A length holds and prints -1: index converts from a signed integer and to text as signed

tslang/include/TypeScript/LowerToLLVM/CastLogicHelper.h

@@ -102,7 +102,8 @@ class CastLogicHelper
 
         if (inType.isIndex() && isResString)
         {
-            return castIntToString(in, tch.getIndexTypeBitwidth(), false);
+            // signed, as index compares and converts to number: `s.length - 3` is -1, not 18446744073709551615
+            return castIntToString(in, tch.getIndexTypeBitwidth(), true);
         }
 
         if (isa<mlir::IntegerType>(inType) && isResString)
@@ -150,6 +151,13 @@ class CastLogicHelper
             }
         }
 
+        // a signed integer keeps its sign as an index: `let n = s.length; n = -1` held 4294967295 under the
+        // zext in castLLVMTypesLogic
+        if (inType.isSignedInteger() && resType.isIndex() && resLLVMType.getIntOrFloatBitWidth() > inType.getIntOrFloatBitWidth())
+        {
+            return rewriter.create<LLVM::SExtOp>(loc, resLLVMType, in);
+        }
+
         // TODO: should be in LLVM cast?
         if (inType.isSignedInteger() && resType.isSignedInteger() && resType.getIntOrFloatBitWidth() > inType.getIntOrFloatBitWidth())
         {

tslang/test/tester/CMakeLists.txt

@@ -392,6 +392,7 @@ tslang_add_test(NAME test-compile-00-union-nullable-value COMMAND test-runner "$
 tslang_add_test(NAME test-compile-00-union-value-equality COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_value_equality.ts")
 tslang_add_test(NAME test-compile-00-strict-equals-mismatched-types COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00strict_equals_mismatched_types.ts")
 tslang_add_test(NAME test-compile-00-length-with-negative-merge COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00length_with_negative_merge.ts")
+tslang_add_test(NAME test-compile-00-length-signed-arithmetic COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00length_signed_arithmetic.ts")
 tslang_add_test(NAME test-compile-00-intersection-type COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00intersection_type.ts")
 tslang_add_test(NAME test-compile-00-intersection-type-generic COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00intersection_type_generic.ts")
 tslang_add_test(NAME test-compile-00-length COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00length.ts")
@@ -867,6 +868,7 @@ tslang_add_test(NAME test-jit-00-union-nullable-value COMMAND test-runner -jit "
 tslang_add_test(NAME test-jit-00-union-value-equality COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_value_equality.ts")
 tslang_add_test(NAME test-jit-00-strict-equals-mismatched-types COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00strict_equals_mismatched_types.ts")
 tslang_add_test(NAME test-jit-00-length-with-negative-merge COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00length_with_negative_merge.ts")
+tslang_add_test(NAME test-jit-00-length-signed-arithmetic COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00length_signed_arithmetic.ts")
 tslang_add_test(NAME test-jit-00-union-global-init COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_global_init.ts")
 tslang_add_test(NAME test-jit-00-intersection-type COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00intersection_type.ts")
 tslang_add_test(NAME test-jit-00-intersection-type-generic COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00intersection_type_generic.ts")

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

@@ -0,0 +1,45 @@
+// A length is an index; it holds, compares and prints as a signed value, as a JS number would:
+// a variable initialised from one keeps -1 when assigned it, and a length minus more is negative
+function assigned(s: string) {
+    let n = s.length;
+    n = -1;
+    assert(n == -1, "n == -1");
+    assert(n < 0, "n < 0");
+    const asNumber: number = n;
+    assert(asNumber == -1, "as number");
+    assert(`${n}` == "-1", "as text");
+    assert("" + n == "-1", "concatenated");
+    const boxed: any = n;
+    assert(`${boxed}` == "-1", "boxed");
+}
+
+function subtracted(s: string, a: number[]) {
+    assert(`${s.length - 3}` == "-1", "string length - 3");
+    assert(`${a.length - 5}` == "-3", "array length - 5");
+    assert(a.length - 5 < 0, "array length - 5 < 0");
+
+    let i = s.length;
+    i--; i--; i--; i--;
+    assert(`${i}` == "-2", "decremented");
+
+    let w = a.length;
+    w -= 5;
+    assert(`${w}` == "-3", "-=");
+}
+
+function countdown(s: string) {
+    let visited = 0;
+    for (let k = s.length - 1; k >= 0; k--) {
+        visited++;
+        if (visited > 10) break;
+    }
+
+    assert(visited == 2, "countdown");
+}
+
+function main() {
+    assigned("ab");
+    subtracted("ab", [1, 2]);
+    countdown("ab");
+    print("done.");
+}