stringify false? and ~(false?) to falsy and truthy respectively

.gitignore

@@ -18,3 +18,6 @@
 __pycache__
 .cache
 .clangd
+compile_commands.json
+flake.nix
+flake.lock

Analysis/include/Luau/ToString.h

@@ -47,6 +47,7 @@ struct ToStringOptions
     bool hideFunctionSelfArgument = false;        // If true, `self: X` will be omitted from the function signature if the function has self
     bool hideTableAliasExpansions = false;        // If true, all table aliases will not be expanded
     bool useQuestionMarks = true;                 // If true, use a postfix ? for options, else write them out as unions that include nil.
+    bool useTruthyFalsy = true;                   // If true, `~(false?)` and `false?` will emit `truthy` and `falsy` respectively.
     bool ignoreSyntheticName = false;             // If true, ignore synthetic names on table types.
     size_t maxTableLength = size_t(FInt::LuauTableTypeMaximumStringifierLength); // Only applied to TableTypes
     size_t maxTypeLength = size_t(FInt::LuauTypeMaximumStringifierLength);

Analysis/src/ToString.cpp

@@ -12,6 +12,7 @@
 #include "Luau/TypePack.h"
 #include "Luau/Type.h"
 #include "Luau/TypeFunction.h"
+#include "Luau/TypeUtils.h"
 #include "Luau/VisitType.h"
 #include "Luau/TypeOrPack.h"
 
@@ -39,6 +40,7 @@ LUAU_FASTFLAG(LuauIntegerType2)
  */
 LUAU_FASTINTVARIABLE(DebugLuauVerboseTypeNames, 0)
 LUAU_FASTFLAGVARIABLE(DebugLuauToStringNoLexicalSort)
+LUAU_FASTFLAGVARIABLE(LuauToStringTruthyFalsy)
 
 namespace Luau
 {
@@ -884,8 +886,14 @@ struct TypeStringifier
         state.emit("*no-refine*");
     }
 
-    void operator()(TypeId, const UnionType& uv)
+    void operator()(TypeId ty, const UnionType& uv)
     {
+        if (FFlag::LuauToStringTruthyFalsy && state.opts.useTruthyFalsy && isApproximatelyFalsyType(ty))
+        {
+            state.emit("falsy");
+            return;
+        }
+
         if (state.hasSeen(&uv))
         {
             state.result.cycle = true;
@@ -1016,7 +1024,10 @@ struct TypeStringifier
             std::string saved = std::move(state.result.name);
             size_t savedSpansSize = state.result.typeSpans.size();
 
-            bool needParens = !state.cycleNames.contains(el) && (get<UnionType>(el) != nullptr || get<FunctionType>(el) != nullptr);
+            // soooo 'falsy' is technically a UnionType :(
+            bool isNonFalsyUnion =
+                get<UnionType>(el) != nullptr && (FFlag::LuauToStringTruthyFalsy && state.opts.useTruthyFalsy ? !isApproximatelyFalsyType(el) : true);
+            bool needParens = !state.cycleNames.contains(el) && (isNonFalsyUnion || get<FunctionType>(el) != nullptr);
 
             if (needParens)
                 state.emit("(");
@@ -1115,8 +1126,14 @@ struct TypeStringifier
         state.emit("never");
     }
 
-    void operator()(TypeId, const NegationType& ntv)
+    void operator()(TypeId ty, const NegationType& ntv)
     {
+        if (FFlag::LuauToStringTruthyFalsy && state.opts.useTruthyFalsy && isApproximatelyTruthyType(ty))
+        {
+            state.emit("truthy");
+            return;
+        }
+
         state.emit("~");
 
         // The precedence of `~` should be less than `|` and `&`.

tests/Normalize.test.cpp

@@ -14,6 +14,7 @@
 LUAU_FASTINT(LuauTypeInferRecursionLimit)
 LUAU_FASTFLAG(LuauIntegerType2)
 LUAU_FASTFLAG(DebugLuauForceOldSolver)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 using namespace Luau;
 
@@ -1111,8 +1112,9 @@ TEST_CASE_FIXTURE(NormalizeFixture, "truthy_table_property_and_optional_table_wi
 
 TEST_CASE_FIXTURE(NormalizeFixture, "free_type_and_not_truthy")
 {
-    ScopedFastFlag sff[] = {
+    ScopedFastFlag sffs[] = {
         {FFlag::DebugLuauForceOldSolver, false}, // Only because it affects the stringification of free types
+        {FFlag::LuauToStringTruthyFalsy, true},
     };
 
