fix(db): infer type in coalesce() instead of returning BasicExpression<any>

Previously coalesce() always returned BasicExpression<any>, losing type
information. Now it uses a generic to infer the non-nullable union of all
argument types, matching the semantic of SQL COALESCE.

Fixes #1341

Author: sleitor

.changeset/fix-coalesce-type.md

@@ -0,0 +1,5 @@
+---
+"@tanstack/db": patch
+---
+
+fix(db): infer type in coalesce() instead of returning BasicExpression<any>

packages/db/src/query/builder/functions.ts

@@ -285,11 +285,13 @@ export function concat(
   )
 }
 
-export function coalesce(...args: Array<ExpressionLike>): BasicExpression<any> {
+export function coalesce<T extends Array<ExpressionLike>>(
+  ...args: T
+): BasicExpression<NonNullable<ExtractType<T[number]>>> {
   return new Func(
     `coalesce`,
     args.map((arg) => toExpression(arg)),
-  )
+  ) as BasicExpression<NonNullable<ExtractType<T[number]>>>
 }
 
 export function add<T1 extends ExpressionLike, T2 extends ExpressionLike>(

packages/db/tests/query/builder/callback-types.test-d.ts

@@ -150,7 +150,7 @@ describe(`Query Builder Callback Types`, () => {
           BasicExpression<number>
         >()
         expectTypeOf(coalesce(user.name, `Unknown`)).toEqualTypeOf<
-          BasicExpression<any>
+          BasicExpression<string>
         >()
 
         return {