Add diagnostic for non-throwing methods implementing a throwing @objc protocol method.

Resolves rdar://problem/60933487

Author: artemcm

include/swift/AST/DiagnosticsSema.def

@@ -2456,6 +2456,9 @@ ERROR(override_throws,none,
 ERROR(override_throws_objc,none,
       "overriding a throwing @objc %select{method|initializer}0 with "
       "a non-throwing %select{method|initializer}0 is not supported", (bool))
+ERROR(satisfy_throws_objc,none,
+      "satisfying a throwing @objc %select{method|initializer}0 with "
+      "a non-throwing %select{method|initializer}0 is not supported", (bool))
 
 ERROR(override_optional_mismatch,none,
       "cannot override %0 %select{parameter|index}1 of type %2 with "

lib/Sema/TypeCheckDeclObjC.cpp

@@ -1605,10 +1605,17 @@ void markAsObjC(ValueDecl *D, ObjCReason reason,
         }
       }
     }
-    
+
     // Attach the foreign error convention.
     if (inheritedConvention) {
-      method->setForeignErrorConvention(*inheritedConvention);
+      // Diagnose if this is a method that does not throw
+      // but inherits an ObjC error convention.
+      if (!method->hasThrows())
+        method->diagnose(diag::satisfy_throws_objc,
+                         isa<ConstructorDecl>(method));
+      else
+        method->setForeignErrorConvention(*inheritedConvention);
+
     } else if (method->hasThrows()) {
       assert(errorConvention && "Missing error convention");
       method->setForeignErrorConvention(*errorConvention);

test/Sema/impl_throw_objc.swift

@@ -0,0 +1,11 @@
+// RUN: %target-typecheck-verify-swift %s
+// REQUIRES: objc_interop
+
+import Foundation
+@objc protocol Tracker {
+    func track(event: String) throws
+}
+
+class ApplicationTracker: NSObject, Tracker {
+    func track(event: String) {} // expected-error {{satisfying a throwing @objc method with a non-throwing method is not supported}}
+}