mergeValue.mergeField: incompatible values

[brianhuffman]

We get a panic from the symbolic backend when we try to :prove a predicate involving a symbolic if-then-else on two records with the same fields written in different orders:

Cryptol> :prove \x -> (if x then { a = True, b = False } else { b = False, a = True }).a
cryptol: You have encountered a bug in Cryptol's implementation.
*** Please create an issue at https://github.com/GaloisInc/cryptol/issues

%< ---------------------------------------------------
  Revision:  ce0365fb80e66bf0aa844b0dac86d6d1777809fb
  Branch:    master (uncommited files present)
  Location:  Cryptol.Symbolic.Value
  Message:   mergeValue.mergeField: incompatible values
CallStack (from HasCallStack):
  panic, called at src/Cryptol/Utils/Panic.hs:21:9 in cryptol-2.8.1-inplace:Cryptol.Utils.Panic
  panic, called at src/Cryptol/Symbolic/Value.hs:148:9 in cryptol-2.8.1-inplace:Cryptol.Symbolic.Value
%< ---------------------------------------------------

The code for mergeValue was assuming that we normalize the field order of all record expressions alphabetically, but it turns out that we don't do that.

Probably the right way to fix problems like this once and for all is to use a Map instead of a list of pairs in the VRecord constructor.