From 482fe75dda86e1c3b5c0bbde90d42c2b48ddeadf Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Sat, 9 Nov 2024 18:28:16 +0300
Subject: [PATCH 1/2] don't skip conversions in array indices in transf

fixes #17958
---
 compiler/transf.nim        | 4 +---
 tests/array/tindexconv.nim | 4 ++++
 2 files changed, 5 insertions(+), 3 deletions(-)
 create mode 100644 tests/array/tindexconv.nim

diff --git a/compiler/transf.nim b/compiler/transf.nim
index 615bff31ee6b..94fdfcfb3d66 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -865,9 +865,7 @@ proc transformArrayAccess(c: PTransf, n: PNode): PNode =
   if n[0].kind == nkSym and n[0].sym.kind == skType:
     result = n
   else:
-    result = newTransNode(n)
-    for i in 0..<n.len:
-      result[i] = transform(c, skipConv(n[i]))
+    result = transformSons(c, n)
 
 proc getMergeOp(n: PNode): PSym =
   case n.kind
diff --git a/tests/array/tindexconv.nim b/tests/array/tindexconv.nim
new file mode 100644
index 000000000000..aa95ed4b672f
--- /dev/null
+++ b/tests/array/tindexconv.nim
@@ -0,0 +1,4 @@
+block: # issue #17958
+  var mem: array[uint8, uint8]
+  let val = 0xffff'u16
+  discard mem[uint8 val]   # Error: unhandled exception: index 65535 not in 0 .. 255 [IndexDefect]

From af45b14b909b806c0cbe683ba476f88af34120a5 Mon Sep 17 00:00:00 2001
From: metagn <metagngn@gmail.com>
Date: Sat, 9 Nov 2024 20:19:01 +0300
Subject: [PATCH 2/2] weird alternative

---
 compiler/transf.nim                  | 11 +++++++++++
 tests/array/tinvalidarrayaccess.nim  |  2 +-
 tests/array/tinvalidarrayaccess2.nim |  2 +-
 tests/js/tindexdefect.nim            |  5 +++--
 4 files changed, 16 insertions(+), 4 deletions(-)

diff --git a/compiler/transf.nim b/compiler/transf.nim
index 94fdfcfb3d66..59e4453fd7fb 100644
--- a/compiler/transf.nim
+++ b/compiler/transf.nim
@@ -866,6 +866,17 @@ proc transformArrayAccess(c: PTransf, n: PNode): PNode =
     result = n
   else:
     result = transformSons(c, n)
+    if n.len >= 2 and result[1].kind in {nkChckRange, nkChckRange64} and
+        n[1].kind in {nkHiddenStdConv, nkHiddenSubConv}:
+      # implicit conversion, was transformed into range check
+      # remove in favor of index check if conversion to array index type
+      # has to be done here because the array index type needs to be relaxed
+      # i.e. a uint32 index can implicitly convert to range[0..3] but not int
+      let arr = skipTypes(n[0].typ, abstractVarRange)
+      if arr.kind == tyArray and
+          firstOrd(c.graph.config, arr) == getOrdValue(result[1][1]) and
+          lastOrd(c.graph.config, arr) == getOrdValue(result[1][2]):
+        result[1] = result[1].skipConv
 
 proc getMergeOp(n: PNode): PSym =
   case n.kind
diff --git a/tests/array/tinvalidarrayaccess.nim b/tests/array/tinvalidarrayaccess.nim
index f8bce45ef365..5656deb781ff 100644
--- a/tests/array/tinvalidarrayaccess.nim
+++ b/tests/array/tinvalidarrayaccess.nim
@@ -1,5 +1,5 @@
 discard """
-  errormsg: "index 2 not in 0 .. 1"
+  errormsg: "conversion from int literal(2) to range 0..1(int) is invalid"
   line: 18
 """
 block:
diff --git a/tests/array/tinvalidarrayaccess2.nim b/tests/array/tinvalidarrayaccess2.nim
index 0a07038344a8..1d8d0cfff586 100644
--- a/tests/array/tinvalidarrayaccess2.nim
+++ b/tests/array/tinvalidarrayaccess2.nim
@@ -1,5 +1,5 @@
 discard """
-  errormsg: "index 3 not in 0 .. 1"
+  errormsg: "conversion from int literal(3) to range 0..1(int) is invalid"
   line: 9
 """
 
diff --git a/tests/js/tindexdefect.nim b/tests/js/tindexdefect.nim
index 37994ec2e749..eea376883836 100644
--- a/tests/js/tindexdefect.nim
+++ b/tests/js/tindexdefect.nim
@@ -5,5 +5,6 @@ discard """
 """
 
 var s = ['a']
-let z = s[10000] == 'a'
-echo z
\ No newline at end of file
+let i = 10000
+let z = s[i] == 'a'
+echo z