{"a", "b"}.as Enumerable(String) crashes the compiler

[aljelly]

Code:

{"a", "b"}.as Enumerable(String)

Result:

Nil assertion failed (NilAssertionError)
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from main
  from __libc_start_main
  from _start
  from ???
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

Playground link

Version:

$ crystal -v
Crystal 0.32.1 [41bd18fbe] (2019-12-18)

LLVM: 8.0.0
Default target: x86_64-unknown-linux-gnu

---

It specifically seems to be with Tuple(String) when the tuple has more than one string. {1, 2}.as Enumerable(Int32) does not crash the compiler, nor does {"a"}.as Enumerable(String).

[rdp]

Not sure why the line numbers aren't showing up. Huh?

On OS X:

$ crystal -v
Crystal 0.32.1 (2019-12-18)

LLVM: 9.0.0
Default target: x86_64-apple-macosx
$ crystal eval '{"a", "b"}.as Enumerable(String)'
Nil assertion failed (NilAssertionError)
  from Nil#not_nil!:NoReturn
  from Crystal::CodeGenVisitor#upcast_distinct<LLVM::Value, Crystal::MixedUnionType, Crystal::Type+>:LLVM::Value
  from Crystal::CodeGenVisitor#upcast<LLVM::Value, Crystal::Type+, Crystal::Type+>:LLVM::Value
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::CodeGenVisitor>:Nil
  from Crystal::ASTNode+@Crystal::ASTNode#accept<Crystal::CodeGenVisitor>: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#eval:NoReturn
  from Crystal::Command#run:(Bool | Crystal::Compiler::Result | Nil)
  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

[evvo]

These works (Crystal 0.32.1) - the problem seems to happen only when there are 2 elements - the compiler somehow assumes that one of the elements could be a Nil:

{"a", "b", "c"}.as Enumerable(String)
{"a"}.as Enumerable(String)
{"a", "b"}.to_a.as Enumerable(String)
{"a", "b", "c", "d"}.as Enumerable(String)
{"a", "b", "c", "d", "e"}.as Enumerable(String)

alias StringNil = String | Nil
{"a", "b"}.as Enumerable(StringNil)

https://carc.in/#/r/8kwv

@rdp , the line numbers are not showing properly for me as well (Ubuntu 16.04, Crystal 0.33.0 [612825a] (2020-02-14)):

$ crystal eval '{"a", "b"}.as Enumerable(String)'
Nil assertion failed (NilAssertionError)
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
  from ???
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

[HertzDevil]

Reduced:

# `Tuple` (built-in type declared by `Crystal::Program`)
struct Foo(*T)
end

# `ENV` (src/env.cr)
module Bar
  extend Enumerable(Foo(String, String))
end

# `Tuple` (src/tuple.cr, this file is required after src/env.cr)
struct Foo(*T)
  include Enumerable(Union(*T))
end

def foo
  Foo(String, String).new.as(Enumerable(String))
  {% p Enumerable(String).includers %} # => [] of ::NoReturn
end

foo

BUG: trying to upcast Enumerable(String) (Crystal::GenericModuleInstanceType) <- Foo(String) (Crystal::GenericClassInstanceType) (Exception)
  from /crystal/src/compiler/crystal/codegen/cast.cr:668:5 in 'upcast_distinct'
  from /crystal/src/compiler/crystal/codegen/cast.cr:523:15 in 'upcast'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:1310:11 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:667:9 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:2246:7 in 'codegen'
  from /crystal/src/compiler/crystal/compiler.cr:173:16 in 'compile'
  from /crystal/src/compiler/crystal/command.cr:220:5 in 'run_command'
  from /crystal/src/compiler/crystal/command.cr:64:5 in '__crystal_main'
  from /crystal/src/crystal/main.cr:115:5 in 'main'
  from src/env/__libc_start_main.c:94:2 in 'libc_start_main_stage2'

Foo(String, String) is not registered as an includer of Enumerable(String), which breaks codegen. The particular variant of the error in the OP happens when Foo is Tuple or NamedTuple and when some reference type also includes Enumerable:

module Bar
  extend Enumerable({String, String})
end

struct Tuple(*T)
  include Enumerable(Union(*T))
end

class Foo2
  include Enumerable(String)
end

def foo
  {"a"}.as(Enumerable(String))
  {"a", "b"}.as(Enumerable(String))
  {% p Enumerable(String).includers %} # => [Foo2, Tuple(String)]
end

foo

Nil assertion failed (NilAssertionError)
  from /crystal/src/nil.cr:108:5 in 'not_nil!'
  from /crystal/src/compiler/crystal/codegen/cast.cr:636:27 in 'upcast_distinct'
  from /crystal/src/compiler/crystal/codegen/cast.cr:523:15 in 'upcast'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:1310:11 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:667:9 in 'accept'
  from /crystal/src/compiler/crystal/codegen/codegen.cr:2246:7 in 'codegen'
  from /crystal/src/compiler/crystal/compiler.cr:173:16 in 'compile'
  from /crystal/src/compiler/crystal/command.cr:220:5 in 'run_command'
  from /crystal/src/compiler/crystal/command.cr:64:5 in '__crystal_main'
  from /crystal/src/crystal/main.cr:115:5 in 'main'
  from src/env/__libc_start_main.c:94:2 in 'libc_start_main_stage2'

Related: #10518