     TypeId freeTy = arena.freshType(getBuiltins(), getGlobalScope());
@@ -1125,7 +1127,7 @@ TEST_CASE_FIXTURE(NormalizeFixture, "free_type_and_not_truthy")
 
     TypeId result = typeFromNormal(*norm);
 
-    CHECK("'a & (false?)" == toString(result));
+    CHECK("'a & falsy" == toString(result));
 }
 
 TEST_CASE_FIXTURE(NormalizeFixture, "free_type_intersection_ordering")

tests/Simplify.test.cpp

@@ -9,6 +9,7 @@
 using namespace Luau;
 
 LUAU_FASTFLAG(DebugLuauForceOldSolver)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 LUAU_DYNAMIC_FASTINT(LuauSimplificationComplexityLimit)
 
 namespace
@@ -482,10 +483,11 @@ TEST_CASE_FIXTURE(SimplifyFixture, "union")
 
 TEST_CASE_FIXTURE(SimplifyFixture, "two_unions")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
     ScopedFastInt sfi{DFInt::LuauSimplificationComplexityLimit, 10};
     TypeId t1 = arena->addType(UnionType{{numberTy, booleanTy, stringTy, nilTy, tableTy}});
 
-    CHECK("false?" == intersectStr(t1, falsyTy));
+    CHECK("falsy" == intersectStr(t1, falsyTy));
 }
 
 TEST_CASE_FIXTURE(SimplifyFixture, "curious_union")

tests/ToString.test.cpp

@@ -13,6 +13,7 @@
 using namespace Luau;
 
 LUAU_FASTFLAG(DebugLuauForceOldSolver)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 TEST_SUITE_BEGIN("ToString");
 
@@ -1062,4 +1063,69 @@ TEST_CASE_FIXTURE(Fixture, "record_type_compositions_generic")
     CHECK_EQ(recordedTyObject, requireTypeAlias("Object"));
 }
 
+TEST_CASE_FIXTURE(BuiltinsFixture, "tostring_truthy_falsy")
+{
+    ScopedFastFlag sffs[] = {
+        {FFlag::DebugLuauForceOldSolver, false},
+        {FFlag::LuauToStringTruthyFalsy, true},
+    };
+
+    CheckResult result = check(R"(
+        type function negate(t: type)
+            return types.negationof(t)
+        end
+        type meow = false?
+        type mrrp = negate<false?>
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ(toString(requireTypeAlias("meow")), "falsy");
+    CHECK_EQ(toString(requireTypeAlias("mrrp")), "truthy");
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "tostring_truthy_falsy_expanded")
+{
+    ScopedFastFlag sffs[] = {
+        {FFlag::DebugLuauForceOldSolver, false},
+        {FFlag::LuauToStringTruthyFalsy, true},
+    };
+
+    CheckResult result = check(R"(
+        type function negate(t: type)
+            return types.negationof(t)
+        end
+        type kya = (false | nil) | false
+        type purr = negate<(false | nil) | false>
+    )");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ(toString(requireTypeAlias("kya")), "falsy");
+    CHECK_EQ(toString(requireTypeAlias("purr")), "truthy");
+}
+
+TEST_CASE_FIXTURE(BuiltinsFixture, "tostring_truthy_falsy_no_parenthesis")
+{
+    ScopedFastFlag sffs[] = {
+        {FFlag::DebugLuauForceOldSolver, false},
+        {FFlag::LuauToStringTruthyFalsy, true},
+    };
+
+    ToStringOptions opts;
+
+    CheckResult result = check(R"(
+		type function negate(t: type)
+		    return types.negationof(t)
+		end
+		type hiss = unknown & (false?)
+		type scratch = unknown & negate<false?>
+	)");
+
+    LUAU_REQUIRE_NO_ERRORS(result);
+
+    CHECK_EQ(toString(requireTypeAlias("hiss")), "falsy & unknown");
+    CHECK_EQ(toString(requireTypeAlias("scratch")), "truthy & unknown");
+}
+
 TEST_SUITE_END();

tests/TypeInfer.functions.test.cpp

@@ -25,6 +25,7 @@ LUAU_FASTFLAG(LuauBidirectionalInferenceVariadics)
 LUAU_FASTFLAG(LuauConstraintGraph)
 LUAU_FASTFLAG(LuauBidirectionalInferenceBetterLambdaHandling)
 LUAU_FASTFLAG(LuauHigherOrderGenericInference)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 TEST_SUITE_BEGIN("TypeInferFunctions");
 
@@ -2888,6 +2889,7 @@ TEST_CASE_FIXTURE(Fixture, "unifier_should_not_bind_free_types")
     ScopedFastFlag sffs[] = {
         {FFlag::LuauRemovePrimitiveTypeConstraintAndSubtypingUnifier, true},
         {FFlag::LuauConstraintGraph, true},
+        {FFlag::LuauToStringTruthyFalsy, true},
     };
 
