MemberQ weird inside Compile

Question

Compile[{{x, _Real, 1}}, MemberQ[x, 2]][{2}]

outputs False.

In fact, it seems to get compiled as False for every input if you look at a CompilePrint. It doesn't call the main evaluator, and MemberQ is included in the Compile`CompilerFunctions[] list

Any ideas?

Answer 1

OK, as J. M. pointed out, it was an issue of types. x was defined to be of Real type, so it was converted and treated as such in the compiled code. Since a 2 can never be a member of a vector of Reals, the compiler hard-coded the result False, which is correct.

Answer 2

Some more insight can be obtained using CompilePrint:

Needs["CompiledFunctionTools`"];
CompilePrint[
 Compile[{{x, _Real, 1}},
  MemberQ[x, 2]]
 ]

(ie, it works out that it's always false due to the type), versus

Needs["CompiledFunctionTools`"];
CompilePrint[
 Compile[{{x, _Integer, 1}},
  MemberQ[x, 2]]
 ]

which does the usual thing.