* cp-tree.h (struct lang_decl_decomp): New type.
	(struct lang_decl): Add u.decomp.
	(LANG_DECL_DECOMP_CHECK): Define.
	(DECL_DECOMPOSITION_P): Note it is set also on the vars
	for user identifiers.
	(DECL_DECOMP_BASE): Define.
	(retrofit_lang_decl): Add extra int = 0 argument.
	* lex.c (retrofit_lang_decl): Add SEL argument, if non-zero
	use it to influence the selector choices and for selector
	0 to non-zero transition copy old content.
	(cxx_dup_lang_specific_decl): Handle DECL_DECOMPOSITION_P.
	* decl.c (poplevel): For DECL_DECOMPOSITION_P, check
	!DECL_DECOMP_BASE instead of !DECL_VALUE_EXPR.  Adjust warning
	wording if decl is a structured binding.
	(cp_finish_decomp): Pass 4 as the new argument to retrofit_lang_decl.
	Set DECL_DECOMP_BASE.  Ignore DECL_READ_P sets from initialization
	of individual variables for tuple structured bindings.
	(grokdeclarator): Pass 4 as the new argument to retrofit_lang_decl.
	Clear DECL_DECOMP_BASE.
	* decl2.c (mark_used): Mark DECL_DECOMP_BASE TREE_USED as well.
	* pt.c (tsubst_decomp_names): Assert DECL_DECOMP_BASE matches what
	is expected.
	* expr.c (mark_exp_read): Recurse on DECL_DECOMP_BASE instead of
	DECL_VALUE_EXPR.

	* g++.dg/cpp1z/decomp29.C (p): New variable.
	(main): Add further tests.

From-SVN: r248483