treat void as a top type, analyzer fix for #28763

pkg/analyzer/lib/src/generated/type_system.dart

Index: pkg/analyzer/lib/src/generated/type_system.dart
diff --git a/pkg/analyzer/lib/src/generated/type_system.dart b/pkg/analyzer/lib/src/generated/type_system.dart
index 1e1d1824caf386948df9cb9b1832c51cb8801a16..50eca45ae4bced7437c0fac9e76124fda427b720 100644
--- a/pkg/analyzer/lib/src/generated/type_system.dart
+++ b/pkg/analyzer/lib/src/generated/type_system.dart
@@ -30,7 +30,7 @@ bool _isTop(DartType t, {bool dynamicIsBottom: false}) {
   if (t.isDartAsyncFutureOr) {
     return _isTop((t as InterfaceType).typeArguments[0]);
   }
-  return (t.isDynamic && !dynamicIsBottom) || t.isObject;
+  return (t.isDynamic && !dynamicIsBottom) || t.isObject || t.isVoid;
 }
 
 typedef bool _GuardedSubtypeChecker<T>(T t1, T t2, Set<Element> visited);
@@ -136,17 +136,6 @@ class StrongTypeSystemImpl extends TypeSystem {
       return type1;
     }
 
-    // Treat void as top-like for GLB. This only comes into play with the
-    // return types of two functions whose GLB is being taken. We allow a
-    // non-void-returning function to subtype a void-returning one, so match
-    // that logic here by treating the non-void arm as the subtype for GLB.
-    if (type1.isVoid) {
-      return type2;
-    }
-    if (type2.isVoid) {
-      return type1;
-    }
-
     // Function types have structural GLB.
     if (type1 is FunctionType && type2 is FunctionType) {
       return _functionGreatestLowerBound(type1, type2);
@@ -889,16 +878,6 @@ class StrongTypeSystemImpl extends TypeSystem {
       return false;
     }
 
-    // Void only appears as the return type of a function, and we handle it
-    // directly in the function subtype rules. We should not get to a point
-    // where we're doing a subtype test on a "bare" void, but just in case we
-    // do, handle it safely.
-    // TODO(rnystrom): Determine how this can ever be reached. If it can't,
-    // remove it.
-    if (t1.isVoid || t2.isVoid) {
-      return t1.isVoid && t2.isVoid;
-    }
-
     // We've eliminated void, dynamic, bottom, type parameters, and FutureOr.
     // The only cases are the combinations of interface type and function type.
 
@@ -1036,14 +1015,6 @@ abstract class TypeSystem {
         _isBottom(type1, dynamicIsBottom: dynamicIsBottom)) {
       return type2;
     }
-    // The least upper bound of void and any type T != dynamic is void.
-    if (type1.isVoid) {
-      return type1;
-    }
-    if (type2.isVoid) {
-      return type2;
-    }
-
     if (type1 is TypeParameterType || type2 is TypeParameterType) {
       return _typeParameterLeastUpperBound(type1, type2);
     }