Sema: Correctly treat overloads in unavailable extensions as unavailable.

Instead of checking for unavailability attributes directly in the solver, which
does not correctly handle members of unavailable extensions, query
`checkDeclarationAvailability()` instead. By using the same underlying logic as
the availability checker the constraint solver can be confident in the accuracy
of this result.

Resolves rdar://87403752.

Author: tshortli

lib/Sema/ConstraintSystem.cpp

@@ -4676,20 +4676,14 @@ void ConstraintSystem::diagnoseFailureFor(SyntacticElementTarget target) {
 
 bool ConstraintSystem::isDeclUnavailable(const Decl *D,
                                          ConstraintLocator *locator) const {
-  // First check whether this declaration is universally unavailable.
-  if (D->getAttrs().isUnavailable(getASTContext()))
-    return true;
-
-  return TypeChecker::isDeclarationUnavailable(D, DC, [&] {
-    SourceLoc loc;
-
-    if (locator) {
-      if (auto anchor = locator->getAnchor())
-        loc = getLoc(anchor);
-    }
+  SourceLoc loc;
+  if (locator) {
+    if (auto anchor = locator->getAnchor())
+      loc = getLoc(anchor);
+  }
 
-    return TypeChecker::overApproximateAvailabilityAtLocation(loc, DC);
-  });
+  auto availabilityContext = TypeChecker::availabilityAtLocation(loc, DC);
+  return checkDeclarationAvailability(D, DC, availabilityContext).has_value();
 }
 
 bool ConstraintSystem::isConformanceUnavailable(ProtocolConformanceRef conformance,

test/Constraints/availability.swift

@@ -64,3 +64,25 @@ extension Box where T == Int {
   // memberwise initializer.
   _ = Box(value: 42)
 }
+
+// rdar://87403752 - ambiguity with member declared in unavailable extension
+struct HasUnavailableExtesion {
+}
+
+@available(*, unavailable)
+extension HasUnavailableExtesion {
+  static var foo: Self = HasUnavailableExtesion()
+}
+
+func test_contextual_member_with_unavailable_extension() {
+  struct A {
+    static var foo: A = A()
+  }
+
+  struct Test {
+    init(_: A) {}
+    init(_: HasUnavailableExtesion) {}
+  }
+
+  _ = Test(.foo) // Ok `A.foo` since `foo` from `HasUnavailableExtesion` is unavailable
+}