[CSApply] Fix an issue with the @objc selector fix-it incorrectly being applied (#20949)

Special-case the diagnostic for members of non-@objc protocols that are referenced in #selector expressions.

Author: theblixguy

include/swift/AST/DiagnosticsSema.def

@@ -550,6 +550,9 @@ ERROR(expr_selector_property_not_settable,none,
       (DescriptiveDeclKind, DeclName))
 ERROR(expr_selector_property_setter_inaccessible,none,
       "setter of %0 %1 is inaccessible", (DescriptiveDeclKind, DeclName))
+ERROR(expr_selector_cannot_be_used,none,
+      "cannot use %0 as a selector because protocol %1 is not exposed to Objective-C",
+      (DeclBaseName, DeclName))
 ERROR(expr_selector_not_objc,none,
       "argument of '#selector' refers to %0 %1 that is not exposed to "
       "Objective-C",

lib/Sema/CSApply.cpp

@@ -4178,6 +4178,17 @@ namespace {
       // The declaration we found must be exposed to Objective-C.
       tc.validateDecl(method);
       if (!method->isObjC()) {
+        // If the method declaration lies in a protocol and we're providing
+        // a default implementation of the method through a protocol extension
+        // and using it as a selector, then bail out as adding @objc to the
+        // protocol might not be the right thing to do and could lead to
+        // problems.
+        if (auto protocolDecl = dyn_cast<ProtocolDecl>(foundDecl->getDeclContext())) {
+          tc.diagnose(E->getLoc(), diag::expr_selector_cannot_be_used,
+                      foundDecl->getBaseName(), protocolDecl->getFullName());
+          return E;
+        }
+        
         tc.diagnose(E->getLoc(), diag::expr_selector_not_objc,
                     foundDecl->getDescriptiveKind(), foundDecl->getFullName())
           .highlight(subExpr->getSourceRange());

test/expr/primary/selector/selector.swift

@@ -144,3 +144,20 @@ case #selector(SR1827.bar)?:
 default:
   break
 }
+
+// SR-9391
+
+protocol SomeProtocol {
+  func someFunction()
+  func anotherFunction()
+}
+
+extension SomeProtocol {
+  func someFunction() {
+    let _ = #selector(anotherFunction) // expected-error {{cannot use 'anotherFunction' as a selector because protocol 'SomeProtocol' is not exposed to Objective-C}} {{none}}
+  }
+
+  func anotherFunction() { 
+    print("Hello world!")
+ }
+}