Sema: Fix doesMemberHaveUnfulfillableConstraintsWithExistentialBase() for generic context

We don't want to pass in the outer generic signature here.

The base type's constraint type is written in terms of archetypes,
and we run generic signature queries against it with types appearing
in the protocol member. Since the protocol member has Self at
depth 0, index 0, prepending the outer generic signature to the
opened existential signature would produce incorrect results.

Author: slavapestov

lib/Sema/ConstraintSystem.cpp

@@ -6023,7 +6023,7 @@ void ConstraintSystem::maybeProduceFallbackDiagnostic(
 /// Because opened archetypes are not part of the surface language, these
 /// constraints render the member inaccessible.
 static bool doesMemberHaveUnfulfillableConstraintsWithExistentialBase(
-    Type baseTy, const ValueDecl *member, const DeclContext *useDC) {
+    Type baseTy, const ValueDecl *member) {
   const auto sig =
       member->getInnermostDeclContext()->getGenericSignatureOfContext();
 
@@ -6060,7 +6060,7 @@ static bool doesMemberHaveUnfulfillableConstraintsWithExistentialBase(
       return Action::Stop;
     }
   } isDependentOnSelfWalker(member->getASTContext().getOpenedArchetypeSignature(
-      baseTy, useDC->getGenericSignatureOfContext()));
+      baseTy, GenericSignature()));
 
   for (const auto &req : sig.getRequirements()) {
     switch (req.getKind()) {
@@ -6121,8 +6121,7 @@ bool ConstraintSystem::isMemberAvailableOnExistential(
   }
 
   if (doesMemberHaveUnfulfillableConstraintsWithExistentialBase(baseTy,
-                                                                member,
-                                                                DC)) {
+                                                                member)) {
     return false;
   }
 

test/decl/protocol/existential_member_accesses_self_assoctype.swift

@@ -560,6 +560,8 @@ extension UnfulfillableGenericRequirements {
                               A: Sequence, A.Element: Sequence,
                               U.A == A.Element.Element {}
   func method7<U>(_: U) where U: UnfulfillableGenericRequirements & Class<Self> {}
+
+  func method8<U>(_: U) where U == Self.A {}
 }
 do {
   let exist: any UnfulfillableGenericRequirements
@@ -579,6 +581,19 @@ do {
   exist.method7(false)
   // expected-error@-1 {{instance method 'method7' requires that 'U' conform to 'UnfulfillableGenericRequirements'}}
   // expected-error@-2 {{member 'method7' cannot be used on value of type 'any UnfulfillableGenericRequirements'; consider using a generic constraint instead}}
+
+  exist.method8(false)
+  // expected-error@-1 {{member 'method8' cannot be used on value of type 'any UnfulfillableGenericRequirements'; consider using a generic constraint instead}}
+}
+
+// Make sure this also works in a generic context!
+struct G<X, Y, Z> {
+  func doIt() {
+    let exist: any UnfulfillableGenericRequirements
+
+    exist.method8(false)
+    // expected-error@-1 {{member 'method8' cannot be used on value of type 'any UnfulfillableGenericRequirements'; consider using a generic constraint instead}}
+  }
 }
 protocol UnfulfillableGenericRequirementsDerived1: UnfulfillableGenericRequirements where A == Bool {}
 protocol UnfulfillableGenericRequirementsDerived2: UnfulfillableGenericRequirements where A == Class<Self> {}