c++: Add C++23 consteval if support - P1938R3 [PR100974]

The following patch implements consteval if support.
There is a new IF_STMT_CONSTEVAL_P flag on IF_STMT and IF_COND is
boolean_false_node to match the non-manifestly constant evaluation
behavior, while constexpr evaluation special-cases it.  Perhaps cleaner
would be to set the condition to __builtin_is_constant_evaluated () call
but we need the IF_STMT_CONSTEVAL_P flag anyway and the IL would be larger.

And I'm not changing the libstdc++ side, where perhaps we could change
std::is_constant_evaluated definition for
 #ifdef __cpp_if_consteval
case to if consteval { return true; } else { return false; }
but we need to keep it defined to __builtin_is_constant_evaluated ()
for C++20 or older.

2021-06-11  Jakub Jelinek  <jakub@redhat.com>

	PR c++/100974
gcc/c-family/
	* c-cppbuiltin.c (c_cpp_builtins): Predefine __cpp_if_consteval for
	-std=c++2b for P1938R3 consteval if support.
gcc/cp/
	* cp-tree.h (struct saved_scope): Add consteval_if_p
	member.  Formatting fix for the discarded_stmt comment.
	(in_consteval_if_p, IF_STMT_CONSTEVAL_P): Define.
	* parser.c (cp_parser_lambda_expression): Temporarily disable
	in_consteval_if_p when parsing lambda body.
	(cp_parser_selection_statement): Parse consteval if.
	* decl.c (struct named_label_entry): Add in_consteval_if member.
	(level_for_consteval_if): New function.
	(poplevel_named_label_1, check_previous_goto_1, check_goto): Handle
	consteval if.
	* constexpr.c (cxx_eval_builtin_function_call): Clarify in comment
	why CP_BUILT_IN_IS_CONSTANT_EVALUATED needs to *non_constant_p
	for !ctx->manifestly_const_eval.
	(cxx_eval_conditional_expression): For IF_STMT_CONSTEVAL_P evaluate
	condition as if it was __builtin_is_constant_evaluated call.
	(potential_constant_expression_1): For IF_STMT_CONSTEVAL_P always
	recurse on both branches.
	* cp-gimplify.c (genericize_if_stmt): Genericize IF_STMT_CONSTEVAL_P
	as the else branch.
	* pt.c (tsubst_expr) <case IF_STMT>: Copy IF_STMT_CONSTEVAL_P.
	Temporarily set in_consteval_if_p when recursing on
	IF_STMT_CONSTEVAL_P then branch.
	(tsubst_lambda_expr): Temporarily disable
	in_consteval_if_p when instantiating lambda body.
	* call.c (immediate_invocation_p): Return false when
	in_consteval_if_p.
gcc/testsuite/
	* g++.dg/cpp23/consteval-if1.C: New test.
	* g++.dg/cpp23/consteval-if2.C: New test.
	* g++.dg/cpp23/consteval-if3.C: New test.
	* g++.dg/cpp23/consteval-if4.C: New test.
	* g++.dg/cpp23/consteval-if5.C: New test.
	* g++.dg/cpp23/consteval-if6.C: New test.
	* g++.dg/cpp23/consteval-if7.C: New test.
	* g++.dg/cpp23/consteval-if8.C: New test.
	* g++.dg/cpp23/consteval-if9.C: New test.
	* g++.dg/cpp23/consteval-if10.C: New test.
	* g++.dg/cpp23/feat-cxx2b.C: Add __cpp_if_consteval tests.