     CheckResult result = check(R"(
@@ -2922,7 +2924,7 @@ TEST_CASE_FIXTURE(Fixture, "unifier_should_not_bind_free_types")
         auto tm2 = get<TypeMismatch>(result.errors[1]);
         REQUIRE(tm2);
         CHECK(toString(tm2->wantedType) == "string");
-        CHECK(toString(tm2->givenType) == "unknown & ~(false?)");
+        CHECK(toString(tm2->givenType) == "truthy & unknown");
     }
 }
 

tests/TypeInfer.intersectionTypes.test.cpp

@@ -13,6 +13,7 @@ LUAU_FASTFLAG(LuauCheckFunctionStatementTypes)
 LUAU_FASTFLAG(DebugLuauForceOldSolver)
 LUAU_FASTFLAG(LuauPropagateFreeTypesIntoUnionAndIntersectionBounds)
 LUAU_FASTFLAG(LuauDropUnionSubtypeReasoning)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 TEST_SUITE_BEGIN("IntersectionTypes");
 
@@ -1498,7 +1499,10 @@ TEST_CASE_FIXTURE(Fixture, "cli_80596_simplify_more_realistic_intersections")
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "narrow_intersection_nevers")
 {
-    ScopedFastFlag sffs{FFlag::DebugLuauForceOldSolver, false};
+    ScopedFastFlag sffs[] = {
+        {FFlag::DebugLuauForceOldSolver, false},
+        {FFlag::LuauToStringTruthyFalsy, true},
+    };
 
     loadDefinition(R"(
         declare extern type Player with
@@ -1513,7 +1517,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "narrow_intersection_nevers")
         end
     )"));
 
-    CHECK_EQ("Player & { read Character: ~(false?) }", toString(requireTypeAtPosition({3, 23})));
+    CHECK_EQ("Player & { read Character: truthy }", toString(requireTypeAtPosition({3, 23})));
 }
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "bounds_propagate_into_free_intersection_bounds")

tests/TypeInfer.operators.test.cpp

@@ -20,11 +20,14 @@ using namespace Luau;
 LUAU_FASTFLAG(DebugLuauForceOldSolver)
 LUAU_FASTFLAG(LuauSolverAgnosticStringification)
 LUAU_FASTFLAG(LuauConcatDoesntAlwaysReturnString)
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 TEST_SUITE_BEGIN("TypeInferOperators");
 
 TEST_CASE_FIXTURE(Fixture, "or_joins_types")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
+
     CheckResult result = check(R"(
         local s = "a" or 10
         local x:string|number = s
@@ -34,7 +37,7 @@ TEST_CASE_FIXTURE(Fixture, "or_joins_types")
     if (!FFlag::DebugLuauForceOldSolver)
     {
         // FIXME: Regression
-        CHECK("(string & ~(false?)) | number" == toString(*requireType("s")));
+        CHECK("(string & truthy) | number" == toString(*requireType("s")));
         CHECK("number | string" == toString(*requireType("x")));
     }
     else
@@ -46,6 +49,8 @@ TEST_CASE_FIXTURE(Fixture, "or_joins_types")
 
 TEST_CASE_FIXTURE(Fixture, "or_joins_types_with_no_extras")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
+
     CheckResult result = check(R"(
         local s = "a" or 10
         local x:number|string = s
@@ -56,7 +61,7 @@ TEST_CASE_FIXTURE(Fixture, "or_joins_types_with_no_extras")
     if (!FFlag::DebugLuauForceOldSolver)
     {
         // FIXME: Regression.
-        CHECK("(string & ~(false?)) | number" == toString(*requireType("s")));
+        CHECK("(string & truthy) | number" == toString(*requireType("s")));
         CHECK("number | string" == toString(*requireType("y")));
     }
     else
@@ -68,6 +73,8 @@ TEST_CASE_FIXTURE(Fixture, "or_joins_types_with_no_extras")
 
