From cfa769fefc86c2f51f5647605622e19a349dd7ee Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Thu, 28 May 2026 05:28:27 +0800
Subject: [PATCH] fixes #22950; Poor error message on cast effect violation
 (#25839)

fixes #22950

This pull request improves the tracking and reporting of effect
annotations (such as `raises`, `tags`, and `forbids`) in pragma blocks,
particularly when using the `cast` pragma. It ensures that the source of
these effect annotations is correctly preserved and referenced, which
improves error reporting and effect analysis. Additionally, a new test
was added to check for violations when using `cast` with effect
annotations.

Effect annotation source tracking and propagation:

* Added new fields (`excSource`, `tagsSource`, `forbidsSource`) to the
`PragmaBlockContext` type to store the original source node for each
effect annotation.
* Updated `castBlock` to set these new source fields when processing
`raises`, `tags`, and `forbids` pragmas, ensuring the source node is
preserved for later error reporting.
* Modified `unapplyBlockContext` to use the stored source node (if
available) when calling `addRaiseEffect`, `addTag`, and `addNotTag`,
improving the accuracy of effect tracking and diagnostics.

Pragma handling improvements:

* Changed the call to `castBlock` in the main pragma processing loop to
pass the entire pragma node, enabling access to the original source for
effect annotations.

Testing:

* Added a new test (`tests/effects/tcast_effect_violation.nim`) to
verify that using `cast(raises: ValueError)` inside a procedure with
`.raises: [].` correctly triggers an error message about an unlisted
exception.
---
 compiler/sempass2.nim                    | 15 ++++++++++-----
 tests/effects/tcast_effect_violation.nim |  8 ++++++++
 2 files changed, 18 insertions(+), 5 deletions(-)
 create mode 100644 tests/effects/tcast_effect_violation.nim

diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index 9c84b721ad..f1e2e69cbe 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -1208,6 +1208,7 @@ type
     enforcedGcSafety, enforceNoSideEffects: bool
     oldExc, oldTags, oldForbids: int
     exc, tags, forbids: PNode
+    excSource, tagsSource, forbidsSource: PNode
 
 proc createBlockContext(tracked: PEffects): PragmaBlockContext =
   var oldForbidsLen = 0
@@ -1230,17 +1231,18 @@ proc unapplyBlockContext(tracked: PEffects; bc: PragmaBlockContext) =
     # anything about 'raises' in the 'cast' at all. Same applies for 'tags'.
     setLen(tracked.exc.sons, bc.oldExc)
     for e in bc.exc:
-      addRaiseEffect(tracked, e, e)
+      addRaiseEffect(tracked, e, if bc.excSource != nil: bc.excSource else: e)
   if bc.tags != nil:
     setLen(tracked.tags.sons, bc.oldTags)
     for t in bc.tags:
-      addTag(tracked, t, t)
+      addTag(tracked, t, if bc.tagsSource != nil: bc.tagsSource else: t)
   if bc.forbids != nil:
     setLen(tracked.forbids.sons, bc.oldForbids)
     for t in bc.forbids:
-      addNotTag(tracked, t, t)
+      addNotTag(tracked, t, if bc.forbidsSource != nil: bc.forbidsSource else: t)
 
-proc castBlock(tracked: PEffects, pragma: PNode, bc: var PragmaBlockContext) =
+proc castBlock(tracked: PEffects, castPragma: PNode, bc: var PragmaBlockContext) =
+  let pragma = castPragma[1]
   case whichPragma(pragma)
   of wGcSafe:
     bc.enforcedGcSafety = true
@@ -1253,6 +1255,7 @@ proc castBlock(tracked: PEffects, pragma: PNode, bc: var PragmaBlockContext) =
     else:
       bc.tags = newNodeI(nkArgList, pragma.info)
       bc.tags.add n
+    bc.tagsSource = castPragma
   of wForbids:
     let n = pragma[1]
     if n.kind in {nkCurly, nkBracket}:
@@ -1260,6 +1263,7 @@ proc castBlock(tracked: PEffects, pragma: PNode, bc: var PragmaBlockContext) =
     else:
       bc.forbids = newNodeI(nkArgList, pragma.info)
       bc.forbids.add n
+    bc.forbidsSource = castPragma
   of wRaises:
     let n = pragma[1]
     if n.kind in {nkCurly, nkBracket}:
@@ -1267,6 +1271,7 @@ proc castBlock(tracked: PEffects, pragma: PNode, bc: var PragmaBlockContext) =
     else:
       bc.exc = newNodeI(nkArgList, pragma.info)
       bc.exc.add n
+    bc.excSource = castPragma
   of wUncheckedAssign:
     discard "handled in sempass1"
   else:
@@ -1520,7 +1525,7 @@ proc track(tracked: PEffects, n: PNode) =
       of wNoSideEffect:
         bc.enforceNoSideEffects = true
       of wCast:
-        castBlock(tracked, pragmaList[i][1], bc)
+        castBlock(tracked, pragmaList[i], bc)
       else:
         discard
     applyBlockContext(tracked, bc)
diff --git a/tests/effects/tcast_effect_violation.nim b/tests/effects/tcast_effect_violation.nim
new file mode 100644
index 0000000000..fe7f93cc4a
--- /dev/null
+++ b/tests/effects/tcast_effect_violation.nim
@@ -0,0 +1,8 @@
+discard """
+  errormsg: "cast(raises: ValueError) can raise an unlisted exception: ValueError"
+  line: 7
+"""
+
+proc fff() {.raises: [].} =
+  {.cast(raises: ValueError).}:
+    discard
\ No newline at end of file