nits

src/common/framework/params_builder.ts

@@ -156,7 +156,7 @@ export class CaseParamsBuilder<CaseP extends {}>
         }
       }
 
-      yield [caseP as DeepReadonly<CaseP>, undefined];
+      yield [caseP as DeepReadonly<typeof caseP>, undefined];
     }
   }
 
@@ -303,7 +303,10 @@ export class SubcaseParamsBuilder<CaseP extends {}, SubcaseP extends {}>
 
       const subcases = Array.from(this.subcases(caseP));
       if (subcases.length) {
-        yield [caseP as DeepReadonly<CaseP>, subcases as DeepReadonly<SubcaseP>[]];
+        yield [
+          caseP as DeepReadonly<typeof caseP>,
+          subcases as DeepReadonly<typeof subcases[number]>[],
+        ];
       }
     }
   }

src/common/internal/test_group.ts

@@ -83,9 +83,11 @@ export function makeTestGroupForUnitTesting<F extends Fixture>(
 /** Parameter name for batch number (see also TestBuilder.batch). */
 const kBatchParamName = 'batch__';
 
-type TestFn<F extends Fixture, P extends {}> = (t: F & { params: P }) => Promise<void> | void;
+type TestFn<F extends Fixture, P extends {}> = (
+  t: F & { params: DeepReadonly<P> }
+) => Promise<void> | void;
 type BeforeAllSubcasesFn<S extends SubcaseBatchState, P extends {}> = (
-  s: S & { params: P }
+  s: S & { params: DeepReadonly<P> }
 ) => Promise<void> | void;
 
 export class TestGroup<F extends Fixture> implements TestGroupBuilder<F> {
@@ -217,7 +219,7 @@ interface TestBuilderWithParams<F extends Fixture, CaseP extends {}, SubcaseP ex
    * Set the test function.
    * @param fn the test function.
    */
-  fn(fn: TestFn<F, DeepReadonly<Merged<CaseP, SubcaseP>>>): void;
+  fn(fn: TestFn<F, Merged<CaseP, SubcaseP>>): void;
   /**
    * Mark the test as unimplemented.
    */

src/common/util/types.ts

@@ -13,7 +13,10 @@ export type TypeEqual<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T
 /* eslint-disable-next-line @typescript-eslint/no-unused-vars */
 export function assertTypeTrue<T extends true>() {}
 
-export type Primitive = string | number | boolean | undefined | null | Function | symbol;
+/**
+ * Deep version of the Readonly<> type, with support for tuples (up to length 7).
+ * <https://gist.github.com/masterkidan/7322752f569b1bba53e0426266768623>
+ */
 export type DeepReadonly<T> = T extends [infer A]
   ? DeepReadonlyObject<[A]>
   : T extends [infer A, infer B]
@@ -40,6 +43,7 @@ export type DeepReadonly<T> = T extends [infer A]
   ? DeepReadonlyArray<A>
   : DeepReadonlyObject<T>;
 
+type Primitive = string | number | boolean | undefined | null | Function | symbol;
 type DeepReadonlyArray<T> = ReadonlyArray<DeepReadonly<T>>;
 type DeepReadonlyObject<T> = { readonly [P in keyof T]: DeepReadonly<T[P]> };