Extension type. Issue 54548. Don't consider Never? to be a bottom type.

Bug: #54548
Change-Id: I56b734c24a84c8188983605eca69de28869b6de0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/345440
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/element/type.dart

@@ -1436,6 +1436,9 @@ class TypeParameterTypeImpl extends TypeImpl implements TypeParameterType {
     Set<TypeParameterElement> seenTypes = {};
     TypeParameterType type = this;
     while (seenTypes.add(type.element)) {
+      if (type.nullabilitySuffix == NullabilitySuffix.question) {
+        return false;
+      }
       var bound = type.bound;
       if (bound is TypeParameterType) {
         type = bound;

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

@@ -982,11 +982,15 @@ class TypeSystemImpl implements TypeSystem {
 
   /// Return `true`  for things in the equivalence class of `Never`.
   bool isBottom(DartType type) {
+    if (type.nullabilitySuffix == NullabilitySuffix.question) {
+      assert(!type.isBottom);
+      return false;
+    }
+
     // BOTTOM(Never) is true
     if (type is NeverType) {
-      var result = type.nullabilitySuffix != NullabilitySuffix.question;
-      assert(type.isBottom == result);
-      return result;
+      assert(type.isBottom);
+      return true;
     }
 
     // BOTTOM(X&T) is true iff BOTTOM(T)

pkg/analyzer/test/src/dart/element/upper_lower_bound_test.dart

@@ -114,7 +114,7 @@ class BoundsHelperPredicatesTest extends _BoundsTestBase {
     T = typeParameter('T', bound: objectQuestion);
 
     isBottom(promotedTypeParameterTypeNone(T, neverNone));
-    isBottom(promotedTypeParameterTypeQuestion(T, neverNone));
+    isNotBottom(promotedTypeParameterTypeQuestion(T, neverNone));
     isBottom(promotedTypeParameterTypeStar(T, neverNone));
 
     isNotBottom(promotedTypeParameterTypeNone(T, neverQuestion));
@@ -124,7 +124,7 @@ class BoundsHelperPredicatesTest extends _BoundsTestBase {
     // BOTTOM(X extends T) is true iff BOTTOM(T)
     T = typeParameter('T', bound: neverNone);
     isBottom(typeParameterTypeNone(T));
-    isBottom(typeParameterTypeQuestion(T));
+    isNotBottom(typeParameterTypeQuestion(T));
     isBottom(typeParameterTypeStar(T));
 
     T = typeParameter('T', bound: neverQuestion);

pkg/analyzer/test/src/diagnostics/extension_type_representation_type_bottom_test.dart

@@ -24,4 +24,37 @@ extension type A(Never it) {}
           5),
     ]);
   }
+
+  test_neverQuestion() async {
+    await assertNoErrorsInCode('''
+extension type A(Never? it) {}
+''');
+  }
+
+  test_typeParameter_never_none() async {
+    await assertErrorsInCode('''
+extension type A<T extends Never>(T it) {}
+''', [
+      error(CompileTimeErrorCode.EXTENSION_TYPE_REPRESENTATION_TYPE_BOTTOM, 34,
+          1),
+    ]);
+  }
+
+  test_typeParameter_never_question() async {
+    await assertNoErrorsInCode('''
+extension type A<T extends Never>(T? it) {}
+''');
+  }
+
+  test_typeParameter_never_question2() async {
+    await assertNoErrorsInCode('''
+extension type A<T extends Never, S extends T>(S? it) {}
+''');
+  }
+
+  test_typeParameter_never_question3() async {
+    await assertNoErrorsInCode('''
+extension type A<T extends Never, S extends T?>(S it) {}
+''');
+  }
 }