* c-common.c (c_common_reswords): Add constinit and __constinit.
	(keyword_is_decl_specifier): Handle RID_CONSTINIT.
	* c-common.h (enum rid): Add RID_CONSTINIT, RID_FIRST_CXX20, and
	RID_LAST_CXX20.
	(D_CXX20): Define.
	* c-cppbuiltin.c (c_cpp_builtins): Define __cpp_constinit.
	* c-format.c (cxx_keywords): Add "constinit".
	* c.opt (Wc++2a-compat, Wc++20-compat): New options.

	* cp-tree.h (TINFO_VAR_DECLARED_CONSTINIT): Define.
	(LOOKUP_CONSTINIT): Define.
	(enum cp_decl_spec): Add ds_constinit.
	* decl.c (check_tag_decl): Give an error for constinit in type
	declarations.
	(check_initializer): Also check LOOKUP_CONSTINIT.
	(cp_finish_decl): Add checking for a constinit declaration.  Set
	TINFO_VAR_DECLARED_CONSTINIT.
	(grokdeclarator): Add checking for a declaration with the constinit
	specifier.
	* lex.c (init_reswords): Handle D_CXX20.
	* parser.c (cp_lexer_get_preprocessor_token): Pass a better location
	to warning_at.  Warn about C++20 keywords.
	(cp_keyword_starts_decl_specifier_p): Handle RID_CONSTINIT.
	(cp_parser_diagnose_invalid_type_name): Add an inform about constinit.
	(cp_parser_decomposition_declaration): Maybe pass LOOKUP_CONSTINIT to
	cp_finish_decl.
	(cp_parser_decl_specifier_seq): Handle RID_CONSTINIT.
	(cp_parser_init_declarator): Maybe pass LOOKUP_CONSTINIT to
	cp_finish_decl.
	(set_and_check_decl_spec_loc): Add "constinit".
	* pt.c (tsubst_decl): Set TINFO_VAR_DECLARED_CONSTINIT.
	(instantiate_decl): Maybe pass LOOKUP_CONSTINIT to cp_finish_decl.
	* typeck2.c (store_init_value): If a constinit variable wasn't
	initialized using a constant initializer, give an error.

	* doc/invoke.texi: Document -Wc++20-compat.

	* g++.dg/cpp2a/constinit1.C: New test.
	* g++.dg/cpp2a/constinit2.C: New test.
	* g++.dg/cpp2a/constinit3.C: New test.
	* g++.dg/cpp2a/constinit4.C: New test.
	* g++.dg/cpp2a/constinit5.C: New test.
	* g++.dg/cpp2a/constinit6.C: New test.
	* g++.dg/cpp2a/constinit7.C: New test.
	* g++.dg/cpp2a/constinit8.C: New test.
	* g++.dg/cpp2a/constinit9.C: New test.
	* g++.dg/cpp2a/constinit10.C: New test.
	* g++.dg/cpp2a/constinit11.C: New test.
	* g++.dg/cpp2a/constinit12.C: New test.

From-SVN: r275008