[bmv2] Compiler Error: Could not find type for <Mux>

[vikramnathan]

I'm compiling this stripped-down p4 program using p4c-bm2-ss and get the following error:
test_bug.p4.txt

terminate called after throwing an instance of 'Util::CompilerBug'
  what():  COMPILER BUG: ../frontends/p4/typeMap.cpp:82
Could not find type for <Mux>(72820)

Aborted (core dumped)

The culprit is the action at line 72, even though it seems permissible based on my reading of the spec. The code works if I do any of the following:

1. Remove line 75
2. Assign a compile-time known value to sub on line 73, i.e. bit<32> _sub = 32w1.
3. Replace line 74 with a non-conditional assignment, i.e. _sub = 32w1.
4. Replacing val[31:0] with val and adjusting the width of _sub accordingly.

Even if the action is invalid for some reason, the compiler should provide a helpful error message.