[CS] Remove invalid DependentMemberType workaround for tuples

We can produce a hole here now without
regressing diagnostics.

Author: hamishknight

lib/Sema/CSSimplify.cpp

@@ -6807,22 +6807,6 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         }
       }
 
-      // If the left-hand side of a 'sequence element' constraint
-      // is a dependent member type without any type variables it
-      // means that conformance check has been "fixed".
-      // Let's record other side of the conversion as a "hole"
-      // to give the solver a chance to continue and avoid
-      // producing diagnostics for both missing conformance and
-      // invalid element type.
-      if (shouldAttemptFixes()) {
-        if (locator.endsWith<LocatorPathElt::SequenceElementType>() &&
-            desugar1->is<DependentMemberType>() &&
-            !desugar1->hasTypeVariable()) {
-          recordPotentialHole(typeVar2);
-          return getTypeMatchSuccess();
-        }
-      }
-
       return formUnsolvedResult();
     }
 

lib/Sema/ConstraintSystem.cpp

@@ -3926,12 +3926,6 @@ struct TypeSimplifier {
         auto conformance = CS.DC->getParentModule()->lookupConformance(
           lookupBaseType, proto);
         if (!conformance) {
-          // FIXME: This regresses diagnostics if removed, but really the
-          // handling of a missing conformance should be the same for
-          // tuples and non-tuples.
-          if (lookupBaseType->is<TupleType>())
-            return DependentMemberType::get(lookupBaseType, assocType);
-
           // If the base type doesn't conform to the associatedtype's protocol,
           // there will be a missing conformance fix applied in diagnostic mode,
           // so the concrete dependent member type is considered a "hole" in

test/Constraints/static_members_on_protocol_in_generic_context.swift

@@ -177,7 +177,7 @@ test_combo(.genericFn(42)) // expected-error {{global function 'test_combo' requ
 
 extension P { // expected-note 13 {{missing same-type requirement on 'Self'}} {{12-12= where Self == <#Type#>}}
   static func generic<T>(_: T) -> T { fatalError() }
-  static func genericWithReqs<T: Collection, Q>(_: T) -> Q where T.Element == Q { // expected-note {{in call to function 'genericWithReqs'}} expected-note {{required by static method 'genericWithReqs' where 'T' = '()'}}
+  static func genericWithReqs<T: Collection, Q>(_: T) -> Q where T.Element == Q { // expected-note {{required by static method 'genericWithReqs' where 'T' = '()'}}
     fatalError()
   }
 }
@@ -217,7 +217,6 @@ _ = P.genericWithReqs([42])
 _ = P.genericWithReqs(())
 // expected-error@-1 {{type '()' cannot conform to 'Collection'}} expected-note@-1 {{only concrete types such as structs, enums and classes can conform to protocols}}
 // expected-error@-2 {{static member 'genericWithReqs' cannot be used on protocol metatype '(any P).Type'}}
-// expected-error@-3 {{generic parameter 'Q' could not be inferred}}
 _ = P[q: ""]
 // expected-error@-1 {{static member 'subscript' cannot be used on protocol metatype '(any P).Type'}}
 _ = P[q: ""].other