From b42cd6e8fc3ec669d67b8b4f2fcf92cb5d0a08d1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Simen=20Kj=C3=A6r=C3=A5s?= <simen.kjeraas@volmax.no>
Date: Mon, 15 Jun 2020 09:02:40 +0200
Subject: [PATCH] Fix issue 20928

---
 std/algorithm/iteration.d | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/std/algorithm/iteration.d b/std/algorithm/iteration.d
index 4cbf903eec2..4ffd0f3532c 100644
--- a/std/algorithm/iteration.d
+++ b/std/algorithm/iteration.d
@@ -630,7 +630,7 @@ private struct MapResult(alias fun, Range)
 
     static if (isRandomAccessRange!R)
     {
-        static if (is(typeof(_input[ulong.max])))
+        static if (is(typeof(Range.init[ulong.max])))
             private alias opIndex_t = ulong;
         else
             private alias opIndex_t = uint;
@@ -870,6 +870,23 @@ private struct MapResult(alias fun, Range)
     assert(m.front == immutable(S)(null));
 }
 
+// Issue 20928
+@safe unittest
+{
+    struct Always3
+    {
+        enum empty = false;
+        auto save() { return this; }
+        long front() { return 3; }
+        void popFront() {}
+        long opIndex(ulong i) { return 3; }
+        long opIndex(ulong i) immutable { return 3; };
+    }
+
+    import std.algorithm.iteration : map;    
+    Always3.init.map!(e => e)[ulong.max];
+}
+
 // each
 /**
 Eagerly iterates over `r` and calls `fun` over _each element.