From 70b28c7d200489f91b6b350325846f1c56fca5db Mon Sep 17 00:00:00 2001
From: Iwo Plaza <iwoplaza@gmail.com>
Date: Fri, 24 Apr 2026 14:47:29 +0200
Subject: [PATCH] fix: ArrayExpression - immediate index access

---
 packages/typegpu/src/tgsl/accessIndex.ts |  6 +-
 packages/typegpu/tests/array.test.ts     | 97 +++++++++++++++++++++++-
 2 files changed, 101 insertions(+), 2 deletions(-)

diff --git a/packages/typegpu/src/tgsl/accessIndex.ts b/packages/typegpu/src/tgsl/accessIndex.ts
index 811c136d0e..129e5fe56b 100644
--- a/packages/typegpu/src/tgsl/accessIndex.ts
+++ b/packages/typegpu/src/tgsl/accessIndex.ts
@@ -13,7 +13,7 @@ import {
 } from '../data/wgslTypes.ts';
 import { isKnownAtComptime } from '../types.ts';
 import { accessProp } from './accessProp.ts';
-import { coerceToSnippet } from './generationHelpers.ts';
+import { ArrayExpression, coerceToSnippet } from './generationHelpers.ts';
 
 const indexableTypeToResult = {
   mat2x2f: vec2f,
@@ -54,6 +54,10 @@ export function accessIndex(target: Snippet, indexArg: Snippet | number): Snippe
       origin = 'runtime';
     }
 
+    if (target.value instanceof ArrayExpression && isKnownAtComptime(index)) {
+      return target.value.elements[index.value as number];
+    }
+
     return snip(
       isKnownAtComptime(target) && isKnownAtComptime(index)
         ? // oxlint-disable-next-line typescript/no-explicit-any -- it's fine, it's there
diff --git a/packages/typegpu/tests/array.test.ts b/packages/typegpu/tests/array.test.ts
index 0eb30bbd72..c4de72d5fc 100644
--- a/packages/typegpu/tests/array.test.ts
+++ b/packages/typegpu/tests/array.test.ts
@@ -1,12 +1,13 @@
 import { attest } from '@ark/attest';
 import { BufferReader, BufferWriter } from 'typed-binary';
-import { describe, expect, expectTypeOf, it } from 'vitest';
+import { describe, expect, expectTypeOf } from 'vitest';
 import { readData, writeData } from '../src/data/dataIO.ts';
 import { d, tgpu } from '../src/index.js';
 import { namespace } from '../src/core/resolve/namespace.ts';
 import { resolve } from '../src/resolutionCtx.ts';
 import type { Infer } from '../src/shared/repr.ts';
 import { arrayLength } from '../src/std/array.ts';
+import { it } from 'typegpu-testing-utility';
 
 describe('array', () => {
   it('produces a visually pleasant type', () => {
@@ -443,6 +444,100 @@ describe('array', () => {
       }"
     `);
   });
+
+  it('array expressions can be indexed into with a comptime-known index', () => {
+    function foo() {
+      'use gpu';
+      const i = 2;
+      const a = [i, 2][0];
+      const b = [i, 2][1];
+    }
+
+    expect(tgpu.resolve([foo])).toMatchInlineSnapshot(`
+      "fn foo() {
+        const i = 2;
+        const a = i;
+        const b = 2i;
+      }"
+    `);
+  });
+
+  it('array expressions can be indexed into with a runtime-known index', () => {
+    function foo() {
+      'use gpu';
+      const i = 0;
+      const a = [1, 2][i];
+    }
+
+    expect(tgpu.resolve([foo])).toMatchInlineSnapshot(`
+      "fn foo() {
+        const i = 0;
+        let a = array<i32, 2>(1, 2)[i];
+      }"
+    `);
+  });
+
+  it('allows picking among references using comptime-known indices', () => {
+    function foo() {
+      'use gpu';
+      const x = d.vec3f(1, 2, 3);
+      // `const y = [x, d.vec3f()]` would throw, but since
+      // we're never constructing the array, it's equivalent
+      // to writing `const y = x;`
+      const y = [x, d.vec3f()][0];
+      return y;
+    }
+
+    expect(tgpu.resolve([foo])).toMatchInlineSnapshot(`
+      "fn foo() -> vec3f {
+        var x = vec3f(1, 2, 3);
+        let y = (&x);
+        return (*y);
+      }"
+    `);
+  });
+
+  it('resolves array expression elements when accessed with comptime-known index', () => {
+    let n = 0;
+    const next = tgpu.comptime(() => n++);
+
+    function foo() {
+      'use gpu';
+      const a = [next(), next()][0];
+      const b = [next(), next()][1];
+    }
+
+    expect(tgpu.resolve([foo])).toMatchInlineSnapshot(`
+      "fn foo() {
+        const a = 0;
+        const b = 3;
+      }"
+    `);
+  });
+
+  it('prunes definitions in array expressions accessed with comptime-known index', ({ root }) => {
+    const u1 = root.createUniform(d.u32, 1);
+    const u2 = root.createUniform(d.u32, 2);
+    const u3 = root.createUniform(d.u32, 3);
+    const u4 = root.createUniform(d.u32, 4);
+
+    function foo() {
+      'use gpu';
+      const a = [u1.$, u2.$][0];
+      const b = [u3.$, u4.$][1];
+    }
+
+    expect(tgpu.resolve([foo])).toMatchInlineSnapshot(`
+      "@group(0) @binding(0) var<uniform> u1: u32;
+
+      @group(0) @binding(1) var<uniform> u4: u32;
+
+      fn foo() {
+        let a = u1;
+        let b = u4;
+      }"
+    `);
+  });
 });
 
 describe('array.length', () => {