19 files changed, 650 insertions, 10 deletions

diff --git a/gcc/c-family/c-cppbuiltin.c b/gcc/c-family/c-cppbuiltin.c
index 42b7604c9ac..f79f939bd10 100644
--- a/gcc/c-family/c-cppbuiltin.c
+++ b/gcc/c-family/c-cppbuiltin.c
@@ -1029,6 +1029,7 @@ c_cpp_builtins (cpp_reader *pfile)
 	{
 	  /* Set feature test macros for C++23.  */
 	  cpp_define (pfile, "__cpp_size_t_suffix=202011L");
+	  cpp_define (pfile, "__cpp_if_consteval=202106L");
 	}
       if (flag_concepts)
         {
diff --git a/gcc/cp/call.c b/gcc/cp/call.c
index d2f6ca872fc..9f03534c20c 100644
--- a/gcc/cp/call.c
+++ b/gcc/cp/call.c
@@ -8840,6 +8840,7 @@ immediate_invocation_p (tree fn, int nargs)
 	      || !DECL_IMMEDIATE_FUNCTION_P (current_function_decl))
 	  && (current_binding_level->kind != sk_function_parms
 	      || !current_binding_level->immediate_fn_ctx_p)
+	  && !in_consteval_if_p
 	  /* As an exception, we defer std::source_location::current ()
 	     invocations until genericization because LWG3396 mandates
 	     special behavior for it.  */
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 01b0c42471d..4f1c3d66129 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -1315,7 +1315,10 @@ cxx_eval_builtin_function_call (const constexpr_ctx *ctx, tree t, tree fun,
     }
 
   /* For __builtin_is_constant_evaluated, defer it if not
-     ctx->manifestly_const_eval, otherwise fold it to true.  */
+     ctx->manifestly_const_eval (as sometimes we try to constant evaluate
+     without manifestly_const_eval even expressions or parts thereof which
+     will later be manifestly const_eval evaluated), otherwise fold it to
+     true.  */
   if (fndecl_built_in_p (fun, CP_BUILT_IN_IS_CONSTANT_EVALUATED,
 			 BUILT_IN_FRONTEND))
     {
@@ -3298,6 +3301,22 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
 					   /*lval*/false,
 					   non_constant_p, overflow_p);
   VERIFY_CONSTANT (val);
+  if (TREE_CODE (t) == IF_STMT && IF_STMT_CONSTEVAL_P (t))
+    {
+      /* Evaluate the condition as if it was
+	 if (__builtin_is_constant_evaluated ()), i.e. defer it if not
+	 ctx->manifestly_const_eval (as sometimes we try to constant evaluate
+	 without manifestly_const_eval even expressions or parts thereof which
+	 will later be manifestly const_eval evaluated), otherwise fold it to
+	 true.  */
+      if (ctx->manifestly_const_eval)
+	val = boolean_true_node;
+      else
+	{
+	  *non_constant_p = true;
+	  return t;
+	}
+    }
   /* Don't VERIFY_CONSTANT the other operands.  */
   if (integer_zerop (val))
     val = TREE_OPERAND (t, 2);
@@ -8809,10 +8828,17 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	return false;
       if (!processing_template_decl)
 	tmp = cxx_eval_outermost_constant_expr (tmp, true);
-      if (integer_zerop (tmp))
-	return RECUR (TREE_OPERAND (t, 2), want_rval);
-      else if (TREE_CODE (tmp) == INTEGER_CST)
-	return RECUR (TREE_OPERAND (t, 1), want_rval);
+      /* potential_constant_expression* isn't told if it is called for
+	 manifestly_const_eval or not, so for consteval if always
+	 process both branches as if the condition is not a known
+	 constant.  */
+      if (TREE_CODE (t) != IF_STMT || !IF_STMT_CONSTEVAL_P (t))
+	{
+	  if (integer_zerop (tmp))
+	    return RECUR (TREE_OPERAND (t, 2), want_rval);
+	  else if (TREE_CODE (tmp) == INTEGER_CST)
+	    return RECUR (TREE_OPERAND (t, 1), want_rval);
+	}
       tmp = *jump_target;
       for (i = 1; i < 3; ++i)
 	{
diff --git a/gcc/cp/cp-gimplify.c b/gcc/cp/cp-gimplify.c
index 9079a5b90ca..96d91b6ef2f 100644
--- a/gcc/cp/cp-gimplify.c
+++ b/gcc/cp/cp-gimplify.c
@@ -161,7 +161,13 @@ genericize_if_stmt (tree *stmt_p)
   if (!else_)
     else_ = build_empty_stmt (locus);
 
