In backwards compatibility mode (that is when the stylesheet specifies version="1.0" an arithmetic expression will fail with a type error if one of the arguments has a statically inferred type of "numeric". An example of an expression that has such a type is a call on sum() when the argument is a path expression (in Saxon-B, where it known that the nodes are all untypedAtomic). The failure is typically a compile time error:
XPTY0004: Arithmetic operator is not defined for arguments of types (numeric, xs:double)
This applies to 9.0 and 9.1. A patch for both these releases is being placed in Subversion.
Test case backwards073 added.