c-strict-syntax-p is a variable defined in `cc-vars.el'.
Its value is

*If non-nil, all syntactic symbols must be found in `c-offsets-alist'.
If the syntactic symbol for a particular line does not match a symbol
in the offsets alist, or if no non-nil offset value can be determined
for a symbol, an error is generated, otherwise no error is reported
and the syntactic symbol is ignored.

This variable is considered obsolete; it doesn't work well with lineup
functions that return nil to support the feature of using lists on
syntactic symbols in `c-offsets-alist'. Please keep it set to nil.

You can customize this variable.