internal error: genMagicExpr: mArrPut

[liori]

When I'm compiling the following MWE file:

type
  TagGenerator = distinct string

proc `[]=`(s: TagGenerator; x: int; b: char): void {.borrow.}

proc next(tg: TagGenerator): void =
  tg[0] = 'x'

nim compiler fails with the following traceback:

bug.nim(7, 5) Error: internal error: genMagicExpr: mArrPut
Traceback (most recent call last)
nim.nim(115)             nim
nim.nim(71)              handleCmdLine
main.nim(254)            mainCommand
main.nim(65)             commandCompileToC
modules.nim(228)         compileProject
modules.nim(173)         compileModule
passes.nim(203)          processModule
passes.nim(137)          processTopLevelStmt
cgen.nim(1221)           myProcess
ccgstmts.nim(1120)       genStmts
ccgexprs.nim(2153)       expr
cgen.nim(789)            genProc
cgen.nim(759)            genProcNoForward
cgen.nim(679)            genProcAux
ccgstmts.nim(1120)       genStmts
ccgexprs.nim(2047)       expr
ccgexprs.nim(1758)       genMagicExpr
msgs.nim(996)            internalError
msgs.nim(970)            liMessage
msgs.nim(842)            handleError
msgs.nim(826)            quit
FAILURE