refactor(experimental): assert single sending signer inside signAndSend function

.changeset/short-jokes-begin.md

@@ -0,0 +1,5 @@
+---
+'@solana/signers': patch
+---
+
+The `signAndSendTransactionMessageWithSigners` function now automatically asserts that the provided transaction message contains a single sending signer and fails otherwise.

packages/signers/README.md

@@ -549,9 +549,9 @@ Here as well, composite transaction signers are treated such that at least one s
 -   `TransactionModifyingSigner`, if no other `TransactionModifyingSigner` exists.
 -   `TransactionPartialSigner`, otherwise.
 
-The provided transaction must be of type `ITransactionWithSingleSendingSigner` meaning that it must contain exactly one `TransactionSendingSigner` inside its account metas. If more than one composite signers implement the `TransactionSendingSigner` interface, one of them will be selected as the sending signer.
+The provided transaction must contain exactly one `TransactionSendingSigner` inside its account metas. If more than one composite signers implement the `TransactionSendingSigner` interface, one of them will be selected as the sending signer. Otherwise, if multiple `TransactionSendingSigners` must be selected, the function will throw an error.
 
-Therefore, you may use the `assertIsTransactionWithSingleSendingSigner()` function to ensure the transaction is of the expected type.
+If you'd like to assert that a transaction makes use of exactly one `TransactionSendingSigner` _before_ calling this function, you may use the `assertIsTransactionWithSingleSendingSigner` function.
 
 ```ts
 assertIsTransactionWithSingleSendingSigner(myTransaction);

packages/signers/src/sign-transaction.ts

@@ -21,7 +21,7 @@ import { isTransactionModifyingSigner, TransactionModifyingSigner } from './tran
 import { isTransactionPartialSigner, TransactionPartialSigner } from './transaction-partial-signer';
 import { isTransactionSendingSigner, TransactionSendingSigner } from './transaction-sending-signer';
 import { isTransactionSigner, TransactionSigner } from './transaction-signer';
-import { ITransactionMessageWithSingleSendingSigner } from './transaction-with-single-sending-signer';
+import { assertIsTransactionMessageWithSingleSendingSigner } from './transaction-with-single-sending-signer';
 
 type CompilableTransactionMessageWithSigners = CompilableTransactionMessage & ITransactionMessageWithSigners;
 
@@ -123,10 +123,10 @@ export async function signTransactionMessageWithSigners<
  * Otherwise, it will send the transaction using the provided fallbackSender.
  */
 export async function signAndSendTransactionMessageWithSigners<
-    TTransactionMessage extends CompilableTransactionMessageWithSigners &
-        ITransactionMessageWithSingleSendingSigner = CompilableTransactionMessageWithSigners &
-        ITransactionMessageWithSingleSendingSigner,
+    TTransactionMessage extends CompilableTransactionMessageWithSigners = CompilableTransactionMessageWithSigners,
 >(transaction: TTransactionMessage, config: { abortSignal?: AbortSignal } = {}): Promise<SignatureBytes> {
+    assertIsTransactionMessageWithSingleSendingSigner(transaction);
+
     const abortSignal = config.abortSignal;
     const { partialSigners, modifyingSigners, sendingSigner } = categorizeTransactionSigners(
         deduplicateSigners(getSignersFromTransactionMessage(transaction).filter(isTransactionSigner)),