From 8a411da772660075e9890437950ab61b00f33fc2 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Mon, 3 Aug 2026 17:49:48 +0800
Subject: [PATCH] fixes #26045; #26046; `when nimvm` leak push options (#26047)

fixes #26045;
fixes #26046

The fix isolates compiler option state while semantically checking each
when nimvm branch.

compiler/semexprs.nim:2745 snapshots the option stack, compiler options,
diagnostics settings, and enabled features. It analyzes one branch and
restores that state in finally. Both the nimvm and else branches use
this function.

This prevents:

```nim
when nimvm:
  {.push overflowChecks: off.}
```

from disabling overflow checks in following runtime code. It also means
a {.pop.} in the opposite branch correctly reports that it has no
corresponding {.push.}.

(cherry picked from commit f6651e6c705300c48f7b846e998a7a02623d151f)
---
 compiler/semexprs.nim                   | 21 +++++++++++++++++++--
 tests/whenstmt/twhen_nimvm_push.nim     | 21 +++++++++++++++++++++
 tests/whenstmt/twhen_nimvm_push_pop.nim |  8 ++++++++
 3 files changed, 48 insertions(+), 2 deletions(-)
 create mode 100644 tests/whenstmt/twhen_nimvm_push.nim
 create mode 100644 tests/whenstmt/twhen_nimvm_push_pop.nim

diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index 6599dd028d..683bde602a 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -2664,6 +2664,22 @@ proc semMagic(c: PContext, n: PNode, s: PSym, flags: TExprFlags; expectedType: P
   else:
     result = semDirectOp(c, n, flags, expectedType)
 
+proc semNimvmBranch(c: PContext, n: PNode, flags: TExprFlags): PNode =
+  let
+    oldOptionStack = c.optionStack[0..^1]
+    oldOptions = c.config.options
+    oldNotes = c.config.notes
+    oldWarningAsErrors = c.config.warningAsErrors
+    oldFeatures = c.features
+  try:
+    result = semExpr(c, n, flags)
+  finally:
+    c.optionStack = oldOptionStack
+    c.config.options = oldOptions
+    c.config.notes = oldNotes
+    c.config.warningAsErrors = oldWarningAsErrors
+    c.features = oldFeatures
+
 proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
   # If semCheck is set to false, ``when`` will return the verbatim AST of
   # the correct branch. Otherwise the AST will be passed through semStmt.
@@ -2700,7 +2716,7 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       checkSonsLen(it, 2, c.config)
       if whenNimvm:
         if semCheck:
-          it[1] = semExpr(c, it[1], flags)
+          it[1] = semNimvmBranch(c, it[1], flags)
           typ = commonType(c, typ, it[1].typ)
         result = n # when nimvm is not elimited until codegen
       elif c.inGenericContext > 0:
@@ -2731,7 +2747,8 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
         discard
       elif result == nil or whenNimvm:
         if semCheck:
-          it[0] = semExpr(c, it[0], flags)
+          it[0] = if whenNimvm: semNimvmBranch(c, it[0], flags)
+                  else: semExpr(c, it[0], flags)
           typ = commonType(c, typ, it[0].typ)
           if typ != nil and typ.kind != tyUntyped:
             it[0] = fitNode(c, typ, it[0], it[0].info)
diff --git a/tests/whenstmt/twhen_nimvm_push.nim b/tests/whenstmt/twhen_nimvm_push.nim
new file mode 100644
index 0000000000..7991239fcc
--- /dev/null
+++ b/tests/whenstmt/twhen_nimvm_push.nim
@@ -0,0 +1,21 @@
+discard """
+  output: "ok"
+"""
+
+var overflowDetected = false
+when nimvm:
+  {.push overflowChecks: off.}
+else:
+  var branchX = high(int)
+  try:
+    inc branchX
+  except OverflowDefect:
+    overflowDetected = true
+
+doAssert overflowDetected
+
+var x = high(int)
+try:
+  inc x
+except OverflowDefect:
+  echo "ok"
diff --git a/tests/whenstmt/twhen_nimvm_push_pop.nim b/tests/whenstmt/twhen_nimvm_push_pop.nim
new file mode 100644
index 0000000000..31f44c4058
--- /dev/null
+++ b/tests/whenstmt/twhen_nimvm_push_pop.nim
@@ -0,0 +1,8 @@
+discard """
+  errormsg: "{.pop.} without a corresponding {.push.}"
+  line: 8
+"""
+
+when nimvm:
+  {.push checks: off.}
+else: {.pop.}