Diagnose *all* cases where a subclass fails to override a required init

In cases where a subclass was unable to synthesize any initializers
(for example, because there were missing designated initializers in the
superclass), we were skipping checking of superclass required
initializers. This meant that we would silently accept subclasses that
cannot be initialized directly (that would produce an error), but
could at runtime be initialized via a required initializer... that
didn't account for the subclass.

Fixes the original problem from #69965,
but not the most minimal one.

Author: DougGregor

lib/Sema/CodeSynthesis.cpp

@@ -826,6 +826,7 @@ createDesignatedInitOverride(ClassDecl *classDecl,
 static void diagnoseMissingRequiredInitializer(
               ClassDecl *classDecl,
               ConstructorDecl *superInitializer,
+              bool downgradeToWarning,
               ASTContext &ctx) {
   // Find the location at which we should insert the new initializer.
   SourceLoc insertionLoc;
@@ -906,6 +907,7 @@ static void diagnoseMissingRequiredInitializer(
   ctx.Diags.diagnose(insertionLoc, diag::required_initializer_missing,
                      superInitializer->getName(),
                      superInitializer->getDeclContext()->getDeclaredInterfaceType())
+    .warnUntilSwiftVersionIf(downgradeToWarning, 6)
     .fixItInsert(insertionLoc, initializerText);
 
   ctx.Diags.diagnose(findNonImplicitRequiredInit(superInitializer),
@@ -1172,10 +1174,11 @@ static void addImplicitInheritedConstructorsToClass(ClassDecl *decl) {
   bool defaultInitable =
       areAllStoredPropertiesDefaultInitializable(ctx.evaluator, decl);
 
-  // We can't define these overrides if we have any uninitialized
-  // stored properties.
-  if (!defaultInitable && !foundDesignatedInit)
-    return;
+  // In cases where we can't define any overrides, we used to suppress
+  // diagnostics about missing required initializers. Now we emit diagnostics,
+  // but downgrade them to warnings prior to Swift 6.
+  bool downgradeRequiredInitsToWarning =
+      !defaultInitable && !foundDesignatedInit;
 
   SmallVector<ConstructorDecl *, 4> nonOverriddenSuperclassCtors;
   collectNonOveriddenSuperclassInits(decl, nonOverriddenSuperclassCtors);
@@ -1187,8 +1190,10 @@ static void addImplicitInheritedConstructorsToClass(ClassDecl *decl) {
       if (superclassCtor->isRequired()) {
         assert(superclassCtor->isInheritable() &&
                "factory initializers cannot be 'required'");
-        if (!decl->inheritsSuperclassInitializers())
-          diagnoseMissingRequiredInitializer(decl, superclassCtor, ctx);
+        if (!decl->inheritsSuperclassInitializers()) {
+          diagnoseMissingRequiredInitializer(
+              decl, superclassCtor, downgradeRequiredInitsToWarning, ctx);
+        }
       }
       continue;
     }
@@ -1204,12 +1209,18 @@ static void addImplicitInheritedConstructorsToClass(ClassDecl *decl) {
 
     // Diagnose a missing override of a required initializer.
     if (superclassCtor->isRequired() && !inheritDesignatedInits) {
-      diagnoseMissingRequiredInitializer(decl, superclassCtor, ctx);
+      diagnoseMissingRequiredInitializer(
+          decl, superclassCtor, downgradeRequiredInitsToWarning, ctx);
       continue;
     }
 
     // A designated or required initializer has not been overridden.
 
+    // We can't define any overrides if we have any uninitialized
+    // stored properties.
+    if (!defaultInitable && !foundDesignatedInit)
+      continue;
+
     bool alreadyDeclared = false;
 
     auto results = decl->lookupDirect(DeclBaseName::createConstructor());

test/decl/class/inheritance_across_modules.swift

@@ -0,0 +1,20 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+
+// RUN: %target-swift-frontend -emit-module-path %t/MyModule.swiftmodule %t/Inputs/MyModule.swift
+
+// RUN: %target-swift-frontend -typecheck -verify -I %t %t/test.swift
+
+//--- Inputs/MyModule.swift
+open class MySuperclassA {
+  required public init() { }
+  internal init(boop: Bool) {}
+}
+
+//--- test.swift
+import MyModule
+
+class MySubclassA: MySuperclassA {
+// expected-warning{{'required' initializer 'init()' must be provided by subclass of 'MySuperclassA'; this is an error in Swift 6}}
+  var hi: String
+}