Sema: Improve redeclaration error for synthesized inits

include/swift/AST/DiagnosticsSema.def

@@ -667,6 +667,9 @@ ERROR(reserved_member_name,none,
       " 'foo.%1' expression", (DeclName, StringRef))
 
 ERROR(invalid_redecl,none,"invalid redeclaration of %0", (DeclName))
+ERROR(invalid_redecl_init,none,
+      "invalid redeclaration of synthesized %select{|memberwise }1%0",
+      (DeclName, bool))
 WARNING(invalid_redecl_swift5_warning,none,
         "redeclaration of %0 is deprecated and will be an error in Swift 5",
         (DeclName))

lib/Sema/TypeCheckDecl.cpp

@@ -914,8 +914,21 @@ static void checkRedeclaration(TypeChecker &tc, ValueDecl *current) {
                     current->getFullName());
         tc.diagnose(other, diag::invalid_redecl_prev, other->getFullName());
       } else {
-        tc.diagnose(current, diag::invalid_redecl, current->getFullName());
-        tc.diagnose(other, diag::invalid_redecl_prev, other->getFullName());
+        const auto *otherInit = dyn_cast<ConstructorDecl>(other);
+        // Provide a better description for implicit initializers.
+        if (otherInit && otherInit->isImplicit()) {
+          // Skip conflicts with inherited initializers, which only happen
+          // when the current declaration is within an extension. The override
+          // checker should have already taken care of emitting a more
+          // productive diagnostic.
+          if (!other->getOverriddenDecl())
+            tc.diagnose(current, diag::invalid_redecl_init,
+                        current->getFullName(),
+                        otherInit->isMemberwiseInitializer());
+        } else {
+          tc.diagnose(current, diag::invalid_redecl, current->getFullName());
+          tc.diagnose(other, diag::invalid_redecl_prev, other->getFullName());
+        }
         markInvalid();
       }
 

test/decl/init/basic_init.swift

@@ -13,8 +13,32 @@ class C {
 typealias t = t // expected-error {{type alias 't' references itself}}
 // expected-note@-1{{type declared here}}
 
+extension Foo {
+  convenience init() {} // expected-error{{invalid redeclaration of synthesized 'init()'}}
+}
 
+class InitClass {
+  init(arg: Bool) {} // expected-note{{add '@objc' to make this declaration overridable}}
+  @objc init(baz: Int) {} // expected-note{{overridden declaration is here}}
+  @objc dynamic init(bar: Int) {}
+}
+class InitSubclass: InitClass {}
+// expected-note@-1 {{'init(bar:)' previously overridden here}}
+// expected-note@-2 {{'init(baz:)' previously overridden here}}
+extension InitSubclass {
+  convenience init(arg: Bool) {} // expected-error{{overriding non-@objc declarations from extensions is not supported}}
+  convenience override init(baz: Int) {}
+  // expected-error@-1 {{cannot override a non-dynamic class declaration from an extension}}
+  // expected-error@-2 {{'init(baz:)' has already been overridden}}
+  convenience override init(bar: Int) {} // expected-error{{'init(bar:)' has already been overridden}}
+}
 
+struct InitStruct {
+  let foo: Int
+}
+extension InitStruct {
+  init(foo: Int) {} // expected-error{{invalid redeclaration of synthesized memberwise 'init(foo:)'}}
+}
 
 // <rdar://problem/17564699> QoI: Structs should get convenience initializers
 struct MyStruct {