We allow primitive fallback values which have mismatched types, but only
if there is a conversion to the target type. Previously we would allow
unsafe conversions (e.g. string to bool), but later had no capacity to
return an error if the conversion failed due to the value of the
fallback being unable to convert to the target type.
This commit makes the more conservative requirement that default
fallback values must have a safe conversion.
66f8d1c1c2