Various set related bugs

[ghost]

when defined(a): # fails
    proc p(T: typedesc): set[T] = discard # "Error: ordinal type expected"
elif defined(b): # fails
    proc p[T](): set[T] = discard
    discard p[int]

    # Traceback (most recent call last)
    # nim.nim(121)             nim
    # nim.nim(77)              handleCmdLine
    # main.nim(163)            mainCommand
    # main.nim(74)             commandCompileToC
    # modules.nim(240)         compileProject
    # modules.nim(180)         compileModule
    # passes.nim(215)          processModule
    # passes.nim(135)          processTopLevelStmt
    # cgen.nim(1297)           myProcess
    # ccgstmts.nim(1126)       genStmts
    # ccgexprs.nim(2084)       expr
    # ccgstmts.nim(1126)       genStmts
    # ccgexprs.nim(2156)       expr
    # cgen.nim(823)            genProc
    # cgen.nim(786)            genProcNoForward
    # cgen.nim(736)            genProcPrototype
    # ccgtypes.nim(848)        genProcHeader
    # ccgtypes.nim(389)        genProcParams
    # ccgtypes.nim(233)        isInvalidReturnType
    # ccgtypes.nim(164)        mapType
    # ccgtypes.nim(151)        mapSetType
    # types.nim(1353)          getSize
    # types.nim(1345)          computeSize
    # types.nim(1292)          computeSizeAux
    # types.nim(1181)          align
    # system.nim(2643)         sysFatal
    # Error: unhandled exception: over- or underflow [OverflowError]
elif defined(c): # works
    proc p[T](): set[T] = discard
    discard p[int8]()

[nc-x]

Case a -

proc p(T: typedesc): set[T] = discard 

This is not a bug AFAIK.
https://nim-lang.org/docs/manual.html#types-set-type says that the basetype of set can only be - int8-int16, uint8/byte-uint16, char, enum whereas typedesc can be any other type as well, and is therefore not an ordinal type.

But yeah, the other cases like

proc p[T](): set[T] = discard
discard p[float]()   # replace float by any other unsupported type in set

crashes one way or the other

[Araq]

Case a is also a bug, the compiler needs to delay that error until set[T] is instantiated.

[metagn]

Case b no longer crashes but gives Error: illegal discard proc, did you mean: p()