Regression: SIGSEGV caused by using is with string

[Vindaar]

I had code similar to

let d = "aaa"
if d is "":
  echo "help"

in a library from when I was learning Nim. Trying to compile on the most recent devel causes a SIGSEGV during compilation.
koch outputs:

Traceback (most recent call last)
nim.nim(133)             nim
nim.nim(97)              handleCmdLine
main.nim(162)            mainCommand
main.nim(73)             commandCompileToC
modules.nim(124)         compileProject
modules.nim(71)          compileModule
passes.nim(194)          processModule
passes.nim(103)          processTopLevelStmt
sem.nim(587)             myProcess
sem.nim(555)             semStmtAndGenerateGenerics
semstmts.nim(1894)       semStmt
semexprs.nim(908)        semExprNoType
semexprs.nim(2510)       semExpr
semstmts.nim(160)        semIf
semexprs.nim(59)         semExprWithType
semexprs.nim(2440)       semExpr
semexprs.nim(1988)       semMagic
semexprs.nim(410)        semIs
semexprs.nim(345)        isOpImpl
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
FAILURE

While is and isnot should be used on types as I know nowadays, this should still not crash the compiler. The regression is also introduced by the recent typedesc merge of: #7681

[Vindaar]

The issue seems to be that the string "" doesn't have the kind field set. In line 345 in semexprs.nim:

  if t1.kind == tyTypeDesc and t2.kind != tyTypeDesc:

trying to access t2.kind is what causes the SIGSEGV.

edit: as a matter of fact t2 is simply nil ( t2.isNil returns true ).

[LemonBoy]

That check is guarded by

  internalAssert c.config,
    n.sonsLen == 3 and
    n[1].typ != nil and
    n[2].kind in {nkStrLit..nkTripleStrLit, nkType}

This means that t2 cannot be a tyTypeDesc. The solutions are:

• Remove the second check in the and
• Run semExpr on the n.sons[2]node anyway before callingisOpImpl`

CC @zah since he touched this proc last

[Vindaar]

Ah, thanks @LemonBoy! I'll create a PR for that.
I'm still pretty unsure about the compiler. As in how to fix issues / what procs to call etc. :) I fixed it locally by doing something like

    let t2 = semTypeNode(c, n[2], nil)
    n.sons[2] = newNodeIT(nkType, n[2].info, t2)

in the else branch of the

  if n[2].kind notin {nkStrLit..nkTripleStrLit}:

Should the call be to semExprWithType (semExpr is not enough, is it?) and only for the case where n.sons[2] is a strLit?

edit: ok, I see now that semExpr works just fine too. I really need to get a better understanding of all these procs :P