c++: consteval blocks

c++: consteval blocks

This patch implements consteval blocks, as specified by P2996.
They aren't very useful without define_aggregate, but having
a reviewed implementation on trunk would be great.

consteval {} can be anywhere where a member-declaration or
block-declaration can be.  The expression corresponding to it is:

  [] -> void static consteval compound-statement ()

and it must be a constant expression.

I've used cp_parser_lambda_expression to take care of most of the
parsing.  Since a consteval block can find itself in a template, we
need a vehicle to carry the block for instantiation.  Rather than
inventing a new tree, I'm using STATIC_ASSERT.

A consteval block can't return a value but that is checked by virtue
of the lambda having a void return type.

	PR c++/120775

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_outermost_constant_expr): Use
	extract_call_expr.
	* cp-tree.h (CONSTEVAL_BLOCK_P, LAMBDA_EXPR_CONSTEVAL_BLOCK_P): Define.
	(finish_static_assert): Adjust declaration.
	(current_nonlambda_function): Likewise.
	* lambda.cc (current_nonlambda_function): New parameter.  Only keep
	iterating if the function represents a consteval block.
	* parser.cc (cp_parser_lambda_expression): New parameter for
	consteval blocks.  Use it.  Set LAMBDA_EXPR_CONSTEVAL_BLOCK_P.
	(cp_parser_lambda_declarator_opt): Likewise.
	(build_empty_string): New.
	(cp_parser_next_tokens_are_consteval_block_p): New.
	(cp_parser_consteval_block): New.
	(cp_parser_block_declaration): Handle consteval blocks.
	(cp_parser_static_assert): Use build_empty_string.
	(cp_parser_member_declaration): Handle consteval blocks.
	* pt.cc (tsubst_stmt): Adjust a call to finish_static_assert.
	* semantics.cc (finish_fname): Warn for consteval blocks.
	(finish_static_assert): New parameter for consteval blocks.  Set
	CONSTEVAL_BLOCK_P.  Evaluate consteval blocks specially.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp26/consteval-block1.C: New test.
	* g++.dg/cpp26/consteval-block2.C: New test.
	* g++.dg/cpp26/consteval-block3.C: New test.
	* g++.dg/cpp26/consteval-block4.C: New test.
	* g++.dg/cpp26/consteval-block5.C: New test.
	* g++.dg/cpp26/consteval-block6.C: New test.
	* g++.dg/cpp26/consteval-block7.C: New test.
	* g++.dg/cpp26/consteval-block8.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>