Devirtualizer: fix a miscompile due to handling of cast instructions.

lib/SILOptimizer/Utils/Devirtualize.cpp

@@ -1147,8 +1147,15 @@ swift::tryDevirtualizeApply(ApplySite applySite, ClassHierarchyAnalysis *cha,
 
     // Try to check if the exact dynamic type of the instance is statically
     // known.
-    if (auto instance = getInstanceWithExactDynamicType(cmi->getOperand(), cha))
-      return tryDevirtualizeClassMethod(fas, instance, cd, ore);
+    if (auto instance = getInstanceWithExactDynamicType(cmi->getOperand(), cha)) {
+      // Update the classDecl, because we are stripping casts more aggressively
+      // in getInstanceWithExactDynamicType than in stripUpCasts.
+      CanType classType = getSelfInstanceType(instance->getType().getASTType());
+      // This should never be null - make the check just to be on the safe side.
+      if (ClassDecl *cd = classType.getClassOrBoundGenericClass())
+        return tryDevirtualizeClassMethod(fas, instance, cd, ore);
+      return {ApplySite(), false};
+    }
 
     if (auto exactTy = getExactDynamicType(cmi->getOperand(), cha)) {
       if (exactTy == cmi->getOperand()->getType())
@@ -1207,8 +1214,11 @@ bool swift::canDevirtualizeApply(FullApplySite applySite,
 
     // Try to check if the exact dynamic type of the instance is statically
     // known.
-    if (auto instance = getInstanceWithExactDynamicType(cmi->getOperand(), cha))
-      return canDevirtualizeClassMethod(applySite, cd);
+    if (auto instance = getInstanceWithExactDynamicType(cmi->getOperand(), cha)) {
+      CanType classType = getSelfInstanceType(instance->getType().getASTType());
+      ClassDecl *cd = classType.getClassOrBoundGenericClass();
+      return cd && canDevirtualizeClassMethod(applySite, cd);
+    }
 
     if (auto exactTy = getExactDynamicType(cmi->getOperand(), cha)) {
       if (exactTy == cmi->getOperand()->getType())

test/SILOptimizer/devirtualize_class_method.swift

@@ -0,0 +1,37 @@
+// RUN: %empty-directory(%t) 
+// RUN: %target-build-swift -O -module-name=test %s -o %t/a.out
+// RUN: %target-run %t/a.out | %FileCheck %s
+// REQUIRES: executable_test
+
+class Base {
+  required init() { }
+
+  class func instance() -> Base {
+    return self.init()
+  }
+}
+
+class Middle: Base {
+  override class func instance() -> Middle {
+    return self.init()
+  }
+}
+
+class Derived: Middle {
+  required init() {
+    super.init()
+    print("init Derived")
+  }
+}
+
+struct Maker<C: Base> {
+  @inline(never)
+  static func create() -> Base {
+    return C.instance()
+  }
+}
+
+// CHECK: init Derived
+// CHECK: test.Derived
+print(Maker<Derived>.create())
+