c++: templated substitution into lambda-expr [PR114393]

c++: templated substitution into lambda-expr [PR114393]

The below testcases use a lambda-expr as a template argument and they
all trip over the below added tsubst_lambda_expr sanity check ultimately
because current_template_parms is empty which causes push_template_decl
to return error_mark_node from the call to begin_lambda_type.  Were it
not for the sanity check this silent error_mark_node result leads to
nonsensical errors down the line, or silent breakage.

In the first testcase, we hit this assert during instantiation of the
dependent alias template-id c1_t<_Data> from instantiate_template, which
clears current_template_parms via push_to_top_level.  Similar story for
the second testcase.  For the third testcase we hit the assert during
partial instantiation of the member template from instantiate_class_template
which similarly calls push_to_top_level.

These testcases illustrate that templated substitution into a lambda-expr
is not always possible, in particular when we lost the relevant template
context.  I experimented with recovering the template context by making
tsubst_lambda_expr fall back to using scope_chain->prev->template_parms if
current_template_parms is empty which worked but seemed like a hack.  I
also experimented with preserving the template context by keeping
current_template_parms set during instantiate_template for a dependent
specialization which also worked but it's at odds with the fact that we
cache dependent specializations (and so they should be independent of
the template context).

So instead of trying to make such substitution work, this patch uses the
extra-args mechanism to defer templated substitution into a lambda-expr
when we lost the relevant template context.

	PR c++/114393
	PR c++/107457
	PR c++/93595

gcc/cp/ChangeLog:

	* cp-tree.h (LAMBDA_EXPR_EXTRA_ARGS): Define.
	(tree_lambda_expr::extra_args): New field.
	* module.cc (trees_out::core_vals) <case LAMBDA_EXPR>: Stream
	LAMBDA_EXPR_EXTRA_ARGS.
	(trees_in::core_vals) <case LAMBDA_EXPR>: Likewise.
	* pt.cc (has_extra_args_mechanism_p): Return true for LAMBDA_EXPR.
	(tree_extra_args): Handle LAMBDA_EXPR.
	(tsubst_lambda_expr): Use LAMBDA_EXPR_EXTRA_ARGS to defer templated
	substitution into a lambda-expr if we lost the template context.
	Add sanity check for error_mark_node result from begin_lambda_type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/lambda-targ2.C: New test.
	* g++.dg/cpp2a/lambda-targ3.C: New test.
	* g++.dg/cpp2a/lambda-targ4.C: New test.

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