c++: substituting fn parm redeclared with dep alias tmpl [PR120224]

c++: substituting fn parm redeclared with dep alias tmpl [PR120224]

Here we declare f twice, the second time around using a dependent
alias template.  Due to alias template transparency these are logically
the same overload.  But now the function type of f (produced from the
first declaration) diverges from the type of its formal parameter
(produced from the subsequent redefinition) in that substituting T=int
succeeds for the function type but not for the formal parameter type.
This eventually causes us to produce an undiagnosed error_mark_node in
the AST of the function call, leading to failure of the sanity check
check added in r14-6343-g0c018a74eb1aff.

Before r14-6343 we would still go on to reject the testcase later at
instantiation time, from regenerate_decl_from_template, making this a
regression.

To fix this, it seems we just need to propagate error_mark_node upon
substitution failure into the type of a PARM_DECL.

	PR c++/120224

gcc/cp/ChangeLog:

	* pt.cc (tsubst_function_decl): Return error_mark_node if
	substituting into the formal parameter list failed.
	(tsubst_decl) <case PARM_DECL>: Return error_mark_node
	upon TREE_TYPE substitution failure, when in a SFINAE
	context.  Return error_mark_node upon DECL_CHAIN substitution
	failure.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/alias-decl-80.C: New test.

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