ResultBuilders use vars instead of lets internally, leading to unexpected parameter packing

lib/Sema/BuilderTransform.cpp

@@ -1597,7 +1597,7 @@ VarDecl *ResultBuilder::buildVar(SourceLoc loc) {
   Identifier name =
       ctx.getIdentifier(("$__builder" + Twine(VarCounter++)).str());
   auto var = new (ctx)
-      VarDecl(/*isStatic=*/false, VarDecl::Introducer::Var, loc, name, DC);
+      VarDecl(/*isStatic=*/false, VarDecl::Introducer::Let, loc, name, DC);
   var->setImplicit();
   return var;
 }

test/Sema/issue-85837.swift

@@ -0,0 +1,36 @@
+// RUN: %target-typecheck-verify-swift
+
+// Verifies fix for Github Issue #85837
+// https://github.com/swiftlang/swift/issues/85837
+//
+// 
+
+@resultBuilder public enum TupleBuilder {
+    public static func buildPartialBlock<T>(first: T) -> (T) {
+        return first
+    }
+
+    public static func buildPartialBlock<each A, B>(accumulated: (repeat each A), next: B) -> (repeat each A, B) {
+        return (repeat each accumulated, next)
+    }
+}
+
+func builder<each A>(@TupleBuilder content: ()->(repeat each A)) -> (repeat each A) {
+    return content()
+}
+
+// Ensure this optimally packs parameters
+let built: (String, Int, String) = builder {
+        "a"
+        2
+        "c"
+    }
+
+/*
+// Ideally we should be able to fall back to this legacy type?
+let legacy: ((String, Int), String) = builder {
+    "a"
+    2
+    "c"
+}
+*/