From a57b0f1417215b11b0beda93b20bd387c78d4aa1 Mon Sep 17 00:00:00 2001
From: Stephen von Takach <steve@place.technology>
Date: Sat, 20 Jun 2026 22:34:27 +1000
Subject: [PATCH] Require `tuple` before `env` in the prelude (#8771)

`{"a", "b"}.as Enumerable(String)` crashed codegen with
`Enumerable::NotFoundError`: `Tuple(String, String)` was missing from
`Enumerable(String)`'s including types.

The cause is require ordering. `Tuple` does `include Indexable(Union(*T))`
(which transitively includes `Enumerable`/`Iterable`), but `tuple` was
required after `env`. `ENV` does `extend Enumerable({String, String})`, which
instantiates `Tuple(String, String)` before `Tuple` gained those module
parents, so its `after_initialize` registered nothing and it never appeared in
`Enumerable(String).including_types`. Codegen then failed to find it when
upcasting. Other arities worked because they were instantiated later.

Requiring `tuple` (and `named_tuple`) in the ordered block right after
`indexable` ensures the includes are processed before any early
instantiation, so all arities register correctly. This replaces the need for
any compiler-side workaround.

A codegen spec covers the original reproducer.
---
 spec/compiler/codegen/cast_spec.cr | 11 +++++++++++
 src/prelude.cr                     |  9 +++++++--
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/spec/compiler/codegen/cast_spec.cr b/spec/compiler/codegen/cast_spec.cr
index cbfba96ff70e..9dcf115329ea 100644
--- a/spec/compiler/codegen/cast_spec.cr
+++ b/spec/compiler/codegen/cast_spec.cr
@@ -497,4 +497,15 @@ describe "Code gen: cast" do
       Base.as(Base | Base.class).as?(Base | Impl).nil?
       CRYSTAL
   end
+
+  it "casts same-type tuple to Enumerable whose splat collapses (#8771)" do
+    # `Tuple(String, String)` is instantiated early (via `ENV`) before
+    # `Tuple` includes `Indexable`, so it used to be missing from
+    # `Enumerable(String).including_types`, crashing this upcast.
+    run(<<-CRYSTAL).to_string.should eq("ab")
+      require "prelude"
+
+      ({"a", "b"}.as(Enumerable(String))).to_a.join
+      CRYSTAL
+  end
 end
diff --git a/src/prelude.cr b/src/prelude.cr
index 267bb7eb1f48..c5beefe4a8aa 100644
--- a/src/prelude.cr
+++ b/src/prelude.cr
@@ -18,6 +18,13 @@ require "iterable"
 require "iterator"
 require "steppable"
 require "indexable"
+# `Tuple` includes `Indexable(Union(*T))`. It must be required here, before any
+# file that can register a tuple as an including type of a module (e.g. `env`'s
+# `extend Enumerable(...)`), otherwise that early instantiation runs with an
+# incomplete parent list and is missing from the module's including types,
+# breaking `as Enumerable(T)` upcasts (#8771).
+require "tuple"
+require "named_tuple"
 require "string"
 require "number"
 require "primitives"
@@ -53,7 +60,6 @@ require "io"
 require "kernel"
 require "math/math"
 require "mutex"
-require "named_tuple"
 require "nil"
 require "humanize"
 require "path"
@@ -78,7 +84,6 @@ require "symbol"
 require "system"
 require "crystal/system/thread"
 require "time"
-require "tuple"
 require "unicode"
 require "union"
 require "va_list"