[Serialization] Filter out XREFs into constrained extensions that should not find declarations there.

The presence of a generic signature in a XREF means that we should only find the result in a (further-constrained) extension with that generic signature. The absence of a generic signature in a XREF means that we should not find the result in a constrained extension. We implemented the former but not the latter, which would lead to deserialization failures if one had both constrained and unconstrained extensions with the same property in them. Methods/initializers weren’t a problem because the generic signature is (redundantly) encoded in their interface type.
(cherry picked from commit 5775732)

Author: DougGregor

lib/Serialization/Deserialization.cpp

@@ -1023,6 +1023,13 @@ static void filterValues(Type expectedTy, Module *expectedModule,
           ->getCanonicalSignature() != expectedGenericSig)
       return true;
 
+    // If we don't expect a specific generic signature, ignore anything from a
+    // constrained extension.
+    if (!expectedGenericSig &&
+        isa<ExtensionDecl>(value->getDeclContext()) &&
+        cast<ExtensionDecl>(value->getDeclContext())->isConstrainedExtension())
+      return true;
+
     // If we're looking at members of a protocol or protocol extension,
     // filter by whether we expect to find something in a protocol extension or
     // not. This lets us distinguish between a protocol member and a protocol

test/Serialization/Inputs/generic_extension_1.swift

@@ -1,3 +1,23 @@
 public extension Array {
   func wobble() -> Element? { return nil }
 }
+
+public protocol P {
+  var property: Int { get }
+}
+
+public protocol Q { }
+
+extension P {
+  public var property: Int { return 0 }
+}
+
+extension P where Self: Q {
+  public var property: Int { return 0 }
+}
+
+public struct ConformsToP: P { }
+
+
+
+

test/Serialization/generic_extension.swift

@@ -11,3 +11,9 @@ import generic_extension_1
 ["a", "b", "c"].wobble()
 
 // CHECK: sil @_TFE19generic_extension_1Sa6wobble{{.*}} : $@convention(method) <τ_0_0> (@guaranteed Array<τ_0_0>) -> @out Optional<τ_0_0>
+
+func useP<T: P>(_ t: T) -> Int { return t.property }
+
+func testUseP(c: ConformsToP) {
+  _ = useP(c)
+}