From 2b2872928b5f8ac1779c980481ecee4a575b4590 Mon Sep 17 00:00:00 2001
From: ringabout <43030857+ringabout@users.noreply.github.com>
Date: Fri, 17 Apr 2026 15:59:22 +0800
Subject: [PATCH] fixes #25469; Conversion from distinct in for forces a copy
 of underlying instance (#25746)

fixes #25469

This pull request introduces an important fix to argument handling in
the compiler's transformation logic and adds a new test to verify
correct behavior with distinct types and ARC memory management.

### Compiler transformation improvements

* Updated `putArgInto` in `compiler/transf.nim` to handle
`nkHiddenStdConv`, `nkHiddenSubConv`, and `nkConv` nodes more
accurately. Now, if the types match (ignoring distinctness and shallow
range differences), the argument is recursively processed; otherwise, it
falls back to a fast assignment. This prevents incorrect assignments
when dealing with type conversions and distinct types.

### Testing for distinct types and ARC

* Added a new test `tdistinct_for_nodup.nim` to ensure correct iteration
and memory management for distinct sequences of large arrays under ARC.
The test checks that the sequence length remains unchanged during
iteration, helping catch regressions related to ARC and distinct types.
---
 compiler/transf.nim               |  5 +++++
 tests/arc/tdistinct_for_nodup.nim | 33 +++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 tests/arc/tdistinct_for_nodup.nim

diff --git a/compiler/transf.nim b/compiler/transf.nim
index 399a8c468a..3059d8fe6b 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -702,6 +702,11 @@ proc putArgInto(arg: PNode, formal: PType): TPutArgInto =
   of nkAddr, nkHiddenAddr:
     result = putArgInto(arg[0], formal)
     if result == paViaIndirection: result = paFastAsgn
+  of nkHiddenStdConv, nkHiddenSubConv, nkConv:
+    if compareTypes(arg.typ, arg[1].typ, dcEqIgnoreDistinct, {IgnoreRangeShallow}):
+      result = putArgInto(arg[1], formal)
+    else:
+      result = paFastAsgn
   of nkCurly, nkBracket:
     for i in 0..<arg.len:
       if putArgInto(arg[i], formal) != paDirectMapping:
diff --git a/tests/arc/tdistinct_for_nodup.nim b/tests/arc/tdistinct_for_nodup.nim
new file mode 100644
index 0000000000..427f6b2b02
--- /dev/null
+++ b/tests/arc/tdistinct_for_nodup.nim
@@ -0,0 +1,33 @@
+discard """
+  cmd: '''nim c --mm:arc --expandArc:foo $file'''
+  nimout: '''
+--expandArc: foo
+
+var broken_cursor
+block :tmp:
+  var i
+  var i_1 = 0
+  let L = len(seq[Large](broken_cursor))
+  block :tmp_1:
+    while i_1 < L:
+      i = seq[Large](broken_cursor)[i_1]
+      discard i
+      {.push, overflowChecks: false.}
+      inc(i_1, 1)
+      {.pop.}
+-- end of expandArc ------------------------
+'''
+"""
+
+
+type
+  Large = array[1024, byte]
+  List = distinct seq[Large]
+
+proc foo =
+  var
+    broken: List
+  for i in seq[Large](broken):
+    discard i
+
+foo()
\ No newline at end of file