-  if (integer_nonzerop (cond) && !TREE_SIDE_EFFECTS (else_))
+  /* consteval if has been verified not to have the then_/else_ blocks
+     entered by gotos/case labels from elsewhere, and as then_ block
+     can contain unfolded immediate function calls, we have to discard
+     the then_ block regardless of whether else_ has side-effects or not.  */
+  if (IF_STMT_CONSTEVAL_P (stmt))
+    stmt = else_;
+  else if (integer_nonzerop (cond) && !TREE_SIDE_EFFECTS (else_))
     stmt = then_;
   else if (integer_zerop (cond) && !TREE_SIDE_EFFECTS (then_))
     stmt = else_;
diff --git a/gcc/cp/cp-tree.h b/gcc/cp/cp-tree.h
index 9ac8b527749..36f99ccf189 100644
--- a/gcc/cp/cp-tree.h
+++ b/gcc/cp/cp-tree.h
@@ -478,6 +478,7 @@ extern GTY(()) tree cp_global_trees[CPTI_MAX];
       AGGR_INIT_ZERO_FIRST (in AGGR_INIT_EXPR)
       CONSTRUCTOR_MUTABLE_POISON (in CONSTRUCTOR)
       OVL_HIDDEN_P (in OVERLOAD)
+      IF_STMT_CONSTEVAL_P (in IF_STMT)
       SWITCH_STMT_NO_BREAK_P (in SWITCH_STMT)
       LAMBDA_EXPR_CAPTURE_OPTIMIZED (in LAMBDA_EXPR)
       IMPLICIT_CONV_EXPR_BRACED_INIT (in IMPLICIT_CONV_EXPR)
@@ -1813,9 +1814,12 @@ struct GTY(()) saved_scope {
   BOOL_BITFIELD x_processing_explicit_instantiation : 1;
   BOOL_BITFIELD need_pop_function_context : 1;
 
-/* Nonzero if we are parsing the discarded statement of a constexpr
-   if-statement.  */
+  /* Nonzero if we are parsing the discarded statement of a constexpr
+     if-statement.  */
   BOOL_BITFIELD discarded_stmt : 1;
+  /* Nonzero if we are parsing or instantiating the compound-statement
+     of consteval if statement.  */
+  BOOL_BITFIELD consteval_if_p : 1;
 
   int unevaluated_operand;
   int inhibit_evaluation_warnings;
@@ -1879,6 +1883,7 @@ extern GTY(()) struct saved_scope *scope_chain;
 #define processing_explicit_instantiation scope_chain->x_processing_explicit_instantiation
 
 #define in_discarded_stmt scope_chain->discarded_stmt
+#define in_consteval_if_p scope_chain->consteval_if_p
 
 #define current_ref_temp_count scope_chain->ref_temp_count
 
@@ -5211,6 +5216,7 @@ more_aggr_init_expr_args_p (const aggr_init_expr_arg_iterator *iter)
 #define ELSE_CLAUSE(NODE)	TREE_OPERAND (IF_STMT_CHECK (NODE), 2)
 #define IF_SCOPE(NODE)		TREE_OPERAND (IF_STMT_CHECK (NODE), 3)
 #define IF_STMT_CONSTEXPR_P(NODE) TREE_LANG_FLAG_0 (IF_STMT_CHECK (NODE))
+#define IF_STMT_CONSTEVAL_P(NODE) TREE_LANG_FLAG_2 (IF_STMT_CHECK (NODE))
 
 /* Like PACK_EXPANSION_EXTRA_ARGS, for constexpr if.  IF_SCOPE is used while
    building an IF_STMT; IF_STMT_EXTRA_ARGS is used after it is complete.  */
diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index cbf647dd569..da254d8bcc2 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -222,6 +222,7 @@ struct GTY((for_user)) named_label_entry {
   bool in_omp_scope;
   bool in_transaction_scope;
   bool in_constexpr_if;
+  bool in_consteval_if;
 };
 
 #define named_labels cp_function_chain->x_named_labels
@@ -491,6 +492,16 @@ level_for_constexpr_if (cp_binding_level *b)
 	  && IF_STMT_CONSTEXPR_P (b->this_entity));
 }
 
