Prepare and_then/map/transform/map_error/or_else for SFINAE-friendliness#99

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Quuxplusone wants to merge 1 commit intoTartanLlama:masterfrom Quuxplusone:sfinae-groundwork

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Commits on Jan 2, 2022

Prepare and_then/map/transform/map_error/or_else for SFINAE-friendliness.
Quuxplusone
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