AST: Fix bug in subclass composition minimization

Now that opened archetype signatures are generalized to encompass outer generic
contexts, the number of requirements in the generic signature is not always
indicative of whether the minimal type is also a composition.

Author: AnthonyLatsis

lib/AST/Type.cpp

@@ -4146,6 +4146,12 @@ CanType ProtocolCompositionType::getMinimalCanonicalType(
   if (superclass)
     MinimalMembers.insert(MinimalMembers.begin(), superclass->getCanonicalType());
 
+  // 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(