Linear types combined with `-fdefer-type-errors` show a poor error message. ``` ghci> :set -XLinearTypes ghci> ap :: (a -> b) -> a %m -> b; ap f x = f x <interactive>:2:35: error: • Couldn't match type ‘m’ with ‘'Many’ arising from multiplicity of ‘x’ ‘m’ is a rigid type variable bound by the type signature for: ap :: forall a b (m :: GHC.Types.Multiplicity). (a -> b) -> a %m -> b at <interactive>:2:1-27 • In an equation for ‘ap’: ap f x = f x • Relevant bindings include ap :: (a -> b) -> a %m -> b (bound at <interactive>:2:30) ghci> :set -fdefer-type-errors ghci> ap :: (a -> b) -> a %m -> b; ap f x = f x <interactive>:4:30: error: Multiplicity coercions are currently not supported (see GHC #19517) <interactive>:4:30: error: Multiplicity coercions are currently not supported (see GHC #19517) ``` The error message is technically correct, but `-fdefer-type-errors` downgrades the type mismatch to a warning. The warning is not shown, while it contains important information. Until #19517 is done, when `-fdefer-type-errors` is on, we should compile multiplicity coercions to runtime errors and convert the error to a warning. Also make the message more user-friendly.