fix #16758 Nim crashes in fixAbstractType (#20855)

* fix #16758 Nim crashes in fixAbstractType

* Update compiler/semexprs.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

compiler/semexprs.nim

@@ -693,6 +693,9 @@ proc semArrayConstr(c: PContext, n: PNode, flags: TExprFlags; expectedType: PTyp
 proc fixAbstractType(c: PContext, n: PNode) =
   for i in 1..<n.len:
     let it = n[i]
+    if it == nil:
+      localError(c.config, n.info, "'$1' has nil child at index $2" % [renderTree(n, {renderNoComments}), $i])
+      return
     # do not get rid of nkHiddenSubConv for OpenArrays, the codegen needs it:
     if it.kind == nkHiddenSubConv and
         skipTypes(it.typ, abstractVar).kind notin {tyOpenArray, tyVarargs}:

tests/macros/t16758.nim

@@ -0,0 +1,38 @@
+discard """
+errormsg: "'blk.p(a)' has nil child at index 1"
+action: reject
+"""
+import macros
+
+type BlockLiteral[T] = object
+  p: T
+
+proc p[T](a:int) = echo 1
+proc p[T](a:string) = echo "a"
+
+iterator arguments(formalParams: NimNode): NimNode =
+  var iParam = 0
+  for i in 1 ..< formalParams.len:
+    let pp = formalParams[i]
+    for j in 0 .. pp.len - 3:
+      yield pp[j]
+      inc iParam
+
+macro implementInvoke(T: typedesc): untyped =
+  let t = getTypeImpl(T)[1]
+
+  let call = newCall(newDotExpr(ident"blk", ident"p"))
+  let params = copyNimTree(t[0])
+  result = newProc(ident"invoke", body = call)
+  # result[2] = newTree(nnkGenericParams,T)
+  for n in arguments(params):
+    call.add(n)
+  
+  params.insert(1, newIdentDefs(ident"blk", newTree(nnkBracketExpr, bindSym"BlockLiteral", T)))
+  result.params = params
+
+proc getInvoke(T: typedesc) =
+  implementInvoke(T)
+
+
+getInvoke(proc(a: int))