The Mac-ward Compatibility project restores compatibility between different versions of the Mac OS development headers.
Issues include:
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Type aliases SRefCon, URefCon, and PRefCon were added to the 10.5 SDK for 64-bit support. Making your code 64-bit-compatible requires using these types, which then makes it incompatible with the 10.4 SDK and earlier.
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The 10.7 SDK omits QuickDraw symbols from the headers even for 32-bit development.
Mac-ward Compatibility defines the missing symbols.