[TypeChecker] TypeChecker::isSubtypeOf should recognize @sendable subtyping

A sendable function can be a subtype of a non-@sendable function,
that is currently established via a fix. `TypeChecker::isSubtypeOf`
should recognize its presence and fail.

Author: xedin

lib/Sema/TypeCheckConstraints.cpp

@@ -1037,6 +1037,16 @@ bool TypeChecker::typesSatisfyConstraint(Type type1, Type type2,
     if (unwrappedIUO)
       *unwrappedIUO = solution->getFixedScore().Data[SK_ForceUnchecked] > 0;
 
+    // Make sure that Sendable vs. no-Sendable mismatches are
+    // failures here to establish subtyping relationship
+    // (unlike in the solver where they are warnings until Swift 6).
+    if (kind == ConstraintKind::Subtype) {
+      if (llvm::any_of(solution->Fixes, [](const auto *fix) {
+            return fix->getKind() == FixKind::AddSendableAttribute;
+          }))
+        return false;
+    }
+
     return true;
   }
 

test/Concurrency/sendable_methods.swift

@@ -228,3 +228,19 @@ do {
     }
   }
 }
+
+do {
+  struct Test {
+    static func fn() {}
+    static func otherFn() {}
+  }
+
+  func fnRet(cond: Bool) -> () -> Void {
+    cond ? Test.fn : Test.otherFn // Ok
+  }
+
+  func forward<T>(_: T) -> T {
+  }
+
+  let _: () -> Void = forward(Test.fn) // Ok
+}