+/* True if B is the level for the condition of a consteval if.  */
+
+static bool
+level_for_consteval_if (cp_binding_level *b)
+{
+  return (b->kind == sk_cond && b->this_entity
+	  && TREE_CODE (b->this_entity) == IF_STMT
+	  && IF_STMT_CONSTEVAL_P (b->this_entity));
+}
+
 /* Update data for defined and undefined labels when leaving a scope.  */
 
 int
@@ -530,6 +541,8 @@ poplevel_named_label_1 (named_label_entry **slot, cp_binding_level *bl)
 	case sk_block:
 	  if (level_for_constexpr_if (bl->level_chain))
 	    ent->in_constexpr_if = true;
+	  else if (level_for_consteval_if (bl->level_chain))
+	    ent->in_consteval_if = true;
 	  break;
 	default:
 	  break;
@@ -3391,6 +3404,7 @@ check_previous_goto_1 (tree decl, cp_binding_level* level, tree names,
   bool complained = false;
   int identified = 0;
   bool saw_eh = false, saw_omp = false, saw_tm = false, saw_cxif = false;
+  bool saw_ceif = false;
 
   if (exited_omp)
     {
@@ -3470,6 +3484,12 @@ check_previous_goto_1 (tree decl, cp_binding_level* level, tree names,
 	      loc = EXPR_LOCATION (b->level_chain->this_entity);
 	      saw_cxif = true;
 	    }
+	  else if (!saw_ceif && level_for_consteval_if (b->level_chain))
+	    {
+	      inf = G_("  enters %<consteval if%> statement");
+	      loc = EXPR_LOCATION (b->level_chain->this_entity);
+	      saw_ceif = true;
+	    }
 	  break;
 
 	default:
@@ -3551,12 +3571,13 @@ check_goto (tree decl)
   unsigned ix;
 
   if (ent->in_try_scope || ent->in_catch_scope || ent->in_transaction_scope
-      || ent->in_constexpr_if
+      || ent->in_constexpr_if || ent->in_consteval_if
       || ent->in_omp_scope || !vec_safe_is_empty (ent->bad_decls))
     {
       diagnostic_t diag_kind = DK_PERMERROR;
       if (ent->in_try_scope || ent->in_catch_scope || ent->in_constexpr_if
-	  || ent->in_transaction_scope || ent->in_omp_scope)
+	  || ent->in_consteval_if || ent->in_transaction_scope
+	  || ent->in_omp_scope)
 	diag_kind = DK_ERROR;
       complained = identify_goto (decl, DECL_SOURCE_LOCATION (decl),
 				  &input_location, diag_kind);
@@ -3602,6 +3623,8 @@ check_goto (tree decl)
 	inform (input_location, "  enters synchronized or atomic statement");
       else if (ent->in_constexpr_if)
 	inform (input_location, "  enters %<constexpr if%> statement");
+      else if (ent->in_consteval_if)
+	inform (input_location, "  enters %<consteval if%> statement");
     }
 
   if (ent->in_omp_scope)
diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index d59a829d0b9..686f98bc4f1 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -10902,6 +10902,11 @@ cp_parser_lambda_expression (cp_parser* parser)
     bool discarded = in_discarded_stmt;
     in_discarded_stmt = 0;
 
+    /* Similarly the body of a lambda in immediate function context is not
+       in immediate function context.  */
+    bool save_in_consteval_if_p = in_consteval_if_p;
+    in_consteval_if_p = false;
+
     /* By virtue of defining a local class, a lambda expression has access to
        the private variables of enclosing classes.  */
 
@@ -10932,6 +10937,7 @@ cp_parser_lambda_expression (cp_parser* parser)
 
     finish_struct (type, /*attributes=*/NULL_TREE);
 
+    in_consteval_if_p = save_in_consteval_if_p;
     in_discarded_stmt = discarded;
 
