Sema: Implement missing part of SE-0346

The proposal states that this should work, but this was never
implemented:

    protocol P<A> {
      associatedtype A
    }

    struct S: P<Int> {}

- Fixes #62906.
- Fixes rdar://91842338.

Author: slavapestov

include/swift/AST/DiagnosticsSema.def

@@ -3616,8 +3616,6 @@ ERROR(inheritance_from_non_protocol,none,
   "inheritance from non-protocol type %0", (Type))
 ERROR(inheritance_from_anyobject,none,
   "only protocols can inherit from 'AnyObject'", ())
-ERROR(inheritance_from_parameterized_protocol,none,
-  "cannot inherit from protocol type with generic argument %0", (Type))
 ERROR(superclass_not_first,none,
   "superclass %0 must appear first in the inheritance clause", (Type))
 ERROR(superclass_not_open,none,

lib/Sema/AssociatedTypeInference.cpp

@@ -439,6 +439,28 @@ static bool isAsyncSequenceFailure(AssociatedTypeDecl *assocType) {
   return assocType->getName() == assocType->getASTContext().Id_Failure;
 }
 
+static Type resolveTypeWitnessViaParameterizedProtocol(
+    Type t, AssociatedTypeDecl *assocType) {
+  if (auto *pct = t->getAs<ProtocolCompositionType>()) {
+    for (auto member : pct->getMembers()) {
+      if (auto result = resolveTypeWitnessViaParameterizedProtocol(
+            member, assocType)) {
+        return result;
+      }
+    }
+  } else if (auto *ppt = t->getAs<ParameterizedProtocolType>()) {
+    auto *proto = ppt->getProtocol();
+    unsigned i = 0;
+    for (auto *otherAssocType : proto->getPrimaryAssociatedTypes()) {
+      if (otherAssocType->getName() == assocType->getName())
+        return ppt->getArgs()[i];
+      ++i;
+    }
+  }
+
+  return Type();
+}
+
 /// Attempt to resolve a type witness via member name lookup.
 static ResolveWitnessResult resolveTypeWitnessViaLookup(
                        NormalProtocolConformance *conformance,
@@ -455,6 +477,23 @@ static ResolveWitnessResult resolveTypeWitnessViaLookup(
     abort();
   }
 
+  // Look for a parameterized protocol type in the conformance context's
+  // inheritance clause.
+  bool deducedFromParameterizedProtocolType = false;
+  auto inherited = (isa<NominalTypeDecl>(dc)
+                    ? cast<NominalTypeDecl>(dc)->getInherited()
+                    : cast<ExtensionDecl>(dc)->getInherited());
+  for (auto index : inherited.getIndices()) {
+    if (auto inheritedTy = inherited.getResolvedType(index)) {
+      if (auto typeWitness = resolveTypeWitnessViaParameterizedProtocol(
+            inheritedTy, assocType)) {
+        recordTypeWitness(conformance, assocType, typeWitness, nullptr);
+        deducedFromParameterizedProtocolType = true;
+      }
+    }
+  }
+
+  // Next, look for a member type declaration with this name.
   NLOptions subOptions = (NL_QualifiedDefault | NL_OnlyTypes |
                           NL_ProtocolMembers | NL_IncludeAttributeImplements);
 
@@ -561,27 +600,40 @@ static ResolveWitnessResult resolveTypeWitnessViaLookup(
     }
   }
 
