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

include/swift/AST/DiagnosticsCommon.def

@@ -95,6 +95,10 @@ NOTE(profile_read_error,none,
 ERROR(generic_signature_not_minimal,none,
       "generic requirement '%0' is redundant in %1", (StringRef, StringRef))
 
+WARNING(protocol_extension_redundant_requirement,none,
+      "requirement of '%1' to '%2' is redundant in an extension of '%0'",
+      (StringRef, StringRef, StringRef))
+
 ERROR(attr_only_on_parameters, none,
       "'%0' may only be used on parameters", (StringRef))
 

lib/Sema/TypeCheckGeneric.cpp

@@ -78,6 +78,23 @@ void checkGenericParamList(TypeChecker &tc,
     RequirementRequest::visitRequirements(owner, resolution.getStage(),
         [&](const Requirement &req, RequirementRepr *reqRepr) {
           auto source = FloatingRequirementSource::forExplicit(reqRepr);
+
+          // If we're extending a protocol and adding a redundant requirement,
+          // for example, `extension Foo where Self: Foo`, then emit a
+          // diagnostic.
+
+          if (auto extDecl = dyn_cast<ExtensionDecl>(owner.dc->getAsDecl())) {
+            auto ownerType = extDecl->getExtendedType();
+            auto selfType = req.getFirstType();
+            auto reqType = req.getSecondType();
+
+            if (ownerType->isExistentialType() && reqType->isEqual(ownerType)) {
+              auto &ctx = extDecl->getASTContext();
+              ctx.Diags.diagnose(extDecl->getLoc(), diag::protocol_extension_redundant_requirement,
+                                 ownerType->getString(), selfType->getString(), reqType->getString());
+            }
+          }
+
           builder->addRequirement(req, reqRepr, source, nullptr,
                                   lookupDC->getParentModule());
           return false;

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 {{requirement of 'Self': 'Foo' is redundant in an extension of 'Foo'}}
+
+protocol Bar {}
+protocol Baz {}
+extension Bar where Self: Baz {} // ok
+extension Bar where Self: Bar, Self: Baz {} // expected-warning {{requirement of 'Self': 'Bar' is redundant in an extension of 'Bar'}}
+
 // ----------------------------------------------------------------------------
 // Protocol extensions with additional requirements
 // ----------------------------------------------------------------------------