Rationale:

The LWG believes length_error is the correct exception to throw.

Proposed resolution:

After [basic.string] paragraph 4 ("The functions described in this clause...") add a new paragraph:

For any string operation, if as a result of the operation, size() would exceed max_size() then the operation throws length_error.

Many string member functions throw if size is getting or exceeding npos. However, I wonder why they don't throw if size is getting or exceeding max_size() instead of npos. May be npos is known at compile time, while max_size() is known at runtime. However, what happens if size exceeds max_size() but not npos, then? It seems the standard lacks some clarifications here.