Compiler crash with to_unsafe and C type

[keidax]

Reproduction:
https://gist.github.com/keidax/890f0a7195f64950effb3a302e879996

On macOS, I get this output:

$ crystal foo_object.cr
Module validation failed: Incorrect number of arguments passed to called function!
  call void @"*Pointer(Void)@Object::to_s<String::Builder>:Nil"(i32 %obj, %"String::Builder"* %self), !dbg !183
 (Exception)
  from Crystal::CodeGenVisitor#finish:Nil
  from Crystal::Compiler#codegen<Crystal::Program, Crystal::ASTNode+, Array(Crystal::Compiler::Source), String>:(Tuple(Array(Crystal::Compiler::CompilationUnit), Array(String)) | Nil)
  from Crystal::Compiler#compile<Array(Crystal::Compiler::Source), String>:Crystal::Compiler::Result
  from Crystal::Command#run_command<Bool>:Nil
  from Crystal::Command#run:(Bool | Crystal::Compiler::Result | Nil)
  from __crystal_main
  from main
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 on macOS Mojave 10.14.6
Output of crystal -v:

Crystal 0.31.1 (2019-10-02)

LLVM: 8.0.1
Default target: x86_64-apple-macosx

[HertzDevil]

This breaks make spec, because the standard library spec does this:

crystal/spec/std/xml/reader_spec.cr

Lines 549 to 554 in 5b8cb31

	describe "#to_unsafe" do
	it "returns a pointer to the underlying LibXML::XMLTextReader" do
	reader = Reader.new("<root/>")
	reader.to_unsafe.should be_a(LibXML::XMLTextReader)
	end
	end

where LibXML::XMLTextReader is defined as:

crystal/src/xml/libxml2.cr

Line 82 in 5b8cb31

type XMLTextReader = Void*

The actual conflict occurs within Spec::BeAExpectation#failure_message. Reduced:

lib LibFoo
  type Foo = Pointer(Void)
end

class Class
  # corresponds to `#to_s(io)`,
  # called by `String::Builder#<<`,
  # called by `String.interpolation(*args : *T) forall T`,
  # called by `Spec::BeAExpectation#failure_message`
  def foo(x)
    x
  end
end

x = 1
LibFoo::Foo.foo(x)
Pointer(Void).foo(x)

But note that the linked spec should fail; LibAsdf::VoidPtr and Pointer(Void) are sibling types, so neither is a subtype of the other. (To make it pass, use alias instead of type.)