https://issues.dlang.org/show_bug.cgi?id=23902

          Issue ID: 23902
           Summary: ImportC error message location for #defines are not
                    precise enough.
           Product: D
           Version: D2
          Hardware: x86
                OS: Mac OS X
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: maxhaton@gmail.com

`#defines(num)` may be given as a location. This is confusing and also extremely unhelpful when anything goes wrong.

The solution is simple: Pack a flag into the Loc struct dmd uses, then store the contextual information about the #define properly.

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