For 'lazy', make "cannot override with a stored property" a warning (#13308)

Previous versions of Swift accidentally treated lazy properties as
computed properties because of how they were implemented. Now that we
check this correctly, we've broken source compatibility. Downgrade the
error to a warning in this case.

(Arguably we could /allow/ overriding with a stored property. The
original concerns were that you could accidentally end up with extra
storage you didn't need, and that observing accessors would behave
differently based on whether or not the property was overriding. But
there's at least no ambiguity for 'lazy', which can't have observing
accessors today.)

rdar://problem/35870371
(cherry picked from commit 7c707ce)

Author: jrose-apple

include/swift/AST/DiagnosticsSema.def

@@ -1877,6 +1877,8 @@ ERROR(override_property_type_mismatch,none,
       (Identifier, Type, Type))
 ERROR(override_with_stored_property,none,
       "cannot override with a stored property %0", (Identifier))
+WARNING(override_with_stored_property_warn,none,
+      "cannot override with a stored property %0", (Identifier))
 ERROR(observing_readonly_property,none,
       "cannot observe read-only property %0; it can't change", (Identifier))
 ERROR(override_mutable_with_readonly_property,none,

lib/Sema/TypeCheckDecl.cpp

@@ -6623,7 +6623,15 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
       // Make sure that the overriding property doesn't have storage.
       if (overrideASD->hasStorage() && !overrideASD->hasObservers()) {
-        TC.diagnose(overrideASD, diag::override_with_stored_property,
+        auto diagID = diag::override_with_stored_property;
+        if (!TC.Context.isSwiftVersionAtLeast(5) &&
+            overrideASD->getAttrs().hasAttribute<LazyAttr>()) {
+          // Swift 4.0 had a bug where lazy properties were considered
+          // computed by the time of this check. Downgrade this diagnostic to
+          // a warning.
+          diagID = diag::override_with_stored_property_warn;
+        }
+        TC.diagnose(overrideASD, diagID,
                     overrideASD->getBaseName().getIdentifier());
         TC.diagnose(baseASD, diag::property_override_here);
         return true;