stricter skip for conversions in array indices in transf

compiler/transf.nim

@@ -865,9 +865,18 @@ 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)
+    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

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]

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:

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
 """
 

tests/js/tindexdefect.nim

@@ -5,5 +5,6 @@ discard """
 """
 
 var s = ['a']
-let z = s[10000] == 'a'
-echo z
+let i = 10000
+let z = s[i] == 'a'
+echo z