     parser->num_template_parameter_lists = saved_num_template_parameter_lists;
@@ -12324,6 +12330,102 @@ cp_parser_selection_statement (cp_parser* parser, bool *if_p,
 		       "%<if constexpr%> only available with "
 		       "%<-std=c++17%> or %<-std=gnu++17%>");
 	  }
+	int ce = 0;
+	if (keyword == RID_IF && !cx)
+	  {
+	    if (cp_lexer_next_token_is_keyword (parser->lexer,
+						RID_CONSTEVAL))
+	      ce = 1;
+	    else if (cp_lexer_next_token_is (parser->lexer, CPP_NOT)
+		     && cp_lexer_nth_token_is_keyword (parser->lexer, 2,
+						       RID_CONSTEVAL))
+	      {
+		ce = -1;
+		cp_lexer_consume_token (parser->lexer);
+	      }
+	  }
+	if (ce)
+	  {
+	    cp_token *tok = cp_lexer_consume_token (parser->lexer);
+	    if (cxx_dialect < cxx23)
+	      pedwarn (tok->location, OPT_Wc__23_extensions,
+		       "%<if consteval%> only available with "
+		       "%<-std=c++2b%> or %<-std=gnu++2b%>");
+
+	    bool save_in_consteval_if_p = in_consteval_if_p;
+	    statement = begin_if_stmt ();
+	    IF_STMT_CONSTEVAL_P (statement) = true;
+	    condition = finish_if_stmt_cond (boolean_false_node, statement);
+
+	    gcc_rich_location richloc = tok->location;
+	    bool non_compound_stmt_p = false;
+	    if (cp_lexer_next_token_is_not (parser->lexer, CPP_OPEN_BRACE))
+	      {
+		non_compound_stmt_p = true;
+		richloc.add_fixit_insert_after (tok->location, "{");
+	      }
+
+	    in_consteval_if_p |= ce > 0;
+	    cp_parser_implicitly_scoped_statement (parser, NULL, guard_tinfo);
+
+	    if (non_compound_stmt_p)
+	      {
+		location_t before_loc
+		  = cp_lexer_peek_token (parser->lexer)->location;
+		richloc.add_fixit_insert_before (before_loc, "}");
+		error_at (&richloc,
+			  "%<if consteval%> requires compound statement");
+		non_compound_stmt_p = false;
+	      }
+
+	    finish_then_clause (statement);
+
+	    /* If the next token is `else', parse the else-clause.  */
+	    if (cp_lexer_next_token_is_keyword (parser->lexer,
+						RID_ELSE))
+	      {
+		cp_token *else_tok = cp_lexer_peek_token (parser->lexer);
+		gcc_rich_location else_richloc = else_tok->location;
+		guard_tinfo = get_token_indent_info (else_tok);
+		/* Consume the `else' keyword.  */
+		cp_lexer_consume_token (parser->lexer);
+
+		begin_else_clause (statement);
+
+		if (cp_lexer_next_token_is_not (parser->lexer, CPP_OPEN_BRACE))
+		  {
+		    non_compound_stmt_p = true;
+		    else_richloc.add_fixit_insert_after (else_tok->location,
+							 "{");
+		  }
+
+		in_consteval_if_p = save_in_consteval_if_p | (ce < 0);
+		cp_parser_implicitly_scoped_statement (parser, NULL,
+						       guard_tinfo);
+
+		if (non_compound_stmt_p)
+		  {
+		    location_t before_loc
+		      = cp_lexer_peek_token (parser->lexer)->location;
+		    else_richloc.add_fixit_insert_before (before_loc, "}");
+		    error_at (&else_richloc,
+			      "%<if consteval%> requires compound statement");
+		  }
+
+		finish_else_clause (statement);
+	      }
+
+	    in_consteval_if_p = save_in_consteval_if_p;
+	    if (ce < 0)
+	      {
+		std::swap (THEN_CLAUSE (statement), ELSE_CLAUSE (statement));
+		if (THEN_CLAUSE (statement) == NULL_TREE)
+		  THEN_CLAUSE (statement) = build_empty_stmt (tok->location);
+	      }
+
+	    finish_if_stmt (statement);
+	    return statement;
+	  }
 
 	/* Look for the `('.  */
 	matching_parens parens;
diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index d87382dba75..b53df9e2ad6 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -18413,6 +18413,7 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
     case IF_STMT:
       stmt = begin_if_stmt ();
       IF_STMT_CONSTEXPR_P (stmt) = IF_STMT_CONSTEXPR_P (t);
+      IF_STMT_CONSTEVAL_P (stmt) = IF_STMT_CONSTEVAL_P (t);
       if (IF_STMT_CONSTEXPR_P (t))
 	args = add_extra_args (IF_STMT_EXTRA_ARGS (t), args);
       tmp = RECUR (IF_COND (t));
@@ -18433,6 +18434,13 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl,
 	}
       if (IF_STMT_CONSTEXPR_P (t) && integer_zerop (tmp))
 	/* Don't instantiate the THEN_CLAUSE. */;
+      else if (IF_STMT_CONSTEVAL_P (t))
+	{
+	  bool save_in_consteval_if_p = in_consteval_if_p;
+	  in_consteval_if_p = true;
+	  RECUR (THEN_CLAUSE (t));
+	  in_consteval_if_p = save_in_consteval_if_p;
+	}
       else
 	{
 	  tree folded = fold_non_dependent_expr (tmp, complain);
@@ -19385,6 +19393,9 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
       local_specialization_stack s (lss_copy);
 
