c++/modules: Merge default arguments [PR99274]

c++/modules: Merge default arguments [PR99274]

When merging a newly imported declaration with an existing declaration
we don't currently propagate new default arguments, which causes issues
when modularising header units.  This patch adds logic to propagate
default arguments to existing declarations on import, and error if the
defaults do not match.

	PR c++/99274

gcc/cp/ChangeLog:

	* module.cc (trees_in::is_matching_decl): Merge default
	arguments.
	* tree.cc (cp_tree_equal) <AGGR_INIT_EXPR>: Handle unification
	of AGGR_INIT_EXPRs with new VAR_DECL slots.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/lambda-7.h: Skip ODR-violating declaration when
	testing ODR deduplication.
	* g++.dg/modules/lambda-7_b.C: Note we're testing ODR
	deduplication.
	* g++.dg/modules/default-arg-1_a.H: New test.
	* g++.dg/modules/default-arg-1_b.C: New test.
	* g++.dg/modules/default-arg-2_a.H: New test.
	* g++.dg/modules/default-arg-2_b.C: New test.
	* g++.dg/modules/default-arg-3.h: New test.
	* g++.dg/modules/default-arg-3_a.H: New test.
	* g++.dg/modules/default-arg-3_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Patrick Palka <ppalka@redhat.com>
Reviewed-by: Jason Merrill <jason@redhat.com>