A union of null and several types can be tested for truth (#540)

`if (x)` with `x: boolean | string | null` failed to compile ("can't
instantiate function. '___cast' not all generic types can be
identified"), and so did `if (enabled && handle)` for a boolean and a
`Handle | null`, whose `&&` makes such a union. The generated union cast
answered `return t` for the null member, which strict null mode refuses
to cast to boolean. Cast to boolean, a null or undefined member is now
false, as castFromAny already does for an `any`.

A null `Handle` carries the class's tag in that union, and the class
member's `instanceof` test read its vtable through the null pointer; for a
truth test of a union with one class, the class member is now just tested
for null.

Reported in docs/bugs/native-ts-graphics-findings.md (issue 6).

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: ASDAlexander77

tslang/lib/TypeScript/MLIRGenCast.cpp

@@ -2272,6 +2272,13 @@ namespace mlirgen
                 }
                 else
                 {
+                    // Tested for truth, a null or undefined member is false, as for an `any`
+                    // (castFromAny). `return t` there cast null to boolean - "can't cast from 'null'
+                    // to 'boolean'" - so a union of null and two or more other types, which is what
+                    // `enabled && handle` makes of a boolean and a `Handle | null`, could not be
+                    // tested in an `if` at all.
+                    auto toBoolean = isa<mlir_ts::BooleanType>(type) || isa<mlir_ts::TypePredicateType>(type);
+
                     // default
                     auto next = false;
                     for (auto& pair : typeOfs)
@@ -2281,7 +2288,19 @@ namespace mlirgen
                         ss << S("if (typeof t == '");
                         ss << stows(pair.getKey().str());
                         ss << S("') ");
-                        if (pair.getKey() == "class")
+                        if (toBoolean && (pair.getKey() == "null" || pair.getKey() == "undefined"))
+                        {
+                            ss << S("return false;\n");
+                        }
+                        else if (toBoolean && pair.getKey() == "class" && classInstances.size() == 1)
+                        {
+                            // tested for truth, an instance is true and null is false - and a null
+                            // instance does carry the class's tag, when a `Class | null` became this
+                            // union (`enabled && handle` with handle null). `instanceof` would read
+                            // its vtable through the null pointer.
+                            ss << S("return t;\n");
+                        }
+                        else if (pair.getKey() == "class")
                         {
                             ss << S("{ \n");
 

tslang/test/tester/CMakeLists.txt

@@ -407,6 +407,7 @@ tslang_add_test(NAME test-compile-00-union-bin-ops-2 COMMAND test-runner "${PROJ
 tslang_add_test(NAME test-compile-00-union-to-any COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_to_any.ts")
 tslang_add_test(NAME test-compile-00-union-errors COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_errors.ts")
 tslang_add_test(NAME test-compile-00-union-null-to-string COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_to_string.ts")
+tslang_add_test(NAME test-compile-00-union-null-truthiness COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_truthiness.ts")
 tslang_add_test(NAME test-compile-00-union-null-undefined COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_undefined.ts")
 tslang_add_test(NAME test-compile-00-union-null-undefined-nonstrict COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_undefined_nonstrict.ts")
 tslang_add_test(NAME test-compile-00-optional-number-compare COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00optional_number_compare.ts")
@@ -931,6 +932,7 @@ tslang_add_test(NAME test-jit-00-union-bin-ops-2 COMMAND test-runner -jit "${PRO
 tslang_add_test(NAME test-jit-00-union-to-any COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_to_any.ts")
 tslang_add_test(NAME test-jit-00-union-errors COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_errors.ts")
 tslang_add_test(NAME test-jit-00-union-null-to-string COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_to_string.ts")
+tslang_add_test(NAME test-jit-00-union-null-truthiness COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_truthiness.ts")
 tslang_add_test(NAME test-jit-00-union-null-undefined COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_undefined.ts")
 tslang_add_test(NAME test-jit-00-union-null-undefined-nonstrict COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00union_null_undefined_nonstrict.ts")
 tslang_add_test(NAME test-jit-00-optional-number-compare COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00optional_number_compare.ts")
@@ -2044,6 +2046,7 @@ set(TSLANG_CORPUS
     00union_bin_ops2.ts
     00union_errors.ts
     00union_null_to_string.ts
+    00union_null_truthiness.ts
     00union_ops.ts
     00union_to_any.ts
     00union_type.ts

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

@@ -0,0 +1,69 @@
+// Testing a union of null and two or more other types for truth failed to compile: "can't
+// instantiate function. '___cast' not all generic types can be identified", after "can't cast
+// from 'null' to 'boolean'" reported at an unrelated place. `boolean && (Handle | null)` is such
+// a union, so `if (enabled && handle)` failed the same way.
+
+class Handle {
+    value: number;
+    constructor() {
+        this.value = 7;
+    }
+}
+
+function use(enabled: boolean, handle: Handle | null): number {
+    if (enabled && handle) {
+        return handle.value;
+    }
+
+    return 0;
+}
+
+function useReversed(enabled: boolean, handle: Handle | null): number {
+    if (handle && enabled) {
+        return 1;
+    }
+
+    return 0;
+}
+
+function truthy(x: boolean | string | null): boolean {
+    if (x) {
+        return true;
+    }
+
+    return false;
+}
+
+function notTruthy(x: boolean | number | null): boolean {
+    return !x;
+}
+
+function andText(e: boolean, s: string | null): number {
+    return e && s ? 1 : 0;
+}
+
+function main() {
+    const h = new Handle();
+    assert(use(true, h) == 7, "use(true, h)");
+    assert(use(false, h) == 0, "use(false, h)");
+    assert(use(true, null) == 0, "use(true, null)");
+    assert(useReversed(true, h) == 1, "useReversed(true, h)");
+    assert(useReversed(true, null) == 0, "useReversed(true, null)");
+
+    assert(truthy("a"), "truthy(\"a\")");
+    assert(!truthy(""), "truthy(\"\")");
+    assert(truthy(true), "truthy(true)");
+    assert(!truthy(false), "truthy(false)");
+    assert(!truthy(null), "truthy(null)");
+
+    assert(notTruthy(null), "!null");
+    assert(notTruthy(0), "!0");
+    assert(!notTruthy(3), "!3");
+    assert(!notTruthy(true), "!true");
+
+    assert(andText(true, "x") == 1, "true && \"x\"");
+    assert(andText(true, null) == 0, "true && null");
+    assert(andText(false, "x") == 0, "false && \"x\"");
+
+    print("done.");
+}