in with a number, a boolean or a bigint on the right is an error

tslang/lib/TypeScript/MLIRGenImpl.h

@@ -5303,6 +5303,17 @@ class MLIRGenImpl
         EXIT_IF_FAILED_OR_NO_VALUE(resultRight)
         auto rightExpressionValue = V(resultRight);
 
+        // a number, a boolean or a bigint has no fields to look up (a string's `length` is an
+        // extension: tests/00in_method_names.ts)
+        auto rightBaseType = mth.getBaseType(rightExpressionValue.getType());
+        if (mth.isNumericType(rightBaseType) || isa<mlir_ts::BooleanType>(rightBaseType)
+            || isa<mlir_ts::BigIntType>(rightBaseType))
+        {
+            emitError(loc(binaryExpressionAST->right), "The right-hand side of an 'in' expression must not be a primitive: '")
+                << to_print(rightExpressionValue.getType()) << "'";
+            return mlir::failure();
+        }
+
         if (isa<mlir_ts::UnionType>(rightExpressionValue.getType()))
         {
             emitError(loc(binaryExpressionAST->right), "not supported");

tslang/test/tester/CMakeLists.txt

@@ -2729,6 +2729,16 @@ set_tests_properties(test-compile-ref-index-store
                      PROPERTIES PASS_REGULAR_EXPRESSION "store\\.ts:8:5: error: indexing a 'Ref<T>' gives a reference to the element; store through it with 'Deref\\(p\\[i\\]\\) = value'"
                                 FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed")
 
+# `in` with a number, a boolean or a bigint on the right is an error: it was quietly false. A
+# string on the right stays an extension (its `length`).
+add_test(NAME test-compile-diagnostic-location-in-primitive
+         COMMAND $<TARGET_FILE:tslang> --emit=obj --no-default-lib -mm=none
+                 "${PROJECT_SOURCE_DIR}/test/tester/diagnostic-location/in_primitive.ts"
+                 -o "${CMAKE_CURRENT_BINARY_DIR}/diagnostic-location-in-primitive.obj")
+set_tests_properties(test-compile-diagnostic-location-in-primitive
+                     PROPERTIES PASS_REGULAR_EXPRESSION "in_primitive\\.ts:7:18: error: The right-hand side of an 'in' expression must not be a primitive: '5'"
+                                FAIL_REGULAR_EXPRESSION "Stack dump|Assertion failed|in_primitive\\.ts:6:")
+
 # A number, string, boolean or array passed where an interface with members is expected is an
 # error; it compiled to a cast lowering could not make, and the callee read garbage. An interface
 # without members still takes any value.

tslang/test/tester/diagnostic-location/in_primitive.ts

@@ -0,0 +1,8 @@
+// The right-hand side of `in` that is a number had no fields to look in, and the test was
+// quietly false.
+
+function main() {
+    const s = "hello";
+    print("length" in s);
+    print("a" in 5);
+}