The argument in Trac #9033 is very compelling: we should not report 20
errors, fix one, and have the other 19 disappear.  They were spurious
in the first place.

The fix was easy; do type-class defaulting uncondionally, rather than
only if there are no insoluble constraints.

See Note [When to do type-class defaulting] in TcSimplify.

Error messages generally improve, especially tc211 which actually
had an example of precisely this phenomenon.