-  // If there are no viable witnesses, and all nonviable candidates came from
-  // protocol extensions, treat this as "missing".
-  if (viable.empty() &&
-      std::find_if(nonViable.begin(), nonViable.end(),
-                   [](const std::pair<TypeDecl *, CheckTypeWitnessResult> &x) {
-                     return x.first->getDeclContext()
-                        ->getSelfProtocolDecl() == nullptr;
-                   }) == nonViable.end())
-    return ResolveWitnessResult::Missing;
+  if (!deducedFromParameterizedProtocolType) {
+    // If there are no viable witnesses, and all nonviable candidates came from
+    // protocol extensions, treat this as "missing".
+    if (viable.empty() &&
+        std::find_if(nonViable.begin(), nonViable.end(),
+                     [](const std::pair<TypeDecl *, CheckTypeWitnessResult> &x) {
+                       return x.first->getDeclContext()
+                          ->getSelfProtocolDecl() == nullptr;
+                     }) == nonViable.end())
+      return ResolveWitnessResult::Missing;
+
+    // If there is a single viable candidate, form a substitution for it.
+    if (viable.size() == 1) {
+      auto interfaceType = viable.front().MemberType;
+      recordTypeWitness(conformance, assocType, interfaceType,
+                        viable.front().Member);
+      return ResolveWitnessResult::Success;
+    }
 
-  // If there is a single viable candidate, form a substitution for it.
-  if (viable.size() == 1) {
-    auto interfaceType = viable.front().MemberType;
-    recordTypeWitness(conformance, assocType, interfaceType,
-                      viable.front().Member);
-    return ResolveWitnessResult::Success;
-  }
+    // Record an error.
+    recordTypeWitness(conformance, assocType,
+                      ErrorType::get(ctx), nullptr);
+  } else {
+    // We deduced the type witness from a parameterized protocol type, so just
+    // make sure there was nothing else.
+    if (viable.size() == 1 &&
+        isa<TypeAliasDecl>(viable[0].Member) &&
+        viable[0].Member->isSynthesized()) {
+      // We found the type alias synthesized above.
+      return ResolveWitnessResult::Success;
+    }
 
-  // Record an error.
-  recordTypeWitness(conformance, assocType,
-                    ErrorType::get(ctx), nullptr);
+    // Otherwise fall through.
+  }
 
   // If we had multiple viable types, diagnose the ambiguity.
   if (!viable.empty()) {

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -78,21 +78,6 @@ using namespace swift;
 
 #define DEBUG_TYPE "TypeCheckDeclPrimary"
 
-static Type containsParameterizedProtocolType(Type inheritedTy) {
-  if (inheritedTy->is<ParameterizedProtocolType>()) {
-    return inheritedTy;
-  }
-
-  if (auto *compositionTy = inheritedTy->getAs<ProtocolCompositionType>()) {
-    for (auto memberTy : compositionTy->getMembers()) {
-      if (auto paramTy = containsParameterizedProtocolType(memberTy))
-        return paramTy;
-    }
-  }
-
-  return Type();
-}
-
 class CheckRepressions {
   llvm::PointerUnion<const TypeDecl *, const ExtensionDecl *> declUnion;
   ASTContext &ctx;
@@ -303,14 +288,6 @@ static void checkInheritanceClause(
       inheritedAnyObject = { i, inherited.getSourceRange() };
     }
 
-    if (auto paramTy = containsParameterizedProtocolType(inheritedTy)) {
-      if (!isa<ProtocolDecl>(decl)) {
-        decl->diagnose(diag::inheritance_from_parameterized_protocol,
-                       paramTy);
-      }
-      continue;
-    }
-
     if (inheritedTy->isConstraintType()) {
       auto layout = inheritedTy->getExistentialLayout();
 

