fixes #14498 [backport:1.2] (#14503)

compiler/parampatterns.nim

@@ -176,6 +176,7 @@ type
     arLocalLValue,            # is an l-value, but local var; must not escape
                               # its stack frame!
     arDiscriminant,           # is a discriminant
+    arLentValue,              # lent value
     arStrange                 # it is a strange beast like 'typedesc[var T]'
 
 proc exprRoot*(n: PNode): PSym =
@@ -260,9 +261,14 @@ proc isAssignable*(owner: PSym, n: PNode; isUnsafeAddr=false): TAssignableResult
       # types that are equal modulo distinction preserve l-value:
       result = isAssignable(owner, n[1], isUnsafeAddr)
   of nkHiddenDeref:
-    if isUnsafeAddr and n[0].typ.kind == tyLent: result = arLValue
+    let n0 = n[0]
-    elif n[0].typ.kind == tyLent: result = arDiscriminant
+    if n0.typ.kind == tyLent:
-    else: result = arLValue
+      if isUnsafeAddr or (n0.kind == nkSym and n0.sym.kind == skResult):
+        result = arLValue
+      else:
+        result = arLentValue
+    else:
+      result = arLValue
   of nkDerefExpr, nkHiddenAddr:
     result = arLValue
   of nkObjUpConv, nkObjDownConv, nkCheckedFieldExpr:

compiler/semexprs.nim

@@ -1712,7 +1712,7 @@ proc semAsgn(c: PContext, n: PNode; mode=asgnNormal): PNode =
   if le == nil:
     localError(c.config, a.info, "expression has no type")
   elif (skipTypes(le, {tyGenericInst, tyAlias, tySink}).kind != tyVar and
-        isAssignable(c, a) == arNone) or
+        isAssignable(c, a) in {arNone, arLentValue}) or
       skipTypes(le, abstractVar).kind in {tyOpenArray, tyVarargs}:
     # Direct assignment to a discriminant is allowed!
     localError(c.config, a.info, errXCannotBeAssignedTo %

tests/lent/tnot_allowed_lent.nim

@@ -7,10 +7,10 @@ type
 
 proc mytest1(s: MyObject, i: int): lent string =
   ## works fine
-  if i < s.x.len - 1 and s.x[i] != "": 
+  if i < s.x.len - 1 and s.x[i] != "":
     result = s.x[i]
   else: raise newException(KeyError, "err1")
-  
+
 proc mytest2(s: MyObject, i: int): lent string =
   ## reject due to if expr
   if i < s.x.len - 1 and s.x[i] != "": s.x[i]

tests/lent/tnot_allowed_lent2.nim

@@ -0,0 +1,14 @@
+discard """
+  errmsg: "'x' cannot be assigned to"
+  line: 10
+"""
+
+proc bug14498 =
+  var a = @['a', 'b', 'c', 'd', 'e', 'f']
+
+  for x in a:
+    x = 'c'
+
+  echo a
+
+bug14498()