Fix issue 12007 - cartesianProduct does'nt work with ranges of immutables

https://d.puremagic.com/issues/show_bug.cgi?id=12007

This makes a tweak to `Zip`'s `save`: Now, it *builds* a new `Zip` object
from the saved ranges, rather than assigning each range individually.

This gives 2 advantages:
1. Better support for `save` (which doesn't actaully guarantee assignability)
2. Avoids useless `opAssign` overhead

std/range.d

@@ -4684,15 +4684,18 @@ struct Zip(Ranges...)
     }
 
     static if (allSatisfy!(isForwardRange, R))
+    {
+        private this(ref Zip other)
+        {
+            foreach (i, Unused; R)
+                ranges[i] = other.ranges[i].save;
+            stoppingPolicy = other.stoppingPolicy;
+        }
         @property Zip save()
         {
-            Zip result = this;
-            foreach (i, Unused; R)
-            {
-                result.ranges[i] = result.ranges[i].save;
-            }
-            return result;
+            return Zip(this);
         }
+    }
 
     private void emplaceIfCan(T)(T* addr)
     {
@@ -5193,6 +5196,21 @@ unittest
     assertThrown(zip(StoppingPolicy.longest, cast(S[]) null, new int[1]).front);
 }
 
+unittest //12007
+{
+    static struct R
+    {
+        enum empty = false;
+        void popFront(){}
+        int front(){return 1;} @property
+        R save(){return this;} @property
+        void opAssign(R) @disable;
+    }
+    R r;
+    auto z = zip(r, r);
+    auto zz = z.save;
+}
+
 /*
     Generate lockstep's opApply function as a mixin string.
     If withIndex is true prepend a size_t index to the delegate.