test/AssociatedTypeInference/type_witness_from_parameterized_protocol.swift

@@ -0,0 +1,56 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P<A, B> {
+  associatedtype A  // expected-note {{multiple matching types named 'A'}}
+  associatedtype B
+
+  func a(_: A)
+}
+
+extension P {
+  func a(_: A) {}
+}
+
+struct S: P<Int, String> {}
+
+func f(_: Int.Type) {}
+func g(_: String.Type) {}
+
+f(S.A.self)
+g(S.B.self)
+
+struct G<T, U>: P<T, U> {}
+
+f(G<Int, String>.A.self)
+g(G<Int, String>.B.self)
+
+protocol Q<B> {
+  associatedtype B  // expected-note {{protocol requires nested type 'B'}}
+}
+
+struct S2: P, Q<String> {
+  func a(_: Int) {}
+}
+
+struct S3: P & Q<String> {
+  func a(_: Int) {}
+}
+
+protocol R {}
+
+struct S4: (P & Q<String>) & R {
+  func a(_: Int) {}
+}
+
+struct Bad: P<Int, Float> { // expected-error {{type 'Bad' does not conform to protocol 'P'}}
+  typealias A = String  // expected-note {{possibly intended match}}
+}
+
+let x = Bad.A.self
+g(x)
+
+struct Circle: Q<Circle.A> {}
+// expected-note@-1 {{'Circle' declared here}}
+// expected-error@-2 {{type 'Circle' does not conform to protocol 'Q'}}
+// expected-note@-3 {{add stubs for conformance}}
+// expected-error@-4 {{'A' is not a member type of struct 'type_witness_from_parameterized_protocol.Circle'}}

test/Generics/rdar94848868.swift

@@ -8,7 +8,6 @@ protocol MyCollectionProtocol: Collection where Iterator == MyCollectionIterator
 struct MyCollectionIterator<MyCollection: MyCollectionProtocol>: IteratorProtocol {
 // expected-note@-1 3{{through reference here}}
     mutating func next() -> MyCollection.Element? {
-// expected-note@-1 2{{through reference here}}
         return nil
     }
 }

test/type/parameterized_protocol.swift

@@ -31,7 +31,6 @@ protocol Invalid5<Element, Element> {
 
 protocol Sequence<Element> {
   associatedtype Element
-  // expected-note@-1 2{{protocol requires nested type 'Element'}}
 }
 
 extension Sequence {
@@ -56,14 +55,6 @@ struct ConcreteEquatableSequence<Element : Equatable> : EquatableSequence {}
 protocol IntSequence : Sequence<Int> {}
 
 
-/// Concrete types cannot inherit from a parameterized protocol
-
-struct SillyStruct : Sequence<Int> {}
-// expected-error@-1 {{cannot inherit from protocol type with generic argument 'Sequence<Int>'}}
-// expected-error@-2 {{type 'SillyStruct' does not conform to protocol 'Sequence'}}
-// expected-note@-3 {{add stubs for conformance}}
-
-
 /// Parameterized protocol in generic parameter inheritance clause
 
 // CHECK-LABEL: parameterized_protocol.(file).IntSequenceWrapper@
@@ -213,12 +204,6 @@ protocol TestCompositionProtocol1 {
   associatedtype S : Sequence<Int> & Sendable
 }
 
-struct TestStructComposition : Sequence<Int> & Sendable {}
-// expected-error@-1 {{cannot inherit from protocol type with generic argument 'Sequence<Int>'}}
-// expected-error@-2 {{type 'TestStructComposition' does not conform to protocol 'Sequence'}}
-// expected-note@-3 {{add stubs for conformance}}
-
-
 /// Conflicts
 
 protocol Pair<X, Y> where Self.X == Self.Y {

validation-test/compiler_crashers_2_fixed/0161-issue-49119.swift

@@ -9,10 +9,9 @@ protocol P {
 struct Type<Param> {}
 extension Type: P where Param: P, Param.A == Type<Param> {
   // expected-error@-1 {{extension of generic struct 'Type' has self-referential generic requirements}}
-  // expected-note@-2 {{through reference here}}
+  // expected-note@-2 2 {{through reference here}}
   // expected-error@-3 {{type 'Type<Param>' does not conform to protocol 'P'}}
   // expected-note@-4 {{add stubs for conformance}}
   typealias A = Param
-  // expected-note@-1 3{{through reference here}}
-  // expected-note@-2 {{possibly intended match 'Type<Param>.A' (aka 'Param') does not conform to 'P'}}
+  // expected-note@-1 {{possibly intended match 'Type<Param>.A' (aka 'Param') does not conform to 'P'}}
 }