fixes #22122; Unclear error message for raise of a complex expression (#25899)

fixes  #22122

The commit fixes a bug in Nim's effects checker where raise statements
with case/if expressions (commonly from template expansion) failed to
track exception types from individual branches.
Problem: addRaiseEffect only saw the outermost expression. When a
template like getTransportError(err) expanded to a case expression
raising 3 different exception types, the compiler only registered the
top-level call — missing the branch-level exceptions.
Fix (2 files):
- compiler/sempass2.nim: Added skipHiddenConv to strip implicit type
coercion nodes (nkHiddenStdConv/nkHiddenSubConv) that hide the control
flow structure. Added addRaiseEffectsFromExpr that recursively walks
into case/if/block/stmtlist expressions to find raise effects in each
branch body. Changed the nkRaiseStmt handler to use this new function.
- tests/effects/tcase_raises.nim: Test with templates that expand to
case expressions raising different exception types, verified via
{.raises: [].} pragma.

compiler/sempass2.nim

@@ -497,6 +497,33 @@ proc addRaiseEffect(a: PEffects, e, comesFrom: PNode) =
     if not isDefectException(e.typ):
       throws(a.exc, e, comesFrom)
 
+proc skipHiddenConv(n: PNode): PNode =
+  result = n
+  while true:
+    case result.kind
+    of nkHiddenStdConv, nkHiddenSubConv:
+      result = result[1]
+    else: break
+
+proc addRaiseEffectsFromExpr(a: PEffects, e, comesFrom: PNode) =
+  if e.isNil:
+    return
+  case e.kind
+  of nkStmtList, nkStmtListExpr, nkBlockStmt, nkBlockExpr:
+    if e.len > 0:
+      addRaiseEffectsFromExpr(a, e.lastSon.skipHiddenConv, comesFrom)
+  of nkIfExpr, nkIfStmt:
+    for branch in items(e):
+      if branch.len > 0:
+        addRaiseEffectsFromExpr(a, branch.lastSon.skipHiddenConv, comesFrom)
+  of nkCaseStmt:
+    for i in 1..<e.len:
+      let branch = e[i]
+      if branch.len > 0:
+        addRaiseEffectsFromExpr(a, branch.lastSon.skipHiddenConv, comesFrom)
+  else:
+    addRaiseEffect(a, e, comesFrom)
+
 proc addTag(a: PEffects, e, comesFrom: PNode) =
   var aa = a.tags
   for i in 0..<aa.len:
@@ -1326,7 +1353,7 @@ proc track(tracked: PEffects, n: PNode) =
     if n[0].kind != nkEmpty:
       n[0].info = n.info
       #throws(tracked.exc, n[0])
-      addRaiseEffect(tracked, n[0], n)
+      addRaiseEffectsFromExpr(tracked, n[0], n)
       for i in 0..<n.safeLen:
         track(tracked, n[i])
       createTypeBoundOps(tracked, n[0].typ, n.info)

tests/effects/tcase_raises.nim

@@ -0,0 +1,74 @@
+from std/os import osLastError, osErrorMsg, OSErrorCode, raiseOSError,
+                   newOSError, `==`
+
+{.push raises: [].}
+
+const
+  EPERM* = OSErrorCode(1)
+  ECONNABORTED* = OSErrorCode(53)
+  ETIMEDOUT* = OSErrorCode(60)
+  ENOTCONN* = OSErrorCode(107)
+  EMFILE* = OSErrorCode(24)
+  ENFILE* = OSErrorCode(23)
+  ENOBUFS* = OSErrorCode(55)
+  ENOMEM* = OSErrorCode(12)
+
+type
+  AsyncError* = object of CatchableError
+  TransportErrorBase* = object of AsyncError
+  TransportOsError* = object of TransportErrorBase
+    code*: OSErrorCode
+  TransportTooManyError* = object of TransportErrorBase
+  TransportAbortedError* = object of TransportErrorBase
+
+template getConnectionAbortedError*(
+           code: OSErrorCode
+         ): ref TransportAbortedError =
+  let msg =
+    case code
+    of OSErrorCode(0), ECONNABORTED:
+      "[ECONNABORTED] Connection has been aborted before being accepted"
+    of EPERM:
+      "[EPERM] Firewall rules forbid connection"
+    of ETIMEDOUT:
+      "[ETIMEDOUT] Operation has been timed out"
+    of ENOTCONN:
+      "[ENOTCONN] Transport endpoint is not connected"
+    else:
+      "[" & $int(code) & "] Connection has been aborted"
+  newException(TransportAbortedError, msg)
+
+template getTransportTooManyError*(
+           code = OSErrorCode(0)
+         ): ref TransportTooManyError =
+  let msg =
+    case code
+    of OSErrorCode(0):
+      "Too many open transports"
+    of EMFILE:
+      "[EMFILE] Too many open files in the process"
+    of ENFILE:
+      "[ENFILE] Too many open files in system"
+    of ENOBUFS:
+      "[ENOBUFS] No buffer space available"
+    of ENOMEM:
+      "[ENOMEM] Not enough memory availble"
+    else:
+      "[" & $int(code) & "] Too many open transports"
+  newException(TransportTooManyError, msg)
+
+template getTransportError*(ecode: OSErrorCode): untyped =
+  case ecode
+  of ECONNABORTED, EPERM, ETIMEDOUT, ENOTCONN:
+    getConnectionAbortedError(ecode)
+  of EMFILE, ENFILE, ENOBUFS, ENOMEM:
+    getTransportTooManyError(ecode)
+  else:
+    (ref TransportOsError)(code: ecode,
+                           msg: "(" & $int(ecode) & ") " & osErrorMsg(ecode))
+
+proc raiseTransportError*(err: OSErrorCode) {.
+     raises: [TransportAbortedError, TransportTooManyError, TransportOsError],
+     noreturn.} =
+  ## Raises transport specific OS error.
+  raise getTransportError(err)