+      bool save_in_consteval_if_p = in_consteval_if_p;
+      in_consteval_if_p = false;
+
       tree body = start_lambda_function (fn, r);
 
       /* Now record them for lookup_init_capture_pack.  */
@@ -19425,6 +19436,8 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 
       finish_lambda_function (body);
 
+      in_consteval_if_p = save_in_consteval_if_p;
+
       if (nested)
 	pop_function_context ();
       else
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if1.C b/gcc/testsuite/g++.dg/cpp23/consteval-if1.C
new file mode 100644
index 00000000000..cff9946bfcd
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if1.C
@@ -0,0 +1,103 @@
+// P1938R3
+// { dg-do run { target c++20 } }
+// { dg-options "" }
+
+extern "C" void abort ();
+
+namespace std {
+  constexpr inline bool
+  is_constant_evaluated () noexcept
+  {
+    if consteval {	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+      return true;
+    } else {
+      return false;
+    }
+  }
+}
+
+consteval int foo (int x) { return x; }
+consteval int bar () { return 2; }
+
+constexpr int
+baz (int x)
+{
+  int r = 0;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (x);
+    }
+  else
+    {
+      r += bar ();
+    }
+  if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 2 * bar ();
+    }
+  else
+    {
+      r += foo (8 * x);
+    }
+  if (std::is_constant_evaluated ())
+    r = -r;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (32 * x);
+    }
+  if not consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 32 * bar ();
+    }
+  return r;
+}
+
+template <typename T>
+constexpr int
+qux (T x)
+{
+  T r = 0;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (x);
+    }
+  else
+    {
+      r += bar ();
+    }
+  if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 2 * bar ();
+    }
+  else
+    {
+      r += foo (8 * x);
+    }
+  if (std::is_constant_evaluated ())
+    r = -r;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (32 * x);
+    }
+  if not consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 32 * bar ();
+    }
+  return r;
+}
+
+constexpr int a = baz (1);
+static_assert (a == 23);
+int b = baz (1);
+constexpr int c = qux (1);
+static_assert (c == 23);
+int d = qux<int> (1);
+
+int
+main ()
+{
+  if (b != 23 || d != 23)
+    abort ();
+  if (baz (1) != 70 || qux (1) != 70 || qux (1LL) != 70)
+    abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if10.C b/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
new file mode 100644
index 00000000000..4c0523fe1d0
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if10.C
@@ -0,0 +1,36 @@
+// P1938R3
+// { dg-do compile { target c++20 } }
+// { dg-options "" }
+
+consteval int foo (int x) { return x; }
+
+constexpr int
+bar (int x)
+{
+  int r = 0;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      auto y = [=] { foo (x); };	// { dg-error "'x' is not a constant expression" }
+      y ();
+    }
+  return r;
+}
+
+template <typename T>
+constexpr T
+baz (T x)
+{
+  T r = 0;
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      auto y = [=] { foo (x); };	// { dg-error "'x' is not a constant expression" }
+      y ();
+    }
+  return r;
+}
+
+int
+qux (int x)
+{
+  return baz (x);
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if2.C b/gcc/testsuite/g++.dg/cpp23/consteval-if2.C
new file mode 100644
index 00000000000..f7053b91c3c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if2.C
@@ -0,0 +1,129 @@
+// P1938R3
+// { dg-do compile { target c++20 } }
+// { dg-options "" }
+
+constexpr bool f()
+{
+  if consteval (true) {}	// { dg-error "'if consteval' requires compound statement" }
+				// { dg-error "expected" "" { target *-*-* } .-1 }
+				// { dg-warning "'if consteval' only available with" "" { target c++20_only } .-2 }
+  if not consteval (false) {}	// { dg-error "'if consteval' requires compound statement" }
+				// { dg-error "expected" "" { target *-*-* } .-1 }
+				// { dg-warning "'if consteval' only available with" "" { target c++20_only } .-2 }
+  if consteval if (true) {}	// { dg-error "'if consteval' requires compound statement" }
+				// { dg-warning "'if consteval' only available with" "" { target c++20_only } .-1 }
