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In some cases it was possible for lookupGlobalOccRn_maybe to return an error, when it should be returning a Nothing. If it called lookupExactOcc_either when there were no matching GlobalRdrElts in the otherwise case, it would return an error message. This could be caused when lookupThName_maybe in Template Haskell was looking in different namespaces (thRdrNameGuesses), guessing different namespaces that the name wasn't guaranteed to be found in. However, by addressing this some more accurate errors were being lost in the conversion to Maybes. So some of the lookup* functions have been shuffled about so that errors should always be ignored in lookup*_maybes, and propagated otherwise. This fixes #18263
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