fixes #25851; ensure --panics:on does not skip nimErr_ check after closure calls (#25855)

fixes #25851

## Summary: `--panics:on` drops `nimErr_` check after closure calls
(#25851)

### Bug

With `--exceptions:goto` and `--panics:on`, the compiler skipped the
`nimErr_` check after indirect closure calls whose result flows directly
into another call (e.g., `result.add elem(src)`). A raise inside the
closure was silently swallowed — the loop continued, and the next
`raise` hit the already-set `nimInErrorMode` flag, overflowing its
`bool` storage into `OverflowDefect`.

### Root Cause

**ast.nim** — `canRaise` checked `fn.typ.n[0].len < effectListLen` first
(false after the expansion) and then `exceptionEffects != nil` (also
false, nil), so it returned `false` — meaning "cannot raise." The C
codegen trusted this and omitted the `nimErr_` check.

### Fix

**ast.nim** — `canRaise` now treats `nil` `exceptionEffects` as "unknown
→ can raise" (`exceptionEffects == nil` as an additional true
condition). This is defense-in-depth: even if some other path expands
the list but leaves `exceptionEffects` nil (e.g., a type with `{.tags.}`
but no `{.raises.}`), the error check is still emitted.

### Test

tclosure_err_panic_goto.nim — exercises the double-trigger pattern
(`drawBool` sets the error flag → closure call must propagate it) with
`matrix: "; --panics:on"` covering both exception modes.

compiler/ast.nim

@@ -1647,9 +1647,13 @@ proc canRaise*(fn: PNode): bool =
     if fn.typ.n[0].kind == nkSym:
       result = false
     else:
+      # A proc-typed value with no explicit raises slot still has
+      # unspecified effects, which sempass2 treats conservatively.
+      # Codegen needs to do the same in order to keep goto-exception
+      # checks after indirect/closure calls.
       result = ((fn.typ.n[0].len < effectListLen) or
-        (fn.typ.n[0][exceptionEffects] != nil and
-        fn.typ.n[0][exceptionEffects].safeLen > 0))
+        fn.typ.n[0][exceptionEffects] == nil or
+        fn.typ.n[0][exceptionEffects].safeLen > 0)
   else:
     result = false
 

tests/ccg/tclosure_err_panic_goto.nim

@@ -0,0 +1,36 @@
+discard """
+  matrix: "; --panics:on"
+"""
+# issue #25851: --panics:on must not drop the nimErr_ check after a closure
+# call whose result is consumed directly (e.g. `result.add elem(src)`).
+# Regression from #25295.
+
+type
+  Overrun = object of CatchableError
+  Source = object
+    data: seq[bool]
+    cursor: int
+  ElemFn = proc(src: var Source): bool {.closure.}
+
+proc drawBool(src: var Source): bool =
+  if src.cursor >= src.data.len: raise newException(Overrun, "exhausted")
+  result = src.data[src.cursor]; inc src.cursor
+
+proc listRun(elem: ElemFn, src: var Source): seq[bool] =
+  result = @[]
+  while true:
+    if not src.drawBool(): break
+    result.add elem(src)        # closure call – the result flows straight
+                                # into `add`, which previously caused the
+                                # compiler to skip the nimErr_ check.
+
+let elem: ElemFn = proc(src: var Source): bool = src.drawBool()
+
+# Both --panics:on and --panics:off must propagate the Overrun.
+var caught = false
+try:
+  var src = Source(data: @[true])
+  discard listRun(elem, src)
+except Overrun:
+  caught = true
+doAssert caught, "Overrun exception was swallowed"