rc: a const interface or bound method made over an element read holds a count

tslang/lib/TypeScript/MLIRGenImpl.h

@@ -1888,8 +1888,8 @@ class MLIRGenImpl
             //
             // Under rc, so does a const read out of a field (`const c = h.c`) or an element (`const c =
             // a[0]`, `const [c] = a`): folded, `h.c = d` or `a[0] = d` freed what `c` still read. So
-            // does an interface or a bound method made over a field read (`const i: I = h.v`,
-            // `const m = h.v.get`), which holds the block as `c` would. Under own the folded read is a
+            // does an interface or a bound method made over such a read (`const i: I = h.v`,
+            // `const m = a[0].get`), which holds the block as `c` would. Under own the folded read is a
             // borrow, and a use after the overwrite is an error there.
             //
             // And a const record whose fields own blocks (`const o = { s: p.child }`): the literal
@@ -1910,7 +1910,8 @@ class MLIRGenImpl
                 || ((isa<mlir_ts::ArrayType>(type) || MLIRTypeHelper::isSharedHandleType(type)
                      || ((readsLocalSlot(initial) || viewsLocalSlot(initial)) && mth.ownsHeapMemory(location, type))
                      || (compileOptions.isRefCounted()
-                         && (readsField(initial) || readsElement(initial) || readsField(viewedValue(initial)))
+                         && (readsField(initial) || readsElement(initial) || readsField(viewedValue(initial))
+                             || readsElement(viewedValue(initial)))
                          && mth.ownsHeapMemory(location, type))
                      || (readsLiteralSlot(mth, location, initial) && isa<mlir_ts::TupleType>(type)
                          && mth.ownsHeapMemory(location, type))

tslang/test/tester/CMakeLists.txt

@@ -392,6 +392,7 @@ tslang_add_test(NAME test-compile-00-shared-global-field-argument COMMAND test-r
 tslang_add_test(NAME test-compile-00-narrowed-assign-other-member COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00narrowed_assign_other_member.ts")
 tslang_add_test(NAME test-compile-00-bound-method-count COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00bound_method_count.ts")
 tslang_add_test(NAME test-compile-00-const-view-of-field COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_field.ts")
+tslang_add_test(NAME test-compile-00-const-view-of-element COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_element.ts")
 tslang_add_test(NAME test-compile-00-const-record-owned-fields COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_record_owned_fields.ts")
 tslang_add_test(NAME test-compile-00-generator-temporaries COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_temporaries.ts")
 tslang_add_test(NAME test-compile-00-array-length-valid COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_valid.ts")
@@ -967,6 +968,7 @@ tslang_add_test(NAME test-jit-00-shared-global-field-argument COMMAND test-runne
 tslang_add_test(NAME test-jit-00-narrowed-assign-other-member COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00narrowed_assign_other_member.ts")
 tslang_add_test(NAME test-jit-00-bound-method-count COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00bound_method_count.ts")
 tslang_add_test(NAME test-jit-00-const-view-of-field COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_field.ts")
+tslang_add_test(NAME test-jit-00-const-view-of-element COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_view_of_element.ts")
 tslang_add_test(NAME test-jit-00-const-record-owned-fields COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_record_owned_fields.ts")
 tslang_add_test(NAME test-jit-00-generator-temporaries COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00generator_temporaries.ts")
 tslang_add_test(NAME test-jit-00-array-length-valid COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_length_valid.ts")
@@ -1932,6 +1934,7 @@ set(TSLANG_CORPUS
     00narrowed_assign_other_member.ts
     00bound_method_count.ts
     00const_view_of_field.ts
+    00const_view_of_element.ts
     00const_record_owned_fields.ts
     00generator_temporaries.ts
     00array_length_valid.ts

tslang/test/tester/tests/00const_view_of_element.ts

@@ -0,0 +1,57 @@
+// A `const` interface or bound method made over an element read keeps the object alive when the
+// element is given a new value, as a `let` does. Folded, the const was the view itself and held no
+// reference of its own: under rc `arr[0] = mkv(..)` freed the object `m()` and `i.get()` then ran
+// on. Each call follows allocations of the same size, so a freed object is reused.
+interface I {
+    get(): number;
+}
+
+class W {
+    constructor(public x: number) {}
+}
+
+class V {
+    items: number[] = [1, 2, 3];
+    constructor(public w: W) {}
+
+    get() {
+        return this.w.x;
+    }
+}
+
+function mkv(n: number) {
+    return new V(new W(n));
+}
+
+function churn(): number {
+    let s: number = 0;
+    const a: V[] = [];
+    for (let i = 0; i < 64; i++) a.push(mkv(1000 + i));
+    for (const e of a) s += e.w.x;
+    return s;
+}
+
+function boundMethod(n: number) {
+    const arr: V[] = [mkv(n), mkv(0)];
+    const m = arr[0].get;
+    arr[0] = mkv(n + 1);
+    churn();
+    return m() == n && arr[0].w.x == n + 1;
+}
+
+function interfaceView(n: number) {
+    const arr: V[] = [mkv(0), mkv(n)];
+    const i: I = arr[1];
+    arr[1] = mkv(n + 1);
+    churn();
+    return i.get() == n && arr[1].w.x == n + 1;
+}
+
+function main() {
+    for (let n = 0; n < 20; n++) {
+        assert(boundMethod(n), "a const bound method over an element");
+        assert(interfaceView(n), "a const interface over an element");
+    }
+
+    print("done.");
+}