feat: Add type inference from queryFn return type

[ho8ae]

Summary

Implements automatic type inference from queryFn return type for query-db-collection, following the same pattern as electric-db-collection.

Changes

• Added InferQueryFnOutput<TQueryFn> type helper to extract item types from queryFn return type
• Implemented ResolveType system with priority order:
  1. Explicit type (highest priority)
  2. Schema inference (second priority)
  3. QueryFn return type inference (third priority)
  4. Fallback to Record<string, unknown>
• Updated QueryCollectionConfig interface to support new type resolution
• Added comprehensive type tests for all inference patterns
• Maintained backward compatibility with existing usage

Examples

New: Type inference from queryFn

const collection = createCollection(
  queryCollectionOptions({
    queryKey: ['todos'],
    queryFn: async () => {
      const response = await fetch('/api/todos')
      return response.json() as Todo[] // Type automatically inferred!
    },
    queryClient,
    getKey: (item) => item.id, // item is typed as Todo
  })
)

Existing: Explicit type (still works)

const collection = createCollection<Todo>(
  queryCollectionOptions({
    queryKey: ['todos'],
    queryFn: async () => fetch('/api/todos').then(r => r.json()),
    queryClient,
    getKey: (item) => item.id,
  })
)

Existing: Schema inference (still works)

const collection = createCollection(
  queryCollectionOptions({
    queryKey: ['todos'],
    queryFn: async () => fetch('/api/todos').then(r => r.json()),
    queryClient,
    schema: todoSchema,
    getKey: (item) => item.id,
  })
)

Testing

• All existing tests pass
• New type inference tests added and passing
• Backward compatibility verified
• Type resolution priority order tested

Type Resolution Priority

The implementation follows the exact same priority order as electric-db-collection:

1. Explicit type - queryCollectionOptions<MyType>(...)
2. Schema inference - schema: mySchema
3. QueryFn inference - queryFn: () => Promise<MyType[]>
4. Fallback - Record<string, unknown>

Related

• Add type inference from queryFn return type in query-db-collection #356
• Follows the same pattern as electric-db-collection's ResolveType implementation
• Maintains consistency with existing TanStack DB patterns

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Latest commit: b629600

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