[CSSimplify] Matching function types with variadic parameters should …

lib/Sema/CSSimplify.cpp

@@ -3411,7 +3411,8 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
       return getTypeMatchFailure(locator);
 
     for (auto pair : matcher.pairs) {
-      auto result = matchTypes(pair.lhs, pair.rhs, subKind, subflags,
+      // Compare the parameter types, taking contravariance into account.
+      auto result = matchTypes(pair.rhs, pair.lhs, subKind, subflags,
                                (func1Params.size() == 1
                                 ? argumentLocator
                                 : argumentLocator.withPathElement(

test/Constraints/variadic_generic_types.swift

@@ -76,4 +76,33 @@ do {
       self.f = f
     }
   }
-}
+}
+
+// rdar://121692664 - compiler doesn't respect contravariance of the variadic function parameters
+do {
+  class Value<T> {
+    init<each V>(_ v: repeat Value<each V>,
+                 transform: @escaping (repeat each V) -> T) {
+    }
+  }
+
+  func coerce(_: Int) {}
+
+  func test(first: Value<Int?>, second: Value<(a: Int, b: Int)>) {
+    let _ = Value(first, second) { first, second in
+      coerce(first)
+      // expected-error@-1 {{value of optional type 'Int?' must be unwrapped to a value of type 'Int'}}
+      // expected-note@-2 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+      // expected-note@-3 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+    }
+
+    // multi-statement closure
+    let _ = Value(first, second) { first, second in
+      _ = 42
+      coerce(first)
+      // expected-error@-1 {{value of optional type 'Int?' must be unwrapped to a value of type 'Int'}}
+      // expected-note@-2 {{coalesce using '??' to provide a default when the optional value contains 'nil'}}
+      // expected-note@-3 {{force-unwrap using '!' to abort execution if the optional value contains 'nil'}}
+    }
+  }
+}