A closure over a const read out of a place captures the value it read

tslang/lib/TypeScript/MLIRGenFunctions.cpp

@@ -2013,6 +2013,21 @@ namespace mlirgen
 
             // review capturing by ref.  it should match storage type
             auto refValue = mcl.GetReferenceFromValue(location, varValue);
+
+            // A copy is taken through the reference only when that reference is the variable's own
+            // storage. A `const` folded into a read (`const v = n.v`, `const v = G`) has none: the
+            // reference is where it was read from, and dereferenced now it gave the closure what the
+            // place holds by the time the closure is made - `n.v = d` in between, and `v` in the
+            // closure was `d`. The value itself is what the const names.
+            auto isOwnStorage = refValue && (refValue.getDefiningOp<mlir_ts::VariableOp>() ||
+                                             refValue.getDefiningOp<mlir_ts::ParamOp>() ||
+                                             refValue.getDefiningOp<mlir_ts::ParamOptionalOp>() ||
+                                             isCapturedCellSlot(refValue));
+            if (refValue && !capturedByRef && !isOwnStorage)
+            {
+                refValue = mlir::Value();
+            }
+
             if (refValue && !capturedByRef)
             {
                 // the box holds a copy, exactly as it does for a value with no reference at all

tslang/test/tester/CMakeLists.txt

@@ -406,6 +406,7 @@ tslang_add_test(NAME test-compile-00-const-element-read COMMAND test-runner "${P
 tslang_add_test(NAME test-compile-00-const-global-read COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_global_read.ts")
 tslang_add_test(NAME test-compile-00-global-argument-reassigned COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_reassigned.ts")
 tslang_add_test(NAME test-compile-00-global-argument-view COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_view.ts")
+tslang_add_test(NAME test-compile-00-closure-const-read COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00closure_const_read.ts")
 tslang_add_test(NAME test-compile-00-element-store-after-value COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00element_store_after_value.ts")
 tslang_add_test(NAME test-compile-00-const-interface-field-read COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_interface_field_read.ts")
 tslang_add_test(NAME test-compile-00-argument-place-overwritten COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00argument_place_overwritten.ts")
@@ -992,6 +993,7 @@ tslang_add_test(NAME test-jit-00-const-element-read COMMAND test-runner -jit "${
 tslang_add_test(NAME test-jit-00-const-global-read COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_global_read.ts")
 tslang_add_test(NAME test-jit-00-global-argument-reassigned COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_reassigned.ts")
 tslang_add_test(NAME test-jit-00-global-argument-view COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00global_argument_view.ts")
+tslang_add_test(NAME test-jit-00-closure-const-read COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00closure_const_read.ts")
 tslang_add_test(NAME test-jit-00-element-store-after-value COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00element_store_after_value.ts")
 tslang_add_test(NAME test-jit-00-const-interface-field-read COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00const_interface_field_read.ts")
 tslang_add_test(NAME test-jit-00-argument-place-overwritten COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00argument_place_overwritten.ts")
@@ -1968,6 +1970,7 @@ set(TSLANG_CORPUS
     00const_global_read.ts
     00global_argument_reassigned.ts
     00global_argument_view.ts
+    00closure_const_read.ts
     00element_store_after_value.ts
     00const_interface_field_read.ts
     00argument_place_overwritten.ts

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

@@ -0,0 +1,45 @@
+// A closure over a `const` read out of a field, an element or a global sees
+// the value the const was given, not what the place holds by the time the closure is made. Folded
+// into the read, the const had no storage of its own, and the capture was taken through the
+// place it was read from: `const v = n.v; n.v = d; const c = () => v.x;` gave `d.x` under gc and
+// none (rc gives such a const storage, and was right).
+class V {
+    constructor(public x: number) {}
+}
+
+class N {
+    v = new V(3);
+    items: V[] = [new V(4)];
+}
+
+let G = new V(5);
+
+function field() {
+    const n = new N();
+    const v = n.v;
+    n.v = new V(2);
+    const c = (): number => v.x;
+    return c();
+}
+
+function element() {
+    const n = new N();
+    const v = n.items[0];
+    n.items[0] = new V(2);
+    const c = (): number => v.x;
+    return c();
+}
+
+function global() {
+    const v = G;
+    G = new V(2);
+    const c = (): number => v.x;
+    return c();
+}
+
+function main() {
+    assert(field() == 3, "a closure over a const read out of a field");
+    assert(element() == 4, "a closure over a const read out of an element");
+    assert(global() == 5, "a closure over a const read out of a global");
+    print("done.");
+}