+  if ! consteval {} else ;	// { dg-error "'if consteval' requires compound statement" }
+				// { dg-warning "'if consteval' only available with" "" { target c++20_only } .-1 }
+  if consteval {} else if (true) {}	// { dg-error "'if consteval' requires compound statement" }
+				// { dg-warning "'if consteval' only available with" "" { target c++20_only } .-1 }
+  if (true)
+    if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+      {
+      }
+    else ;			// { dg-error "'if consteval' requires compound statement" }
+  return false;
+}
+
+consteval int foo (int x) { return x; }
+consteval int bar () { return 2; }
+
+constexpr int
+baz (int x)
+{
+  int r = 0;
+  if not consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (x);	// { dg-error "'x' is not a constant expression" }
+    }
+  else
+    {
+      r += bar ();
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 2 * bar ();
+    }
+  else
+    {
+      r += foo (8 * x);	// { dg-error "'x' is not a constant expression" }
+    }
+  if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (32 * x);// { dg-error "'x' is not a constant expression" }
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 32 * bar ();
+    }
+  return r;
+}
+
+template <typename T>
+constexpr int
+qux (int x)
+{
+  int r = 0;
+  if not consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (x);	// { dg-error "'x' is not a constant expression" }
+    }
+  else
+    {
+      r += bar ();
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 2 * bar ();
+    }
+  else
+    {
+      r += foo (8 * x);	// { dg-error "is not a constant expression" }
+    }
+  if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (32 * x);// { dg-error "is not a constant expression" }
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 32 * bar ();
+    }
+  return r;
+}
+
+template <typename T>
+constexpr T
+corge (T x)
+{
+  T r = 0;
+  if not consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (x);	// { dg-error "'x' is not a constant expression" }
+    }
+  else
+    {
+      r += bar ();
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 2 * bar ();
+    }
+  else
+    {
+      r += foo (8 * x);	// { dg-error "is not a constant expression" }
+    }
+  if ! consteval	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += foo (32 * x);// { dg-error "is not a constant expression" }
+    }
+  if consteval		// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+    {
+      r += 32 * bar ();
+    }
+  return r;
+}
+
+int
+garply (int x)
+{
+  return corge (x);
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if3.C b/gcc/testsuite/g++.dg/cpp23/consteval-if3.C
new file mode 100644
index 00000000000..9c7e3aa7f60
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if3.C
@@ -0,0 +1,73 @@
+// P1938R3
+// { dg-do run { target c++20 } }
+// { dg-options "" }
+
+constexpr inline bool
+is_constant_evaluated () noexcept
+{
+  if consteval { return true; } else { return false; }	// { dg-warning "'if consteval' only available with" "" { target c++20_only } }
+}
+
+template<int N> struct X { int v = N; };
+X<is_constant_evaluated ()> x; // type X<true>
+int y = 4;
+int a = is_constant_evaluated () ? y : 1; // initializes a to 1
+int b = is_constant_evaluated () ? 2 : y; // initializes b to 2
+int c = y + (is_constant_evaluated () ? 2 : y); // initializes c to 2*y
+int d = is_constant_evaluated (); // initializes d to 1
+int e = d + is_constant_evaluated (); // initializes e to 1 + 0
+
+struct false_type { static constexpr bool value = false; };
+struct true_type { static constexpr bool value = true; };
+template<class T, class U>
+struct is_same : false_type {};
+template<class T>
+struct is_same<T, T> : true_type {};
+
+constexpr int
+foo (int x)
+{
+  const int n = is_constant_evaluated () ? 13 : 17; // n == 13
+  int m = is_constant_evaluated () ? 13 : 17; // m might be 13 or 17 (see below)
+  char arr[n] = {}; // char[13]
+  return m + sizeof (arr) + x;
+}
+
+constexpr int
+bar ()
+{
+  const int n = is_constant_evaluated() ? 13 : 17;
+  X<n> x1;
+  X<is_constant_evaluated() ? 13 : 17> x2;
+  static_assert (is_same<decltype (x1), decltype (x2)>::value, "x1/x2's type");
+  return x1.v + x2.v;
+}
+
+int p = foo (0); // m == 13; initialized to 26
