reset inTypeofContext in generic instantiations (#24229)

fixes #24228, refs #22022

As described in
https://github.com/nim-lang/Nim/issues/24228#issuecomment-2392462221,
instantiating generic routines inside `typeof` causes all code inside to
be treated as being in a typeof context, and thus preventing compile
time proc folding, causing issues when code is generated for the
instantiated routine. Now, instantiated generic procs are treated as
never being inside a `typeof` context.

This is probably an arbitrary special case and more issues with the
`typeof` behavior from #22022 are likely. Ideally this behavior would be
removed but it's necessary to accomodate the current [proc `declval` in
the package `stew`](https://github.com/status-im/nim-stew/pull/190), at
least without changes to `compileTime` that would either break other
code (making it not eagerly fold by default) or still require a change
in stew (adding an option to disable the eager folding).

Alternatively we could also make the eager folding opt-in only for
generic compileTime procs so that #22022 breaks nothing whatsoever, but
a universal solution would be better. Edit: Done in #24230 via
experimental switch

(cherry picked from commit ea9811a4d2)

compiler/seminst.nim

@@ -342,7 +342,11 @@ proc generateInstance(c: PContext, fn: PSym, pt: TIdTable,
   if c.instCounter > 50:
     globalError(c.config, info, "generic instantiation too nested")
   inc c.instCounter
-  defer: dec c.instCounter
+  let currentTypeofContext = c.inTypeofContext
+  c.inTypeofContext = 0
+  defer:
+    dec c.instCounter
+    c.inTypeofContext = currentTypeofContext
   # careful! we copy the whole AST including the possibly nil body!
   var n = copyTree(fn.ast)
   # NOTE: for access of private fields within generics from a different module

tests/vm/tgenericcompiletimeproc.nim

@@ -34,3 +34,24 @@ block: # issue #24150, related regression
   discard compiles(y(w int))
   proc s(): int {.compileTime.} = discard
   discard s()
+
+block: # issue #24228, also related regression
+  proc d(_: static[string]) = discard $0
+  proc m(_: static[string]) = d("")
+  iterator v(): int {.closure.} =
+    template a(n: untyped) =
+      when typeof(n) is void:
+        n
+      else:
+        n
+    a(m(""))
+  let _ = v
+
+block: # issue #24228 simplified
+  proc d[T]() =
+    let x = $0
+  proc v() =
+    when typeof(d[string]()) is void:
+      d[string]()
+  v()
+