Merge pull request #58554 from AnthonyLatsis/min-comp-fix

AST: Fix bug in subclass composition minimization

Author: AnthonyLatsis

lib/AST/Type.cpp

@@ -4115,7 +4115,6 @@ CanType ProtocolCompositionType::getMinimalCanonicalType(
     return Reqs.front().getSecondType()->getCanonicalType();
   }
 
-  Type superclass;
   llvm::SmallVector<Type, 2> MinimalMembers;
   bool MinimalHasExplicitAnyObject = false;
   auto ifaceTy = Sig.getGenericParams().back();
@@ -4126,10 +4125,6 @@ CanType ProtocolCompositionType::getMinimalCanonicalType(
 
     switch (Req.getKind()) {
     case RequirementKind::Superclass:
-      assert((!superclass || superclass->isEqual(Req.getSecondType()))
-             && "Multiple distinct superclass constraints!");
-      superclass = Req.getSecondType();
-      break;
     case RequirementKind::Conformance:
       MinimalMembers.push_back(Req.getSecondType());
       break;
@@ -4141,10 +4136,16 @@ CanType ProtocolCompositionType::getMinimalCanonicalType(
     }
   }
 
-  // Ensure superclass bounds appear first regardless of their order among
-  // the signature's requirements.
-  if (superclass)
-    MinimalMembers.insert(MinimalMembers.begin(), superclass->getCanonicalType());
+  // A superclass constraint is always retained and must appear first in the
+  // members list.
+  assert(Composition->getMembers().front()->getClassOrBoundGenericClass() ==
+         MinimalMembers.front()->getClassOrBoundGenericClass());
+
+  // If we are left with a single member and no layout constraint, the member
+  // is the minimal type. Also, note that a protocol composition cannot be
+  // constructed with a single member unless there is a layout constraint.
+  if (MinimalMembers.size() == 1 && !MinimalHasExplicitAnyObject)
+    return CanType(MinimalMembers.front());
 
   // The resulting composition is necessarily canonical.
   return CanType(build(Ctx, MinimalMembers, MinimalHasExplicitAnyObject));

test/type/subclass_composition.swift

@@ -42,6 +42,8 @@ typealias OtherAndP2 = Other & P2
 
 protocol P3 : class {}
 
+protocol P4 {}
+
 struct Unrelated {}
 
 //
@@ -70,6 +72,18 @@ func alreadyConforms(_: P3 & AnyObject) {} // expected-error {{invalid redeclara
 func alreadyConformsMeta(_: P3.Type) {} // expected-note {{'alreadyConformsMeta' previously declared here}}
 func alreadyConformsMeta(_: (P3 & AnyObject).Type) {} // expected-error {{invalid redeclaration of 'alreadyConformsMeta'}}
 
+func notARedeclaration(_: P4) {}
+func notARedeclaration(_: P4 & AnyObject) {}
+
+do {
+  class C: P4 {}
+  struct S<T: P4> {
+    // Don't crash when computing minimal compositions inside a generic context.
+    func redeclaration(_: C & P4) {} // expected-note {{'redeclaration' previously declared here}}
+    func redeclaration(_: C & P4) {} // expected-error {{invalid redeclaration of 'redeclaration'}}
+  }
+}
+
 // SE-0156 stipulates that a composition can contain multiple classes, as long
 // as they are all the same.
 func basicDiagnostics(