Sema: Mark type witness candidates nonviable when contextual requirements are incompatible

Author: AnthonyLatsis

lib/Sema/TypeCheckProtocol.cpp

@@ -3795,6 +3795,20 @@ ResolveWitnessResult ConformanceChecker::resolveTypeWitnessViaLookup(
       if (typeAliasDecl->getDeclaredInterfaceType()->is<UnboundGenericType>())
         continue;
 
+    // A candidate is nonviable if its context has requirements not
+    // satisfied by those of the confomance context.
+    {
+      const auto candidateCtxSig = candidate.Member->getDeclContext()
+          ->getGenericSignatureOfContext();
+      const auto conformanceCtxSig = DC->getGenericSignatureOfContext();
+
+      if(candidateCtxSig && conformanceCtxSig &&
+         !candidateCtxSig->requirementsNotSatisfiedBy(
+             conformanceCtxSig).empty()) {
+        continue;
+      }
+    }
+
     // Check this type against the protocol requirements.
     if (auto checkResult =
             checkTypeWitness(DC, Proto, assocType, candidate.MemberType)) {

test/Generics/conditional_conformances.swift

@@ -433,7 +433,7 @@ func sr_10992_bar(_ fn: (SR_10992_P) -> Void) {
 
 // SR-7516
 
-protocol SR_7516_P1 { associatedtype X = Void } // expected-note {{protocol requires nested type 'X'; do you want to add it?}}
+protocol SR_7516_P1 { associatedtype X = Void } // expected-note 4{{protocol requires nested type 'X'; do you want to add it?}}
 protocol SR_7516_P2 { associatedtype X = Bool }
 protocol SR_7516_P3 { associatedtype X = Void }
 protocol SR_7516_P4 { associatedtype X } // expected-note {{protocol requires nested type 'X'; do you want to add it?}}
@@ -514,3 +514,19 @@ extension SR_7516_S5: SR_7516_P1 {
 // DEFAULT_TYPE_WITNESS-NEXT: internal typealias X = Void
 }
 extension SR_7516_S5: SR_7516_P4 where T: Equatable {}
+
+struct SR_7516_S6<T> {}
+extension SR_7516_S6: SR_7516_P2 where T == Void {}
+extension SR_7516_S6: SR_7516_P1 where T == Never { // expected-error {{type 'SR_7516_S6<T>' does not conform to protocol 'SR_7516_P1'}}
+}
+
+struct SR_7516_S7<T> {}
+extension SR_7516_S7: SR_7516_P1 where T == Void {} // expected-error {{type 'SR_7516_S7<T>' does not conform to protocol 'SR_7516_P1'}}
+extension SR_7516_S7: SR_7516_P2 where T == Never {
+  typealias X = Bool
+}
+
+struct SR_7516_S8<T>: SR_7516_P1 {} // expected-error {{type 'SR_7516_S8<T>' does not conform to protocol 'SR_7516_P1'}}
+extension SR_7516_S8: SR_7516_P2 where T == Never {
+  typealias X = Bool
+}