Rationale (September, 2008):

It is unclear what effect the provision of “unique mappings” has or if a conforming program could detect the failure of an implementation to do so. There has been a significant effort to synchronize this clause with the corresponding section of the C99 Standard, and given the lack of perceptible impact of the proposed change, there is insufficient motivation to introduce a new divergence in the wording.

Additional notes (October, 2006):

WG14 takes no position on this proposed change.

Proposed resolution (April, 2006):

Change 15.3 [cpp.include] paragraph 5 as indicated:

The implementation provides unique mappings between the delimited sequence and the external source file name for sequences consisting of one or more nondigits or digits (5.10 [lex.name]), optionally followed by a period (.) and a single nondigit...

(Clark Nelson will discuss this revision with WG14.)

In language imported directly from the C Standard, 15.3 [cpp.include] paragraph 5 says,

The implementation provides unique mappings for sequences consisting of one or more nondigits (5.10 [lex.name]) followed by a period (.) and a single nondigit.

This is clearly intended to support C header names like stdio.h. However, C++ has header names like cstdio that do not conform to this pattern but still presumably require “unique mappings.”