser, lambda_p);
 }
 
 /*---------------------------------------------------------------------------
@@ -32082,7 +31996,7 @@ cp_parser_compound_requirement (cp_parser *parser)
 	      return error_mark_node;
 	    }
 	}
-      else if (!flag_concepts_ts)
+      else
 	/* P1452R2 removed the trailing-return-type option.  */
 	error_at (type_loc,
 		  "return-type-requirement is not a type-constraint");
@@ -33228,110 +33142,6 @@ cp_parser_template_declaration_after_parameters (cp_parser* parser,
     vec_safe_push (unparsed_funs_with_definitions, decl);
 }
 
-/* Parse a template introduction header for a template-declaration.  Returns
-   false if tentative parse fails.  */
-
-static bool
-cp_parser_template_introduction (cp_parser* parser, bool member_p)
-{
-  cp_parser_parse_tentatively (parser);
-
-  tree saved_scope = parser->scope;
-  tree saved_object_scope = parser->object_scope;
-  tree saved_qualifying_scope = parser->qualifying_scope;
-  bool saved_colon_corrects_to_scope_p = parser->colon_corrects_to_scope_p;
-
-  cp_token *start_token = cp_lexer_peek_token (parser->lexer);
-
-  /* In classes don't parse valid unnamed bitfields as invalid
-     template introductions.  */
-  if (member_p)
-    parser->colon_corrects_to_scope_p = false;
-
-  /* Look for the optional `::' operator.  */
-  cp_parser_global_scope_opt (parser,
-			      /*current_scope_valid_p=*/false);
-  /* Look for the nested-name-specifier.  */
-  cp_parser_nested_name_specifier_opt (parser,
-				       /*typename_keyword_p=*/false,
-				       /*check_dependency_p=*/true,
-				       /*type_p=*/false,
-				       /*is_declaration=*/false);
-
-  cp_token *token = cp_lexer_peek_token (parser->lexer);
-  tree concept_name = cp_parser_identifier (parser);
-
-  /* Look up the concept for which we will be matching
-     template parameters.  */
-  tree tmpl_decl = cp_parser_lookup_name_simple (parser, concept_name,
-						 token->location);
-  parser->scope = saved_scope;
-  parser->object_scope = saved_object_scope;
-  parser->qualifying_scope = saved_qualifying_scope;
-  parser->colon_corrects_to_scope_p = saved_colon_corrects_to_scope_p;
-
-  if (concept_name == error_mark_node
-      || (seen_error () && !concept_definition_p (tmpl_decl)))
-    cp_parser_simulate_error (parser);
-
-  /* Look for opening brace for introduction.  */
-  matching_braces braces;
-  braces.require_open (parser);
-  location_t open_loc = input_location;
-
-  if (!cp_parser_parse_definitely (parser))
-    return false;
-
-  push_deferring_access_checks (dk_deferred);
-
-  /* Build vector of placeholder parameters and grab
-     matching identifiers.  */
-  tree introduction_list = cp_parser_introduction_list (parser);
-
-  /* Look for closing brace for introduction.  */
-  if (!braces.require_close (parser))
-    return true;
-
-  /* The introduction-list shall not be empty.  */
-  int nargs = TREE_VEC_LENGTH (introduction_list);
-  if (nargs == 0)
-    {
-      /* In cp_parser_introduction_list we have already issued an error.  */
-      return true;
-    }
-
-  if (tmpl_decl == error_mark_node)
-    {
-      cp_parser_name_lookup_error (parser, concept_name, tmpl_decl, NLE_NULL,
-				   token->location);
-      return true;
-    }
-
-  /* Build and associate the constraint.  */
-  location_t introduction_loc = make_location (open_loc,
-					       start_token->location,
-					       parser->lexer);
-  tree parms = finish_template_introduction (tmpl_decl,
-					     introduction_list,
-					     introduction_loc);
-  if (parms && parms != error_mark_node)
-    {
-      if (!flag_concepts_ts)
-	pedwarn (introduction_loc, 0, "template-introductions"
-		 " are not part of C++20 concepts; use %qs to enable",
-		 "-fconcepts-ts");
-
-      cp_parser_template_declaration_after_parameters (parser, parms,
-						       member_p);
-      return true;
-    }
-
-  if (parms == NULL_TREE)
-    error_at (token->location, "no matching concept for template-introduction");
-
-  return true;
-}
-
 /* Parse a normal template-declaration following the template keyword.  */
 
 static void
@@ -33423,8 +33233,6 @@ cp_parser_template_declaration_after_export (cp_parser* parser, bool member_p)
       cp_parser_explicit_template_declaration (parser, member_p);
       return true;
     }
-  else if (flag_concepts)
-    return cp_parser_template_introduction (parser, member_p);
 
   return false;
 }