Sema: Specify compatibility margin for access_control_non_objc_open_member

This diagnostic was staged in
#38991 as a warning to not break
source. Give it a reasonable margin and, in turn, convey that it is in
fact a compatibility warning and will become an error.

Author: AnthonyLatsis

include/swift/AST/DiagnosticsSema.def

@@ -2176,7 +2176,7 @@ ERROR(access_control_extension_open,none,
 ERROR(access_control_open_bad_decl,none,
       "only classes and overridable class members can be declared 'open';"
       " use 'public'", ())
-WARNING(access_control_non_objc_open_member,none,
+ERROR(access_control_non_objc_open_member,none,
       "non-'@objc' %0 in extensions cannot be overridden; use 'public' instead",
       (DescriptiveDeclKind))
 ERROR(access_control_requires_package_name, none,

lib/Sema/TypeCheckAttr.cpp

@@ -1329,7 +1329,8 @@ void AttributeChecker::visitAccessControlAttr(AccessControlAttr *attr) {
             diagnose(attr->getLocation(),
                      diag::access_control_non_objc_open_member,
                      VD->getDescriptiveKind())
-                .fixItReplace(attr->getRange(), "public");
+                .fixItReplace(attr->getRange(), "public")
+                .warnUntilSwiftVersion(7);
           }
         }
       }

test/attr/open_objc.swift

@@ -9,11 +9,11 @@ open class InvalidOpenExtension {
   open var openVar: Int { 3 } // No warning: Pure Swift properties can be open inside the class declaration.
 }
 extension InvalidOpenExtension {
-  open func nonObjcOpenMethod() { } // expected-warning {{non-'@objc' instance method in extensions cannot be overridden; use 'public' instead}} {{3-7=public}}
-  open class func nonObjcOpenMethod() { } // expected-warning {{non-'@objc' class method in extensions cannot be overridden; use 'public' instead}} {{3-7=public}}
-  open var nonObjcOpenVar: Int { 3 }  // expected-warning {{non-'@objc' property in extensions cannot be overridden; use 'public' instead}} {{3-7=public}}
-  open subscript(_ index: Int) -> Int { 3 } // expected-warning {{non-'@objc' subscript in extensions cannot be overridden; use 'public' instead}} {{3-7=public}}
-  open convenience init(index: Int) { self.init() } // expected-error {{only classes and overridable class members can be declared 'open'; use 'public'}} // expected-warning {{non-'@objc' initializer in extensions cannot be overridden; use 'public' instead}}
+  open func nonObjcOpenMethod() { } // expected-warning {{non-'@objc' instance method in extensions cannot be overridden; use 'public' instead; this will be an error in a future Swift language mode}} {{3-7=public}}
+  open class func nonObjcOpenMethod() { } // expected-warning {{non-'@objc' class method in extensions cannot be overridden; use 'public' instead; this will be an error in a future Swift language mode}} {{3-7=public}}
+  open var nonObjcOpenVar: Int { 3 }  // expected-warning {{non-'@objc' property in extensions cannot be overridden; use 'public' instead; this will be an error in a future Swift language mode}} {{3-7=public}}
+  open subscript(_ index: Int) -> Int { 3 } // expected-warning {{non-'@objc' subscript in extensions cannot be overridden; use 'public' instead; this will be an error in a future Swift language mode}} {{3-7=public}}
+  open convenience init(index: Int) { self.init() } // expected-error {{only classes and overridable class members can be declared 'open'; use 'public'}} // expected-warning {{non-'@objc' initializer in extensions cannot be overridden; use 'public' instead; this will be an error in a future Swift language mode}}
 
   @objc open func objcOpenMethod() { } // No warning: @objc methods can be open inside extensions. 
   @objc open var objcOpenVar: Int { 3 } // No warning: @objc methods can be open inside extensions.