Sema: Fix logic error in sanitizeProtocolRequirements()

If P is our protocol and it has an associated type A, we can't just
fold any dependent member type T.[Q]A to T.[P]A; this only makes
sense if T conforms to P.

Fixes rdar://problem/122587920.

Author: slavapestov

lib/Sema/AssociatedTypeInference.cpp

@@ -2516,15 +2516,15 @@ static void sanitizeProtocolRequirements(
   sanitizeType = [&](Type outerType) {
     return outerType.transformRec([&](TypeBase *type) -> llvm::Optional<Type> {
       if (auto depMemTy = dyn_cast<DependentMemberType>(type)) {
-        if (!depMemTy->getAssocType() ||
-            depMemTy->getAssocType()->getProtocol() != proto) {
+        if ((!depMemTy->getAssocType() ||
+             depMemTy->getAssocType()->getProtocol() != proto) &&
+            proto->getGenericSignature()->requiresProtocol(depMemTy->getBase(), proto)) {
 
           if (auto *assocType = proto->getAssociatedType(depMemTy->getName())) {
             Type sanitizedBase = sanitizeType(depMemTy->getBase());
             if (!sanitizedBase)
               return Type();
-            return Type(DependentMemberType::get(sanitizedBase,
-                                                  assocType));
+            return Type(DependentMemberType::get(sanitizedBase, assocType));
           }
 
           if (depMemTy->getBase()->is<GenericTypeParamType>())

test/decl/protocol/req/rdar122587920.swift

@@ -0,0 +1,30 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift -disable-experimental-associated-type-inference
+
+struct S<Element> {}
+
+extension S: Sequence, IteratorProtocol {
+  mutating func next() -> Element?? {
+    fatalError()
+  }
+}
+
+extension S: Collection {
+  var startIndex: Int {
+    fatalError()
+  }
+
+  var endIndex: Int {
+    fatalError()
+  }
+
+  subscript(_ index: Int) -> Element? {
+    fatalError()
+  }
+
+  func index(after index: Int) -> Int {
+    fatalError()
+  }
+}
+
+let x: S<Int>.Type = S<Int>.Iterator.self