PR c++/79654
	* decl.c (cp_finish_decomp): Don't set decl's type to error_mark_node
	on error.
	* pt.c (tsubst_decomp_names): Return error_mark_node if the first
	decl after the decomposition artificial decl has error_mark_node.
	* decl2.c (prune_vars_needing_no_initialization): Use error_operand_p
	instead of just == error_mark_node comparison.

	* g++.dg/cpp1z/decomp26.C: New test.

Co-Authored-By: Paolo Carlini <paolo.carlini@oracle.com>

From-SVN: r245639