c++: Add -fimplicit-constexpr

With each successive C++ standard the restrictions on the use of the
constexpr keyword for functions get weaker and weaker; it recently occurred
to me that it is heading toward the same fate as the C register keyword,
which was once useful for optimization but became obsolete.  Similarly, it
seems to me that we should be able to just treat inlines as constexpr
functions and not make people add the extra keyword everywhere.

There were a lot of testcase changes needed; many disabling errors about
non-constexpr functions that are now constexpr, and many disabling implicit
constexpr so that the tests can check the same thing as before, whether
that's mangling or whatever.

gcc/c-family/ChangeLog:

	* c.opt: Add -fimplicit-constexpr.
	* c-cppbuiltin.c: Define __cpp_implicit_constexpr.
	* c-opts.c (c_common_post_options): Disable below C++14.

gcc/cp/ChangeLog:

	* cp-tree.h (struct lang_decl_fn): Add implicit_constexpr.
	(decl_implicit_constexpr_p): New.
	* class.c (type_maybe_constexpr_destructor): Use
	TYPE_HAS_TRIVIAL_DESTRUCTOR and maybe_constexpr_fn.
	(finalize_literal_type_property): Simplify.
	* constexpr.c (is_valid_constexpr_fn): Check for dtor.
	(maybe_save_constexpr_fundef): Try to set DECL_DECLARED_CONSTEXPR_P
	on inlines.
	(cxx_eval_call_expression): Use maybe_constexpr_fn.
	(maybe_constexpr_fn): Handle flag_implicit_constexpr.
	(var_in_maybe_constexpr_fn): Use maybe_constexpr_fn.
	(potential_constant_expression_1): Likewise.
	(decl_implicit_constexpr_p): New.
	* decl.c (validate_constexpr_redeclaration): Allow change with
	-fimplicit-constexpr.
	(grok_special_member_properties): Use maybe_constexpr_fn.
	* error.c (dump_function_decl): Don't print 'constexpr'
	if it's implicit.
	* Make-lang.in (check-c++-all): Update.

libstdc++-v3/ChangeLog:

	* testsuite/20_util/to_address/1_neg.cc: Adjust error.
	* testsuite/26_numerics/random/concept.cc: Adjust asserts.

gcc/testsuite/ChangeLog:

	* lib/g++-dg.exp: Handle "impcx".
	* lib/target-supports.exp
	(check_effective_target_implicit_constexpr): New.
	* g++.dg/abi/abi-tag16.C:
	* g++.dg/abi/abi-tag18a.C:
	* g++.dg/abi/guard4.C:
	* g++.dg/abi/lambda-defarg1.C:
	* g++.dg/abi/mangle26.C:
	* g++.dg/cpp0x/constexpr-diag3.C:
	* g++.dg/cpp0x/constexpr-ex1.C:
	* g++.dg/cpp0x/constexpr-ice5.C:
	* g++.dg/cpp0x/constexpr-incomplete2.C:
	* g++.dg/cpp0x/constexpr-memfn1.C:
	* g++.dg/cpp0x/constexpr-neg3.C:
	* g++.dg/cpp0x/constexpr-specialization.C:
	* g++.dg/cpp0x/inh-ctor19.C:
	* g++.dg/cpp0x/inh-ctor30.C:
	* g++.dg/cpp0x/lambda/lambda-mangle3.C:
	* g++.dg/cpp0x/lambda/lambda-mangle5.C:
	* g++.dg/cpp1y/auto-fn12.C:
	* g++.dg/cpp1y/constexpr-loop5.C:
	* g++.dg/cpp1z/constexpr-lambda7.C:
	* g++.dg/cpp2a/constexpr-dtor3.C:
	* g++.dg/cpp2a/constexpr-new13.C:
	* g++.dg/cpp2a/constinit11.C:
	* g++.dg/cpp2a/constinit12.C:
	* g++.dg/cpp2a/constinit14.C:
	* g++.dg/cpp2a/constinit15.C:
	* g++.dg/cpp2a/spaceship-constexpr1.C:
	* g++.dg/cpp2a/spaceship-eq3.C:
	* g++.dg/cpp2a/udlit-class-nttp-neg2.C:
	* g++.dg/debug/dwarf2/auto1.C:
	* g++.dg/debug/dwarf2/cdtor-1.C:
	* g++.dg/debug/dwarf2/lambda1.C:
	* g++.dg/debug/dwarf2/pr54508.C:
	* g++.dg/debug/dwarf2/pubnames-2.C:
	* g++.dg/debug/dwarf2/pubnames-3.C:
	* g++.dg/ext/is_literal_type3.C:
	* g++.dg/ext/visibility/template7.C:
	* g++.dg/gcov/gcov-12.C:
	* g++.dg/gcov/gcov-2.C:
	* g++.dg/ipa/devirt-35.C:
	* g++.dg/ipa/devirt-36.C:
	* g++.dg/ipa/devirt-37.C:
	* g++.dg/ipa/devirt-44.C:
	* g++.dg/ipa/imm-devirt-1.C:
	* g++.dg/lookup/builtin5.C:
	* g++.dg/lto/inline-crossmodule-1_0.C:
	* g++.dg/modules/enum-1_a.C:
	* g++.dg/modules/fn-inline-1_c.C:
	* g++.dg/modules/pmf-1_b.C:
	* g++.dg/modules/used-1_c.C:
	* g++.dg/tls/thread_local11.C:
	* g++.dg/tls/thread_local11a.C:
	* g++.dg/tm/pr46653.C:
	* g++.dg/ubsan/pr70035.C:
	* g++.old-deja/g++.other/delete6.C:
	* g++.dg/modules/pmf-1_a.H:
	Adjust for implicit constexpr.

2 files changed, 6 insertions, 1 deletions

diff --git a/gcc/testsuite/lib/g++-dg.exp b/gcc/testsuite/lib/g++-dg.exp
index c360770a7a2..fd06d278faa 100644
--- a/gcc/testsuite/lib/g++-dg.exp
+++ b/gcc/testsuite/lib/g++-dg.exp
@@ -57,7 +57,8 @@ proc g++-dg-runtest { testcases flags default-extra-flags } {
 	    set option_list { }
 	    foreach x $std_list {
 		# Handle "concepts" as C++17 plus Concepts TS.
-		if { $x eq "concepts" } then { set x "17 -fconcepts" }
+		if { $x eq "concepts" } then { set x "17 -fconcepts"
+		} elseif { $x eq "impcx" } then { set x "23 -fimplicit-constexpr" }
 		lappend option_list "${std_prefix}$x"
 	    }
 	} else {
diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
index 8cbda192fe0..c928d99a14b 100644
--- a/gcc/testsuite/lib/target-supports.exp
+++ b/gcc/testsuite/lib/target-supports.exp
@@ -10232,6 +10232,10 @@ proc check_effective_target_concepts { } {
     return [check-flags { "" { } { -fconcepts } }]
 }
 
+proc check_effective_target_implicit_constexpr { } {
+    return [check-flags { "" { } { -fimplicit-constexpr } }]
+}
+
 # Return 1 if expensive testcases should be run.
 
 proc check_effective_target_run_expensive_tests { } {