Add some nullability tests for Generate Type and fix one bug

Closes dotnet#30316

src/EditorFeatures/CSharpTest/Diagnostics/GenerateType/GenerateTypeTests.cs

@@ -444,6 +444,27 @@ private class Goo
 index: 2);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
+        public async Task TestGenerateClassFromNullableFieldDeclarationIntoSameType()
+        {
+            await TestInRegularAndScriptAsync(
+@"#nullable enable
+class Class
+{
+    [|Goo?|] f;
+}",
+@"#nullable enable
+class Class
+{
+    Goo? f;
+
+    private class Goo
+    {
+    }
+}",
+index: 2);
+        }
+
         [WpfFact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
         public async Task TestGenerateClassFromFieldDeclarationIntoGlobalNamespace()
         {
@@ -1385,6 +1406,78 @@ public T(int v1, string v2)
 index: 1);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
+        public async Task GenerateWithNullableParameter()
+        {
+            await TestInRegularAndScriptAsync(
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        string? s = null;
+        new [|T|](s);
+    }
+}",
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        string? s = null;
+        new [|T|](s);
+    }
+}
+
+internal class T
+{
+    private string? s;
+
+    public T(string? s)
+    {
+        this.s = s;
+    }
+}",
+parseOptions: TestOptions.Regular8WithNullableAnalysis,
+index: 1);
+        }
+
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
+        public async Task GenerateWithNullableParameterThatIsNotNull()
+        {
+            await TestInRegularAndScriptAsync(
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        string? s = ""asdf"";
+        new [|T|](s);
+    }
+}",
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        string? s = ""asdf"";
+        new [|T|](s);
+    }
+}
+
+internal class T
+{
+    private string s;
+
+    public T(string s)
+    {
+        this.s = s;
+    }
+}",
+parseOptions: TestOptions.Regular8WithNullableAnalysis,
+index: 1);
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
         public async Task GenerateWithNamedParameter()
         {
@@ -2109,6 +2202,41 @@ protected Base(out int o)
 index: 1);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
+        public async Task GenerateWithDelegatingConstructorAssigningToNullableField()
+        {
+            await TestInRegularAndScriptAsync(
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        Base? b = new [|T|]();
+    }
+}
+
+class Base
+{
+}",
+@"#nullable enable
+class Class
+{
+    void M()
+    {
+        Base? b = new [|T|]();
+    }
+}
+
+internal class T : Base
+{
+}
+
+class Base
+{
+}",
+index: 1);
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsGenerateType)]
         public async Task GenerateWithNonDelegatingConstructor1()
         {

src/Features/Core/Portable/GenerateType/AbstractGenerateTypeService.State.cs

@@ -245,7 +245,9 @@ private void SetBaseType(INamedTypeSymbol baseType)
                     return;
                 }
 
-                this.BaseTypeOrInterfaceOpt = baseType;
+                // Strip off top-level nullability since we can't put top-level nullability into the base list. We will still include nested nullability
+                // if you're deriving some interface like IEnumerable<string?>.
+                this.BaseTypeOrInterfaceOpt = baseType.WithNullability(NullableAnnotation.NotApplicable);
             }
 
             private bool GenerateStruct(TService service, SemanticModel semanticModel, CancellationToken cancellationToken)