Added test to verify intellisense on Enumerable.Join parameter types

src/Compilers/CSharp/Test/Semantic/Semantics/MethodTypeInferenceTests.cs

@@ -845,5 +845,131 @@ void Test(C<char> c)
             Assert.Equal(SpecialType.System_Char, method.TypeArguments.Single().SpecialType);
             Assert.Equal("void C<System.Char>.M<System.Char>(System.Func<System.Char, System.Char> f1, System.Func<System.Int64, System.Char> f2, params System.Int32[] a)", method.ToTestDisplayString());
         }
+
+        [WorkItem(8712, "https://github.com/dotnet/roslyn/issues/8712")]
+        [Fact]
+        public void EnumerableJoinIntellisenseForParameterTypesShouldPopOutAutoComplete_1()
+        {
+            var source = @"
+using System.Collections.Generic;
+using System.Linq;
+
+public class Book
+{
+    public int AuthorId { get; set; }
+    public string Title { get; set; }
+}
+
+public class Author
+{
+    public int Id { get; set; }
+    public string Name { get; set; }
+}
+
+public class Test
+{
+    public static void NoIntellisenseInEnumerableJoin()
+    {
+        IEnumerable<Book> books = null;
+        IEnumerable<Author> authors = null;
+
+        var test = books.Join(authors, b => b.    // !!Fails here!!
+    }
+}";
+
+            var compilation = CreateCSharpCompilation(source);
+            var tree = compilation.SyntaxTrees.Single();
+            var model = compilation.GetSemanticModel(tree);
+
+            var book = (IdentifierNameSyntax)tree.GetRoot().DescendantTokens().Last(t => t.Text == "b").Parent;
+            var bookType = model.GetTypeInfo(book).Type;
+
+            Assert.Equal("Book", bookType.Name);
+        }
+
+        [WorkItem(8712, "https://github.com/dotnet/roslyn/issues/8712")]
+        [Fact]
+        public void EnumerableJoinIntellisenseForParameterTypesShouldPopOutAutoComplete_2()
+        {
+            var source = @"
+using System.Collections.Generic;
+using System.Linq;
+
+public class Book
+{
+    public int AuthorId { get; set; }
+    public string Title { get; set; }
+}
+
+public class Author
+{
+    public int Id { get; set; }
+    public string Name { get; set; }
+}
+
+public class Test
+{
+    public static void NoIntellisenseInEnumerableJoin()
+    {
+        IEnumerable<Book> books = null;
+        IEnumerable<Author> authors = null;
+
+        var test = books.Join(authors, b => b.AuthorId, a => a.    // !!Fails here!!
+    }
+}";
+
+            var compilation = CreateCSharpCompilation(source);
+            var tree = compilation.SyntaxTrees.Single();
+            var model = compilation.GetSemanticModel(tree);
+
+            var author = (IdentifierNameSyntax)tree.GetRoot().DescendantTokens().Last(t => t.Text == "a").Parent;
+            var authorType = model.GetTypeInfo(author).Type;
+
+            Assert.Equal("Author", authorType.Name);
+        }
+
+        [WorkItem(8712, "https://github.com/dotnet/roslyn/issues/8712")]
+        [Fact]
+        public void EnumerableJoinIntellisenseForParameterTypesShouldPopOutAutoComplete_3()
+        {
+            var source = @"
+using System.Collections.Generic;
+using System.Linq;
+
+public class Book
+{
+    public int AuthorId { get; set; }
+    public string Title { get; set; }
+}
+
+public class Author
+{
+    public int Id { get; set; }
+    public string Name { get; set; }
+}
+
+public class Test
+{
+    public static void NoIntellisenseInEnumerableJoin()
+    {
+        IEnumerable<Book> books = null;
+        IEnumerable<Author> authors = null;
+
+        var test = books.Join(authors, b => b.AuthorId, a => a.Id, (bookResult, authorResult) => new { bookResult, authorResult });
+    }
+}";
+
+            var compilation = CreateCSharpCompilation(source).VerifyDiagnostics();
+            var tree = compilation.SyntaxTrees.Single();
+            var model = compilation.GetSemanticModel(tree);
+
+            var bookResult = (IdentifierNameSyntax)tree.GetRoot().DescendantTokens().Last(t => t.Text == "bookResult").Parent;
+            var bookResultType = model.GetTypeInfo(bookResult).Type;
+            Assert.Equal("Book", bookResultType.Name);
+
+            var authorResult = (IdentifierNameSyntax)tree.GetRoot().DescendantTokens().Last(t => t.Text == "authorResult").Parent;
+            var authorResultType = model.GetTypeInfo(authorResult).Type;
+            Assert.Equal("Author", authorResultType.Name);
+        }
     }
 }