Context reduction stack overflow without undecidable instances
When using type families whose reduction surely terminates (and which are accepted without undecidable instances) GHC still seems to set the context stack reduction limit. See the included code.
The fixed limit on context stack size prevents us from doing type-level arithmetic on arbitrarily-sized type-level numerals.
If the context-size limit is applied whether undecidable instance extension is used or not, one may wonder of the purpose of the undecidable instance extension.