Module validation failed: Incorrect number of arguments passed to called function!
  %0 = call %"String::Builder"* @"*Pointer(Void)@Object::to_s<String::Builder>:String::Builder"(i32 %obj, %"String::Builder"* %self), !dbg !148

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Error: you've found a bug in the Crystal compiler. Please open an issue, including source code that will allow us to reproduce the bug: https://github.com/crystal-lang/crystal/issues

This is due to:

describe CuDNN do
  it "allocates memory" do
    p = ::CuDNN.malloc(100)
    p.should be_a(Pointer(Void))
  end
end

which calls
module CuDNN
  extend self
  def malloc(size)
    check_success(LibCUDA.malloc(out ptr, size))
    return ptr
  end
end

which calls

@[Link("cudart")]
lib LibCUDA
  enum ErrorT
    Success
  end
  fun malloc = cudaMalloc(ptr : Void**, size : LibC::Int) : ErrorT
end