[CSDiagnostics] Add a couple of asserts to RequirementFailure

This will make it slightly easier to spot cases where we've resolved
to the wrong affected decl.

Author: hamishknight

lib/Sema/CSDiagnostics.cpp

@@ -310,7 +310,12 @@ Type RequirementFailure::getOwnerType() const {
 const GenericContext *RequirementFailure::getGenericContext() const {
   if (auto *genericCtx = AffectedDecl->getAsGenericContext())
     return genericCtx;
-  return AffectedDecl->getDeclContext()->getAsDecl()->getAsGenericContext();
+
+  auto parentDecl = AffectedDecl->getDeclContext()->getAsDecl();
+  if (!parentDecl)
+    return nullptr;
+
+  return parentDecl->getAsGenericContext();
 }
 
 const Requirement &RequirementFailure::getRequirement() const {

lib/Sema/CSDiagnostics.h

@@ -236,6 +236,10 @@ class RequirementFailure : public FailureDiagnostic {
     assert(locator);
     assert(isConditional() || Signature);
     assert(AffectedDecl);
+    assert(getRequirementDC() &&
+           "Couldn't find where the requirement came from?");
+    assert(getGenericContext() &&
+           "Affected decl not within a generic context?");
 
     auto reqElt = locator->castLastElementTo<LocatorPathElt::AnyRequirement>();
     assert(reqElt.getRequirementKind() == kind);