[Issue 24429] New: [ImportC] improve error message for size_t

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24429

          Issue ID: 24429
           Summary: [ImportC] improve error message for size_t
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: minor
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: lance at lancebachmeier.com

If you compile C code that uses size_t, you'll get the error message:

Error: undefined identifier `size_t`

It would help to add something to that error message telling the user they need
#include <stddef.h> in the C file. This is slightly confusing because size_t is
built into D, so it's kind of surprising when DMD says it doesn't know what
size_t is.

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