Do not narrow a typed value to Opaque on typeof "function"

tslang/lib/TypeScript/MLIRGenImpl.h

@@ -3307,6 +3307,16 @@ class MLIRGenImpl
         else
         {
             if (inverse) return mlir::failure();
+
+            // `typeof x === "function"` (also "class", "interface", "object") names no type, only Opaque.
+            // The type x already has is more precise: casting would lose it - a function could no longer
+            // be called in the branch - and for a generic function nobody instantiated it references a
+            // function that is never emitted.
+            if (isa<mlir_ts::OpaqueType>(safeType))
+            {
+                return mlir::success();
+            }
+
             CAST_A(result, location, safeType, exprValue, genContext);
             castedValue = V(result);
         }

tslang/test/tester/CMakeLists.txt

@@ -171,6 +171,7 @@ add_test(NAME test-compile-00-funcs-nesting-generic COMMAND test-runner "${PROJE
 add_test(NAME test-compile-00-funcs-nesting-capture COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_nesting_capture.ts")
 add_test(NAME test-compile-00-funcs-hybrid-null-this COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_hybrid_null_this.ts")
 add_test(NAME test-compile-00-typeof-static-fold COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_static_fold.ts")
+add_test(NAME test-compile-00-typeof-function-narrowing COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_function_narrowing.ts")
 add_test(NAME test-compile-00-typeof-static-fold-conditions COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_static_fold_conditions.ts")
 add_test(NAME test-compile-00-array-move-wide-elements COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_move_wide_elements.ts")
 add_test(NAME test-compile-00-funcs-expression-generic COMMAND test-runner "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_expression_generic.ts")
@@ -589,6 +590,7 @@ add_test(NAME test-jit-00-funcs-nesting-generic COMMAND test-runner -jit "${PROJ
 add_test(NAME test-jit-00-funcs-nesting-capture COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_nesting_capture.ts")
 add_test(NAME test-jit-00-funcs-hybrid-null-this COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_hybrid_null_this.ts")
 add_test(NAME test-jit-00-typeof-static-fold COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_static_fold.ts")
+add_test(NAME test-jit-00-typeof-function-narrowing COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_function_narrowing.ts")
 add_test(NAME test-jit-00-typeof-static-fold-conditions COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00typeof_static_fold_conditions.ts")
 add_test(NAME test-jit-00-array-move-wide-elements COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00array_move_wide_elements.ts")
 add_test(NAME test-jit-00-funcs-expression-generic COMMAND test-runner -jit "${PROJECT_SOURCE_DIR}/test/tester/tests/00funcs_expression_generic.ts")
@@ -1576,6 +1578,7 @@ set(TSLANG_CORPUS
     00type_aliases_in_generics.ts
     00type_guard_function.ts
     00typed_array.ts
+    00typeof_function_narrowing.ts
     00typeof_static_fold.ts
     00typeof_static_fold_conditions.ts
     00types_indexedaccesstype.ts

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

@@ -0,0 +1,41 @@
+// `typeof f === "function"` names no type of its own, only Opaque. A value whose type is known is
+// not narrowed to it: that lost the function type, so the function could not be called in the
+// branch, and for a generic function nobody instantiated it referenced a function never emitted.
+function one() {
+    return 1;
+}
+
+function id<T>(x: T) {
+    return x;
+}
+
+function main() {
+    let calls = 0;
+
+    if (typeof one === "function") {
+        assert(one() == 1, "plain function is still callable");
+        calls++;
+    }
+
+    if (typeof id === "function") {
+        calls++;
+    }
+
+    if (typeof id === "function") {
+        assert(id(2) == 2, "generic function is still callable");
+        calls++;
+    }
+
+    const f = () => 3;
+    assert(typeof f === "function" ? f() == 3 : false, "arrow function in ?:");
+
+    // `any` is still narrowed at run time
+    let a: any = one;
+    if (typeof a === "function") {
+        calls++;
+    }
+
+    assert(calls == 4, "every branch ran");
+
+    print("done.");
+}