[Sema] Emit a diagnostic when extending a protocol with a redundant requirement

include/swift/AST/DiagnosticsCommon.def

@@ -95,6 +95,9 @@ NOTE(profile_read_error,none,
 ERROR(generic_signature_not_minimal,none,
       "generic requirement '%0' is redundant in %1", (StringRef, StringRef))
 
+WARNING(extension_redundant_requirement,none,
+"redundant requirement in extension", ())
+
 ERROR(attr_only_on_parameters, none,
       "'%0' may only be used on parameters", (StringRef))
 

lib/AST/TypeCheckRequests.cpp

@@ -334,6 +334,22 @@ bool RequirementRequest::visitRequirements(
     // Resolve to a requirement.
     auto req = evaluator(RequirementRequest{owner, index, stage});
     if (req) {
+
+      // If we're in an extension declaration and if we are adding a
+      // redundant requirement (for example, `extension Foo where Self: Foo`)
+      // then emit a diagnostic.
+
+      // FIXME: Leads to duplicate diagnostic emissions
+      if (auto extDecl = dyn_cast<ExtensionDecl>(owner.dc->getAsDecl())) {
+        auto ownerType = extDecl->getExtendedType();
+        auto reqType = req->getSecondType();
+
+        if (reqType->isEqual(ownerType)) {
+          auto &ctx = extDecl->getASTContext();
+          ctx.Diags.diagnose(extDecl->getLoc(), diag::extension_redundant_requirement);
+        }
+      }
+
       // Invoke the callback. If it returns true, we're done.
       if (callback(*req, &requirements[index]))
         return true;

test/decl/ext/protocol.swift

@@ -176,6 +176,18 @@ extension S1 {
   }
 }
 
+// ----------------------------------------------------------------------------
+// Protocol extensions with redundant requirements
+// ----------------------------------------------------------------------------
+
+protocol Foo {}
+extension Foo where Self: Foo {} // expected-warning {{redundant requirement in extension}}
+
+protocol Bar {}
+protocol Baz {}
+extension Bar where Self: Baz {} // ok
+extension Bar where Self: Bar, Self: Baz {} // expected-warning {{redundant requirement in extension}}
+
 // ----------------------------------------------------------------------------
 // Protocol extensions with additional requirements
 // ----------------------------------------------------------------------------