Sema: Generic parameter packs cannot witness associated type requirements

Author: slavapestov

lib/Sema/TypeCheckProtocolInference.cpp

@@ -923,6 +923,10 @@ AssociatedTypeInference::computeAbstractTypeWitness(
   // If there is a generic parameter of the named type, use that.
   if (auto genericSig = dc->getGenericSignatureOfContext()) {
     for (auto gp : genericSig.getInnermostGenericParams()) {
+      // Packs cannot witness associated type requirements.
+      if (gp->isParameterPack())
+        continue;
+
       if (gp->getName() == assocType->getName())
         return AbstractTypeWitness(assocType, gp);
     }

test/decl/protocol/conforms/variadic_generic_type.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature VariadicGenerics
+
+// Because of -enable-experimental-feature VariadicGenerics
+// REQUIRES: asserts
+
+// Generic parameter packs cannot witness associated type requirements
+protocol HasAssoc {
+  associatedtype A
+  // expected-note@-1 {{protocol requires nested type 'A'; do you want to add it?}}
+}
+
+struct HasPack<each A>: HasAssoc {}
+// expected-error@-1 {{type 'HasPack<repeat each A>' does not conform to protocol 'HasAssoc'}}
+
+protocol P {}
+
+protocol HasPackRequirements {
+  func doStuff1<each U>(_ value: repeat each U) -> (repeat Array<each U>)
+  func doStuff2<each U>(_ value: repeat each U) -> (repeat Array<each U>)
+}
+
+extension HasPackRequirements {
+  func doStuff1<each U>(_ value: repeat each U) -> (repeat Array<each U>) {
+    return (repeat [each value])
+  }
+}
+
+struct ConformsPackRequirements<each T>: HasPackRequirements {
+  func doStuff2<each U>(_ value: repeat each U) -> (repeat Array<each U>) {
+    return (repeat [each value])
+  }
+}
+
+