[Sema] Avoid a couple of downstream diags with invalid same-type reqs

Avoid conformance failures and member lookup errors for generic
signatures with invalid same-type requirements.

lib/Sema/TypeCheckProtocol.cpp

@@ -1156,6 +1156,15 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
       getOrCreateRequirementEnvironment(reqEnvCache, dc, reqSig, proto,
                                         covariantSelf, conformance);
 
+  auto reqSubMap = reqEnvironment.getRequirementToWitnessThunkSubs();
+
+  Type selfTy = proto->getSelfInterfaceType().subst(reqSubMap);
+  if (selfTy->hasError()) {
+    return RequirementMatch(witness, MatchKind::WitnessInvalid,
+                            witnessType, reqEnvironment,
+                            /*optionalAdjustments*/ {});
+  }
+
   // Set up the constraint system for matching.
   auto setup =
       [&]() -> std::tuple<std::optional<RequirementMatch>, Type, Type, Type, Type> {
@@ -1163,10 +1172,6 @@ swift::matchWitness(WitnessChecker::RequirementEnvironmentCache &reqEnvCache,
     // the required type and the witness type.
     cs.emplace(dc, ConstraintSystemFlags::AllowFixes);
 
-    auto reqSubMap = reqEnvironment.getRequirementToWitnessThunkSubs();
-
-    Type selfTy = proto->getSelfInterfaceType().subst(reqSubMap);
-
     // Open up the type of the requirement.
     reqLocator =
         cs->getConstraintLocator(req, ConstraintLocator::ProtocolRequirement);

lib/Sema/TypeCheckType.cpp

@@ -235,6 +235,13 @@ Type TypeResolution::resolveDependentMemberType(
     // Record the type we found.
     repr->setValue(nestedType, nullptr);
   } else {
+    // If we have a concrete equivalence to an error type, avoid diagnosing the
+    // missing member.
+    if (auto concreteBase = genericSig->getConcreteType(baseTy)) {
+      if (concreteBase->hasError())
+        return ErrorType::get(baseTy);
+    }
+
     // Resolve the base to a potential archetype.
     // Perform typo correction.
     TypoCorrectionResults corrections(repr->getNameRef(), repr->getNameLoc());

test/Generics/invalid.swift

@@ -154,3 +154,13 @@ func errorMessageVariants(x: X<Int>, x2: X<Bool> = X<Int>()) -> X<Bool> {
   let _: X<Int>.Foo = X<Bool>.Foo() // expected-error {{cannot convert parent type 'X<Bool>' to expected type 'X<Int>'}}
   return x // expected-error {{cannot convert return expression of type 'X<Int>' to return type 'X<Bool>'}}
 }
+
+protocol P2 {
+  func foo()
+}
+
+// Make sure we don't diagnose the conformance failure or 'T.K'.
+struct HasInvalidSameType<T>: P2 where T == Undefined, T.K == Int {
+  // expected-error@-1 {{cannot find type 'Undefined' in scope}}
+  func foo() {}
+}