Fix l-value computation for subscript call. (#5177)

1 files changed, 25 insertions, 0 deletions

diff --git a/tests/bugs/gh-4971.slang b/tests/bugs/gh-4971.slang
new file mode 100644
index 000000000..948e43385
--- /dev/null
+++ b/tests/bugs/gh-4971.slang
@@ -0,0 +1,25 @@
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK):-vk -output-using-type
+//TEST(compute):COMPARE_COMPUTE(filecheck-buffer=CHECK):-d3d11 -output-using-type
+
+struct Test {
+    RWStructuredBuffer<int> val;
+    __subscript(int x, int y)->int
+    {
+        get { return val[x * 3 + y]; }
+        [nonmutating] set { val[x * 3 + y] = newValue; }
+    }
+}
+Test test;
+
+//TEST_INPUT: set test = {out ubuffer(data=[0 0 0 0 0 0 0 0 0], stride=4)};
+
+[numthreads(1, 1, 1)]
+void computeMain()
+{
+    // test[0,0] should be a valid l-value here because although `test` is
+    // a read-only parameter, the `set` accessor is marked as `nonmutating`,
+    // which means that it can be called even when `test` is not mutable.
+
+    // CHECK: 1
+    test[0,0] = 1;
+}
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