fix deref/addr pair deleting assignment location in C++ (#24280)

fixes #24274

The code in the `if` branch replaces the current destination `d` with a
new one. But the location `d` can be an assignment location, in which
case the provided expression isn't generated. To fix this, don't trigger
this code for when the location already exists. An alternative would be
to call `putIntoDest` in this case as is done below.

compiler/ccgexprs.nim

@@ -758,7 +758,10 @@ proc genDeref(p: BProc, e: PNode, d: var TLoc) =
     if typ.kind in {tyUserTypeClass, tyUserTypeClassInst} and typ.isResolvedUserTypeClass:
       typ = typ.last
     typ = typ.skipTypes(abstractInstOwned)
-    if typ.kind in {tyVar} and tfVarIsPtr notin typ.flags and p.module.compileToCpp and e[0].kind == nkHiddenAddr:
+    if typ.kind in {tyVar} and tfVarIsPtr notin typ.flags and
+        p.module.compileToCpp and e[0].kind == nkHiddenAddr and
+        # don't override existing location:
+        d.k == locNone:
       d = initLocExprSingleUse(p, e[0][0])
       return
     else:

tests/ccgbugs/tvararrayiter.nim

@@ -0,0 +1,21 @@
+discard """
+  targets: "c cpp"
+"""
+
+block: # issue #24274
+  iterator foo[T](x: var T): var T =
+    yield x
+
+  var x: array[3, char]
+  for a in foo(x):
+    let b = a
+  
+  var y: array[3, char] = ['a', 'b', 'c']
+  for a in foo(y):
+    let b = a
+    doAssert a[0] == 'a'
+    doAssert a[1] == 'b'
+    doAssert a[2] == 'c'
+    doAssert b[0] == 'a'
+    doAssert b[1] == 'b'
+    doAssert b[2] == 'c'