 TEST_CASE_FIXTURE(Fixture, "or_joins_types_with_no_superfluous_union")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
+
     CheckResult result = check(R"(
         local s = "a" or "b"
         local x:string = s
@@ -77,7 +84,7 @@ TEST_CASE_FIXTURE(Fixture, "or_joins_types_with_no_superfluous_union")
     if (!FFlag::DebugLuauForceOldSolver)
     {
         // FIXME: Regression
-        CHECK("(string & ~(false?)) | string" == toString(requireType("s")));
+        CHECK("(string & truthy) | string" == toString(requireType("s")));
     }
     else
         CHECK("string" == toString(requireType("s")));

tests/TypeInfer.refinements.test.cpp

@@ -12,6 +12,7 @@ LUAU_FASTFLAG(DebugLuauForceOldSolver)
 LUAU_FASTFLAG(DebugLuauAssertOnForcedConstraint)
 LUAU_FASTFLAG(LuauRemovePrimitiveTypeConstraintAndSubtypingUnifier)
 LUAU_FASTFLAG(LuauIndexingIntoErrorGivesError);
+LUAU_FASTFLAG(LuauToStringTruthyFalsy)
 
 using namespace Luau;
 
@@ -1035,6 +1036,8 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "either_number_or_string")
 
 TEST_CASE_FIXTURE(Fixture, "not_t_or_some_prop_of_t")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
+
     CheckResult result = check(R"(
         local function f(t: {x: boolean}?)
             if not t or t.x then
@@ -1054,7 +1057,7 @@ TEST_CASE_FIXTURE(Fixture, "not_t_or_some_prop_of_t")
         // ... which we can't _quite_ refine into the type it ought to be:
         //
         //  { write x: boolean, read x: true } | nil
-        CHECK_EQ("({ read x: ~(false?) } & { x: boolean })?", toString(requireTypeAtPosition({3, 28})));
+        CHECK_EQ("({ read x: truthy } & { x: boolean })?", toString(requireTypeAtPosition({3, 28})));
     }
     else
         CHECK_EQ("{ x: boolean }?", toString(requireTypeAtPosition({3, 28})));
@@ -1663,6 +1666,7 @@ TEST_CASE_FIXTURE(RefinementExternTypeFixture, "isa_type_refinement_must_be_know
 
 TEST_CASE_FIXTURE(RefinementExternTypeFixture, "asserting_optional_properties_should_not_refine_extern_types_to_never")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
 
     CheckResult result = check(R"(
         local weld: WeldConstraint = nil :: any
@@ -1676,7 +1680,7 @@ TEST_CASE_FIXTURE(RefinementExternTypeFixture, "asserting_optional_properties_sh
     LUAU_REQUIRE_NO_ERRORS(result);
 
     if (!FFlag::DebugLuauForceOldSolver)
-        CHECK_EQ("WeldConstraint & { read Part1: ~(false?) }", toString(requireTypeAtPosition({3, 15})));
+        CHECK_EQ("WeldConstraint & { read Part1: truthy }", toString(requireTypeAtPosition({3, 15})));
     else
         CHECK_EQ("WeldConstraint", toString(requireTypeAtPosition({3, 15})));
     CHECK_EQ("Vector3", toString(requireTypeAtPosition({6, 29})));
@@ -2566,6 +2570,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "nonnil_refinement_on_generic")
 
 TEST_CASE_FIXTURE(BuiltinsFixture, "truthy_refinement_on_generic")
 {
+    ScopedFastFlag sff{FFlag::LuauToStringTruthyFalsy, true};
     CheckResult result = check(R"(
         local function printOptional<T>(item: T?, printer: (T) -> string): string
             if item then
@@ -2578,7 +2583,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "truthy_refinement_on_generic")
 
     LUAU_REQUIRE_NO_ERRORS(result);
     if (!FFlag::DebugLuauForceOldSolver)
-        CHECK_EQ("T & ~(false?)", toString(requireTypeAtPosition({3, 31})));
+        CHECK_EQ("T & truthy", toString(requireTypeAtPosition({3, 31})));
     else
         CHECK_EQ("T", toString(requireTypeAtPosition({3, 31})));
 }
@@ -2883,6 +2888,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "refinements_from_and_should_not_refine_to_ne
 {
     ScopedFastFlag sffs[] = {
         {FFlag::DebugLuauForceOldSolver, false},
+        {FFlag::LuauToStringTruthyFalsy, true},
     };
 
     loadDefinition(R"(
@@ -2906,7 +2912,7 @@ TEST_CASE_FIXTURE(BuiltinsFixture, "refinements_from_and_should_not_refine_to_ne
 
     LUAU_REQUIRE_NO_ERRORS(results);
 
-    CHECK_EQ("(Config & { read KeyboardEnabled: false? }) | (Config & { read MouseEnabled: false? })", toString(requireTypeAtPosition({6, 24})));
+    CHECK_EQ("(Config & { read KeyboardEnabled: falsy }) | (Config & { read MouseEnabled: falsy })", toString(requireTypeAtPosition({6, 24})));
 }
 
 TEST_CASE_FIXTURE(Fixture, "force_simplify_constraint_doesnt_drop_blocked_type")