fix segfault in effect tracking for sym node with nil type (#24114)

fixes #24112

Sym nodes in templates that could be open are [given `nil`
type](https://github.com/nim-lang/Nim/blob/22d2cf217597468ace8ba540d6990b1f6d8a816a/compiler/semtempl.nim#L274)
when `--experimentalOpenSym` is disabled so that they can be semchecked
to give a warning since #24007. The first nodes of object constructors
(in this case) and in type conversions don't replace their first node
(the symbol) with a typechecked one, they only call `semTypeNode` on it
and leave it as is.

Effect tracking checks if the type of a sym node has a destructor to
check if the node type should be replaced with the sym type. But this
causes a segfault when the type of the node is nil. To fix this, we
always set the node type to the sym type if the node type is nil.

Alternatively `semObjConstr` and `semConv` could be changed to set the
type of their first node to the found type but I'm not sure if this
would break anything. They could call `semExprWithType` on the first
node but `semTypeNode` would still have to be called (maybe call it
before?). This isn't a problem if the sym node has a type but is just
nested in `nkOpenSym` or `nkOpenSymChoice` which have nil type instead
(i.e. with openSym enabled), so maybe this still is the "most general"
solution, I don't know.

compiler/sempass2.nim

@@ -1210,7 +1210,7 @@ proc track(tracked: PEffects, n: PNode) =
     if n.sym.typ != nil and tfHasAsgn in n.sym.typ.flags:
       tracked.owner.flags.incl sfInjectDestructors
       # bug #15038: ensure consistency
-      if not hasDestructor(n.typ) and sameType(n.typ, n.sym.typ): n.typ = n.sym.typ
+      if n.typ == nil or (not hasDestructor(n.typ) and sameType(n.typ, n.sym.typ)): n.typ = n.sym.typ
   of nkHiddenAddr, nkAddr:
     if n[0].kind == nkSym and isLocalSym(tracked, n[0].sym) and
           n.typ.kind notin {tyVar, tyLent}:

tests/template/t24112.nim

@@ -0,0 +1,19 @@
+discard """
+  matrix: "--skipParentCfg --filenames:legacyRelProj --hints:off"
+  action: reject
+"""
+
+# issue #24112, needs --experimental:openSym disabled
+
+block: # simplified
+  type
+    SomeObj = ref object # Doesn't error if you make SomeObj be non-ref
+  template foo = yield SomeObj()
+  when compiles(foo): discard
+
+import std/asyncdispatch
+block:
+  proc someProc(): Future[void] {.async.} = discard
+  proc foo() =
+    await someProc() #[tt.Error
+                  ^ Can only 'await' inside a proc marked as 'async'. Use 'waitFor' when calling an 'async' proc in a non-async scope instead]#