From 2c68ae817f6ac6a1908be8fa05be56a1a5c32958 Mon Sep 17 00:00:00 2001
From: ASDAlexander77 <duzhar@googlemail.com>
Date: Tue, 13 Oct 2026 17:37:36 +0100
Subject: [PATCH] A store to an indexed Ref<T> names the form that stores

`bytes[i]` of a `Ref<u8>` is the element's reference, as a read gives it
(`Deref(bytes[i])`), so `bytes[i] = 65` has nothing to store to and
reported "saving to constant object". It now says to store through the
reference: `Deref(bytes[i]) = value`, which works.

Found writing a std::string's characters into a tuple's bytes
(TSLANG_ISSUES.md 17).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 tslang/lib/TypeScript/MLIRGenImpl.h         |  7 +++++++
 tslang/test/tester/CMakeLists.txt           | 10 ++++++++++
 tslang/test/tester/ref-index-store/store.ts |  9 +++++++++
 3 files changed, 26 insertions(+)
 create mode 100644 tslang/test/tester/ref-index-store/store.ts

diff --git a/tslang/lib/TypeScript/MLIRGenImpl.h b/tslang/lib/TypeScript/MLIRGenImpl.h
index c4b942158..fd6f64934 100644
--- a/tslang/lib/TypeScript/MLIRGenImpl.h
+++ b/tslang/lib/TypeScript/MLIRGenImpl.h
@@ -6192,6 +6192,13 @@ class MLIRGenImpl
         else
         {
             LLVM_DEBUG(dbgs() << "\n!! left expr.: " << leftExpressionValueBeforeCast << " ...\n";);
+            // `p[i]` of a `Ref<T>` is the reference to the element (as a read gives it), not the element
+            if (leftExpressionValueBeforeCast.getDefiningOp<mlir_ts::PointerOffsetRefOp>())
+            {
+                emitError(location, "indexing a 'Ref<T>' gives a reference to the element; store through it with 'Deref(p[i]) = value'");
+                return mlir::failure();
+            }
+
             emitError(location, "saving to constant object");
             return mlir::failure();
         }
diff --git a/tslang/test/tester/CMakeLists.txt b/tslang/test/tester/CMakeLists.txt
index 891d8b56e..9f77f13a2 100644
--- a/tslang/test/tester/CMakeLists.txt
+++ b/tslang/test/tester/CMakeLists.txt
@@ -2699,6 +2699,16 @@ add_test(NAME test-compile-integer-literal-truncation-quiet
 set_tests_properties(test-compile-integer-literal-truncation-quiet
                      PROPERTIES FAIL_REGULAR_EXPRESSION "warning:|error:|Stack dump|Assertion failed")
 
+# A store to an indexed Ref<T> (`bytes[i] = x`) names the form that stores, `Deref(bytes[i]) = x`,
+# instead of "saving to constant object".
+add_test(NAME test-compile-ref-index-store
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/ref-index-store/store.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/ref-index-store.obj")
+set_tests_properties(test-compile-ref-index-store
+                     PROPERTIES PASS_REGULAR_EXPRESSION "store\\.ts:8:5: error: indexing a 'Ref<T>' gives a reference to the element; store through it with 'Deref\\(p\\[i\\]\\) = value'"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
+
 # A number, string, boolean or array passed where an interface with members is expected is an
 # error; it compiled to a cast lowering could not make, and the callee read garbage. An interface
 # without members still takes any value.
diff --git a/tslang/test/tester/ref-index-store/store.ts b/tslang/test/tester/ref-index-store/store.ts
new file mode 100644
index 000000000..0dd2fa3ae
--- /dev/null
+++ b/tslang/test/tester/ref-index-store/store.ts
@@ -0,0 +1,9 @@
+// Indexing a Ref<T> gives the element's reference, as a read does (`Deref(bytes[i])`), so a store
+// to it is an error that names the form that stores: `Deref(bytes[i]) = value`.
+type Str = [buf: [_0: u64, _1: u64], size: index];
+
+function main() {
+    let value: Str = [[0, 0], 0];
+    let bytes = <Ref<u8>><Opaque>Ref(value);
+    bytes[0] = 65;
+}