fixes #25553; Invalid codegen for accessing tuple in array (#25555)

fixes #25553

(cherry picked from commit 4566ffaca9)

compiler/sigmatch.nim

@@ -1673,7 +1673,7 @@ proc typeRel(c: var TCandidate, f, aOrig: PType,
 
     let roota = if skipBoth or deptha > depthf: a.skipGenericAlias else: a
     let rootf = if skipBoth or depthf > deptha: f.skipGenericAlias else: f
-    
+
     if f.isConcept:
       result = enterConceptMatch(c, rootf, roota, flags)
     elif a.kind == tyGenericInst:
@@ -2187,9 +2187,9 @@ proc implicitConv(kind: TNodeKind, f: PType, arg: PNode, m: TCandidate,
       result.typ() = errorType(c)
   else:
     result.typ() = f.skipTypes({tySink})
-  # keep varness
+  # keep varness, but don't wrap lent types with var
   if arg.typ != nil and arg.typ.kind == tyVar:
-    result.typ() = toVar(result.typ, tyVar, c.idgen)
+    result.typ() = toVar(result.typ.skipTypes({tyLent}), tyVar, c.idgen)
     # copy the tfVarIsPtr flag
     result.typ.flags = arg.typ.flags
   else:

tests/types/tlent_var.nim

@@ -23,3 +23,18 @@ proc varProc(x: var int) =
 doAssert: not compiles(test_lent(x) = 1)
 doAssert: not compiles(varProc(test_lent(x)))
 
+type X = tuple[a: int, b: int]
+
+type ArrayBuf*[N: static int, T] = object
+  buf*: array[N, T]
+
+var v: ArrayBuf[32, X]
+
+
+# proc `[]`*[N, T](b: var ArrayBuf[N, T], i: BackwardsIndex): lent T = # works
+#   b.buf[i]
+
+template `[]`*[N, T](b: var ArrayBuf[N, T], i: BackwardsIndex): lent T =
+  b.buf[i]
+
+doAssert $v[^4] == "(a: 0, b: 0)"