+int q = p + foo (0); // m == 17 for this call; initialized to 56
+static_assert (bar () == 26, "bar");
+
+struct S { int a, b; };
+
+S s = { is_constant_evaluated () ? 2 : 3, y };
+S t = { is_constant_evaluated () ? 2 : 3, 4 };
+
+static_assert (is_same<decltype (x), X<true> >::value, "x's type");
+
+int
+main ()
+{
+  if (a != 1 || b != 2 || c != 8 || d != 1 || e != 1 || p != 26 || q != 56)
+    __builtin_abort ();
+  if (s.a != 3 || s.b != 4 || t.a != 2 || t.b != 4)
+    __builtin_abort ();
+  if (foo (y) != 34)
+    __builtin_abort ();
+#if __cplusplus >= 201703L
+  if constexpr (foo (0) != 26)
+    __builtin_abort ();
+#endif
+  constexpr int w = foo (0);
+  if (w != 26)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if4.C b/gcc/testsuite/g++.dg/cpp23/consteval-if4.C
new file mode 100644
index 00000000000..2760c1c2d97
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if4.C
@@ -0,0 +1,44 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+void f()
+{
+  goto l;			// { dg-message "from here" }
+  if consteval			// { dg-message "enters 'consteval if'" }
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+}
+
+void g()
+{
+  goto l;			// { dg-message "from here" }
+  if not consteval		// { dg-message "enters 'consteval if'" }
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+}
+
+void h()
+{
+  goto l;			// { dg-message "from here" }
+  if consteval			// { dg-message "enters 'consteval if'" }
+    {
+    }
+  else
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+}
+
+void i()
+{
+  goto l;			// { dg-message "from here" }
+  if not consteval		// { dg-message "enters 'consteval if'" }
+    {
+    }
+  else
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if5.C b/gcc/testsuite/g++.dg/cpp23/consteval-if5.C
new file mode 100644
index 00000000000..b1c6192705c
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if5.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+void f()
+{
+  if consteval			// { dg-message "enters 'consteval if'" }
+    {
+      goto l;			// { dg-message "from here" }
+    }
+  else
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if6.C b/gcc/testsuite/g++.dg/cpp23/consteval-if6.C
new file mode 100644
index 00000000000..2b92cccb832
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if6.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+void f()
+{
+  if consteval
+    {
+      goto l;
+    l:;
+    }
+  else
+    {
+      goto l2;
+    l2:;
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if7.C b/gcc/testsuite/g++.dg/cpp23/consteval-if7.C
new file mode 100644
index 00000000000..ab9da084015
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if7.C
@@ -0,0 +1,16 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+void f()
+{
+  if not consteval
+    {
+    l:;
+      goto l;
+    }
+  else
+    {
+    l2:;
+      goto l2;
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if8.C b/gcc/testsuite/g++.dg/cpp23/consteval-if8.C
new file mode 100644
index 00000000000..36adaf92a7f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if8.C
@@ -0,0 +1,14 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+void f()
+{
+  if consteval
+    {
+    l:;				// { dg-error "jump to label" }
+    }
+  else
+    {
+      goto l;			// { dg-message "from here" }
+    }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/consteval-if9.C b/gcc/testsuite/g++.dg/cpp23/consteval-if9.C
new file mode 100644
index 00000000000..1c7db1ac906
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp23/consteval-if9.C
@@ -0,0 +1,11 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-w" }
+
+constexpr void f(int i)
+{
+  switch (i)
+    if consteval		// { dg-message "enters 'consteval if'" }
+      {
+      case 42:;			// { dg-error "jump to case label" }
+      }
+}
diff --git a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
index 283a8bae31e..7070f598d39 100644
--- a/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
+++ b/gcc/testsuite/g++.dg/cpp23/feat-cxx2b.C
@@ -545,3 +545,9 @@
 #elif __cpp_size_t_suffix != 202011
 #  error "__cpp_size_t_suffix != 202011"
 #endif
+
+#ifndef __cpp_if_consteval
+#  error "__cpp_if_consteval"
+#elif __cpp_if_consteval != 202106
+#  error "__cpp_if_consteval != 202106"
+#endif