By: Tom Sydney Kerckhove <syd@cs-syd.eu>
Mutation: skip-calls-to for ConstConstructor
A function whose result is always the same constructor makes every call to it
an equivalent mutant: replacing the call with that constructor cannot change
anything, so no test can kill it. A delegating
instance PersistFieldSql RepoType where
sqlType Proxy = sqlType (Proxy :: Proxy Text)
is the shape that keeps coming up, sqlType answering with a constant of its
type by definition.
Deciding that from the compiler is out of reach here in a way the Map.empty
case was not. The head of that expression is the class-method selector, not
the instance's method, so there is no unfolding to read: it would take walking
the typechecker's evidence to the PersistFieldSql Text dictionary, pulling its
DFunUnfolding, indexing out the method and beta-reducing the selector against
it -- a Core evaluator inside a typechecker plugin, silently doing nothing
whenever any link in that chain is missing.
So this is config, as it already is for the other two operators whose
equivalent mutants come from a semantic property: symmetry for
SwitchFunctionArguments, identity for ElideCall. Same key, same name matching
(bare or qualified, by defining module or by import module). A surviving
mutation on a call now carries a mitigation naming the key, so the report
points at it.
The key skips calls, not definitions: the body of a listed function stays
under mutation testing, which the example pins.