js compile crash for generic proc that has return type marked by `exportc`

[litlighilit]

Nim Version

2.3.1
2.*

Description

static: assert defined(js)
proc f(_: SomeFloat): int{.exportc.} = discard

Current Output

Error: unhandled exception: index 7 not in 0 .. 6 [IndexDefect]

Expected Output

(compiles with no error)

Known Workarounds

Delete one of these properties:

• exportc
• generic
• has return type
• (js backend)

Additional Information

real world example: nimpylib/nimpylib#43

Traceback:

compiler/jsgen.nim:3063

genSym accesses n[namePos] (and the line above it checks proc has sfExportc and no sfCompilerProc)
→ genProcForSymIfNeeded →attachProc →

genProc checks
if prc.typ.returnType != nil and sfPure notin prc.flags:

and then call gen(p, prc.ast[resultPos], a) where prc is n[namePos]

But n[namePos].sym.ast.sons.len != 8

So n[namePos].sym.ast[resultPos](resultPos = 7) raises IndexDefect

compiler/pipelines

However, we can find that even in C backend, the node that'll be passed as n[namePos].sym.ast in JS backend still doesn't contain Item#7