Refining table properties to generate readonly discriminants

Analysis/src/ConstraintGenerator.cpp

@@ -45,6 +45,7 @@ LUAU_FASTFLAGVARIABLE(LuauInferLocalTypesInMultipleAssignments)
 LUAU_FASTFLAGVARIABLE(LuauDoNotLeakNilInRefinement)
 LUAU_FASTFLAGVARIABLE(LuauExtraFollows)
 LUAU_FASTFLAG(LuauUserTypeFunTypecheck)
+LUAU_FASTFLAGVARIABLE(LuauRefineJustTheReadProperty)
 
 namespace Luau
 {
@@ -529,11 +530,18 @@ void ConstraintGenerator::computeRefinement(
 
             TypeId nextDiscriminantTy = arena->addType(TableType{});
             NotNull<TableType> table{getMutable<TableType>(nextDiscriminantTy)};
-            // When we fully support read-write properties (i.e. when we allow properties with
-            // completely disparate read and write types), then the following property can be
-            // set to read-only since refinements only tell us about what we read. This cannot
-            // be allowed yet though because it causes read and write types to diverge.
-            table->props[*key->propName] = Property::rw(discriminantTy);
+            if (FFlag::LuauRefineJustTheReadProperty)
+            {
+                table->props[*key->propName] = Property::readonly(discriminantTy);
+            }
+            else
+            {
+                // When we fully support read-write properties (i.e. when we allow properties with
+                // completely disparate read and write types), then the following property can be
+                // set to read-only since refinements only tell us about what we read. This cannot
+                // be allowed yet though because it causes read and write types to diverge.
+                table->props[*key->propName] = Property::rw(discriminantTy);
+            }
             table->scope = scope.get();
             table->state = TableState::Sealed;
 
@@ -547,7 +555,7 @@ void ConstraintGenerator::computeRefinement(
             refis->get(proposition->key->def)->shouldAppendNilType =
                 (sense || !eq) && containsSubscriptedDefinition(proposition->key->def) && !proposition->implicitFromCall;
         }
-        else 
+        else
         {
             refis->get(proposition->key->def)->shouldAppendNilType = (sense || !eq) && containsSubscriptedDefinition(proposition->key->def);
         }

tests/TypeInfer.refinements.test.cpp

@@ -14,6 +14,7 @@ LUAU_FASTFLAG(LuauIntersectNotNil)
 LUAU_FASTFLAG(LuauSkipNoRefineDuringRefinement)
 LUAU_FASTFLAG(LuauFunctionCallsAreNotNilable)
 LUAU_FASTFLAG(LuauDoNotLeakNilInRefinement)
+LUAU_FASTFLAG(LuauRefineJustTheReadProperty)
 
 using namespace Luau;
 
@@ -2538,7 +2539,6 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "function_calls_are_not_nillable")
             return nil
         end
     )"));
-
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "oss_1528_method_calls_are_not_nillable")
@@ -2562,4 +2562,49 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "oss_1528_method_calls_are_not_nillable")
     )"));
 }
 
+TEST_CASE_FIXTURE(Fixture, "refine_just_the_read_property")
+{
+    ScopedFastFlag sff[]{
+        {FFlag::LuauSolverV2, true},
+        {FFlag::LuauRefineJustTheReadProperty, true},
+    };
+
+    CheckResult result = check(R"(
+        type Foo = { p: boolean }
+
+        function f(x: Foo)
+            if x.p == true then return end
+
+            x.p = true
+            x.p = false
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    // The first check is correct because it reflects the state of the program by that point.
+    // The second check is not. It fails to account for transitive typestate change from the
+    // previous statement.
+    CHECK_EQ("Foo & { read p: ~true }", toString(requireTypeAtPosition({6, 12})));
+    CHECK_EQ("Foo & { read p: ~true }", toString(requireTypeAtPosition({7, 12})));
+}
+
+TEST_CASE_FIXTURE(Fixture, "refine_just_the_read_property_typestate")
+{
+    ScopedFastFlag sff[]{
+        {FFlag::LuauSolverV2, true},
+        {FFlag::LuauRefineJustTheReadProperty, true},
+    };
+
+    CheckResult result = check(R"(
+        type Foo = { p: {}? }
+
+        function f(x: Foo)
+            if not x.p then x.p = {} end
+        end
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+}
+
 TEST_SUITE_END();