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In --make mode there is a single EPS whose eps_fam_inst_env holds all type family instances that have been read from interface files from external packages for any reason. The type checker uses this field pervasively via tcGetFamInstEnvs.

When compiling multiple files it's fairly easy to set up a situation in which

1. the first module to be compiled A loads an interface file O.hi from another package containing an orphan family instance (say, because it imports the module directly);
2. a later module B uses this instance to reduce a type, even though B has no dependency at all on the module O defining the instance.

(The only tricky bit in arranging this is that since B cannot depend on A, a little good fortune is needed for GHC to decide to compile B after A.)

Now another module could import B and obtain a function whose definition relies on a type family instance that is not visible from B at all, compromising type safety.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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