Internal interpreter error when reading constant with invalid initializer

[HertzDevil]

Constants defined in an interpreter session, like in compiled code, are not type-checked until the constant is read. If a compilation error occurs during initialization, subsequent reads of the same constant produce an internal error:

icr:1:0> A = 1.foo
=> nil
icr:2:0> A
error in line 1
Error: undefined method 'foo' for Int32
icr:3:0> A
Nil assertion failed (NilAssertionError)
  from src/nil.cr:108:5 in 'not_nil!'
  from src/compiler/crystal/interpreter/compiler.cr:1453:16 in 'get_const_index_and_compiled_def'
  from src/compiler/crystal/interpreter/compiler.cr:2604:27 in 'initialize_const_if_needed'
  from src/compiler/crystal/interpreter/compiler.cr:1435:17 in 'visit'
  from src/compiler/crystal/syntax/visitor.cr:27:12 in 'accept'
  from src/compiler/crystal/interpreter/compiler.cr:157:5 in 'compile'
  from src/compiler/crystal/interpreter/interpreter.cr:215:5 in 'interpret'
  from src/compiler/crystal/interpreter/repl.cr:100:5 in 'interpret'
  from src/compiler/crystal/interpreter/repl.cr:40:17 in 'run'
  from src/compiler/crystal/command/repl.cr:37:7 in 'repl'
  from src/compiler/crystal/command.cr:100:7 in 'run'
  from src/compiler/crystal/command.cr:51:5 in 'run'
  from src/compiler/crystal/command.cr:50:3 in 'run'
  from src/compiler/crystal.cr:11:1 in '__crystal_main'
  from src/crystal/main.cr:115:5 in 'main_user_code'
  from src/crystal/main.cr:101:7 in 'main'
  from src/crystal/main.cr:127:3 in 'main'
  from /lib/x86_64-linux-gnu/libc.so.6 in '??'
  from /lib/x86_64-linux-gnu/libc.so.6 in '__libc_start_main'
  from /home/quinton/crystal/crystal/.build/crystal in '_start'
  from ???

The delayed evaluation matches the behavior of compiled code, but it does look a little strange in a REPL:

icr:1:0> A = B # okay
=> nil
icr:2:0> B = 1
=> nil
icr:3:0> A
=> 1

The interpreter doesn't check the same initializer again after the first time it is evaluated:

icr:1:0> A = B
=> nil
icr:2:0> A
error in line 1
Error: undefined constant B

Did you mean 'A'?
icr:3:0> B = 1
=> nil
icr:4:0> A
Nil assertion failed (NilAssertionError)