[Concurrency] Downgrade @preconcurrency @Sendable conversion failures

to warnings even in Swift 6.

Author: hborla

lib/Sema/CSFix.cpp

@@ -294,10 +294,16 @@ getConcurrencyFixBehavior(ConstraintSystem &cs, ConstraintKind constraintKind,
 
   // For a @preconcurrency callee outside of a strict concurrency
   // context, ignore.
-  if (cs.hasPreconcurrencyCallee(locator) &&
-      !contextRequiresStrictConcurrencyChecking(
-          cs.DC, GetClosureType{cs}, ClosureIsolatedByPreconcurrency{cs}))
+  if (cs.hasPreconcurrencyCallee(locator)) {
+    // Preconcurrency failures are always downgraded to warnings, even in
+    // Swift 6 mode.
+    if (contextRequiresStrictConcurrencyChecking(
+            cs.DC, GetClosureType{cs}, ClosureIsolatedByPreconcurrency{cs})) {
+      return FixBehavior::DowngradeToWarning;
+    }
+
     return FixBehavior::Suppress;
+  }
 
   // Otherwise, warn until Swift 6.
   if (!cs.getASTContext().LangOpts.isSwiftVersionAtLeast(6))

test/Concurrency/predates_concurrency_swift6.swift

@@ -180,3 +180,14 @@ do {
     }
   }
 }
+
+
+
+@preconcurrency
+func withSendableClosure(_: @Sendable () -> Void) {}
+
+func conversionDowngrade() {
+  let ns: () -> Void = {}
+  withSendableClosure(ns)
+  // expected-warning@-1 {{converting non-sendable function value to '@Sendable () -> Void' may introduce data races}}
+}