Compiler crash when attempting to mutate a val field

[SeanTAllen]

program:

actor Main
  var niclas:String = "Niclas"
  
  new create(env: Env) =>
    start()
    
  be start() =>
    _start()
    
  fun _start() =>
    niclas = "Hedhman"

backtrace:

* thread #1, name = 'ponyc', stop reason = signal SIGABRT
  * frame #0: 0x00007ffff7d14a7c libc.so.6`pthread_kill + 300
    frame #1: 0x00007ffff7cc0476 libc.so.6`raise + 22
    frame #2: 0x00007ffff7ca67f3 libc.so.6`abort + 211
    frame #3: 0x0000555555c86d9b ponyc`ponyint_assert_fail(expr="0", file="/home/sean/code/ponylang/ponyc/src/libponyc/type/cap.c", line=543, func="cap_fetch") at ponyassert.c:65:3
    frame #4: 0x0000555555c17f1f ponyc`cap_fetch(type=0x00007ffff5a9f240) at cap.c:543:3
    frame #5: 0x0000555555c17f55 ponyc`cap_single(type=0x00007ffff5a9f240) at cap.c:549:16
    frame #6: 0x0000555555c7f065 ponyc`safe_to_mutate(ast=0x00007ffff72d4540) at safeto.c:219:24
    frame #7: 0x0000555555c6f598 ponyc`expr_assign(opt=0x00007fffffffe578, ast=0x00007ffff72d3b00) at operator.c:446:21
    frame #8: 0x0000555555be2fe3 ponyc`pass_expr(astp=0x00007fffffffe060, options=0x00007fffffffe578) at expr.c:576:29
    frame #9: 0x0000555555be3d14 ponyc`ast_visit(ast=0x00007fffffffe060, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:466:12
    frame #10: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe0e0, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #11: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe160, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #12: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe1e0, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #13: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe260, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #14: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe2e0, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #15: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe360, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #16: 0x0000555555be3c54 ponyc`ast_visit(ast=0x00007fffffffe458, pre=(ponyc`pass_pre_expr at expr.c:528), post=(ponyc`pass_expr at expr.c:549), options=0x00007fffffffe578, pass=PASS_EXPR) at pass.c:437:14
    frame #17: 0x0000555555be47d6 ponyc`visit_pass(astp=0x00007fffffffe458, options=0x00007fffffffe578, last_pass=PASS_ALL, out_r=0x00007fffffffe423, pass=PASS_EXPR, pre_fn=(ponyc`pass_pre_expr at expr.c:528), post_fn=(ponyc`pass_expr at expr.c:549)) at pass.c:178:6
    frame #18: 0x0000555555be419a ponyc`ast_passes(astp=0x00007fffffffe458, options=0x00007fffffffe578, last=PASS_ALL) at pass.c:271:7
    frame #19: 0x0000555555be3de2 ponyc`ast_passes_program(ast=0x00007ffff7c3bd00, options=0x00007fffffffe578) at pass.c:327:10
    frame #20: 0x0000555555c06878 ponyc`program_load(path=".", opt=0x00007fffffffe578) at package.c:916:7

[SeanTAllen]

So the issue is cap_fetch we only handle TK_NOMINAL and TK_TYPEPARAMREF but we have a TK_ARROW at that point.

@jasoncarr0 this is the code you just added. I'm not sure of the proper fix. Hopefully this is enough triaging to make it an easy fix for you.