PR c++/60531 - Wrong error about unresolved overloaded function

PR c++/60531 - Wrong error about unresolved overloaded function

For PR60531, GCC wrongly rejects function templates with explicitly
specified template arguments as overloaded. They are resolved by
resolve_nondeduced_context, which is normally called by
cp_default_conversion through decay_conversion, but the latter have
extra effects making them unusable here. Calling the former directly
does work.

	* typeck.c (cp_build_binary_op): See if overload can be resolved.
	(cp_build_unary_op): Ditto.

	* g++.dg/template/operator15.C: New test.

From-SVN: r271910