Add Validate(scheme) and use for RemoteSignInScheme not self validation

src/Microsoft.AspNetCore.Authentication/AuthenticationBuilder.cs

@@ -97,10 +97,6 @@ namespace Microsoft.AspNetCore.Authentication
             public void PostConfigure(string name, TOptions options)
             {
                 options.SignInScheme = options.SignInScheme ?? _authOptions.DefaultSignInScheme ?? _authOptions.DefaultScheme;
-                if (string.Equals(options.SignInScheme, name, StringComparison.Ordinal))
-                {
-                    throw new InvalidOperationException(Resources.Exception_RemoteSignInSchemeCannotBeSelf);
-                }
             }
         }
     }

src/Microsoft.AspNetCore.Authentication/AuthenticationHandler.cs

@@ -87,7 +87,7 @@ namespace Microsoft.AspNetCore.Authentication
             Context = context;
 
             Options = OptionsMonitor.Get(Scheme.Name) ?? new TOptions();
-            Options.Validate();
+            Options.Validate(Scheme.Name);
 
             await InitializeEventsAsync();
             await InitializeHandlerAsync();

src/Microsoft.AspNetCore.Authentication/AuthenticationSchemeOptions.cs

@@ -13,9 +13,14 @@ namespace Microsoft.AspNetCore.Authentication
         /// <summary>
         /// Check that the options are valid. Should throw an exception if things are not ok.
         /// </summary>
-        public virtual void Validate()
+        public virtual void Validate() { }
-        {
+
-        }
+        /// <summary>
+        /// Checks that the options are valid for a specific scheme
+        /// </summary>
+        /// <param name="scheme">The scheme being validated.</param>
+        public virtual void Validate(string scheme)
+            => Validate();
 
         /// <summary>
         /// Gets or sets the issuer that should be used for any claims that are created

src/Microsoft.AspNetCore.Authentication/RemoteAuthenticationOptions.cs

@@ -32,6 +32,19 @@ namespace Microsoft.AspNetCore.Authentication
             };
         }
 
+        /// <summary>
+        /// Checks that the options are valid for a specific scheme
+        /// </summary>
+        /// <param name="scheme">The scheme being validated.</param>
+        public override void Validate(string scheme)
+        {
+            base.Validate(scheme);
+            if (string.Equals(scheme, SignInScheme, StringComparison.Ordinal))
+            {
+                throw new InvalidOperationException(Resources.Exception_RemoteSignInSchemeCannotBeSelf);
+            }
+        }
+
         /// <summary>
         /// Check that the options are valid.  Should throw an exception if things are not ok.
         /// </summary>

test/Microsoft.AspNetCore.Authentication.Test/FacebookTests.cs

@@ -29,7 +29,11 @@ namespace Microsoft.AspNetCore.Authentication.Facebook
         {
             var server = CreateServer(
                 app => { },
-                services => services.AddAuthentication().AddFacebook(o => o.SignInScheme = FacebookDefaults.AuthenticationScheme),
+                services => services.AddAuthentication().AddFacebook(o => {
+                    o.AppId = "whatever";
+                    o.AppSecret = "whatever";
+                    o.SignInScheme = FacebookDefaults.AuthenticationScheme;
+                }),
                 context => 
                 {
                     // Gross
@@ -45,7 +49,10 @@ namespace Microsoft.AspNetCore.Authentication.Facebook
         {
             var server = CreateServer(
                 app => { },
-                services => services.AddAuthentication(o => o.DefaultScheme = FacebookDefaults.AuthenticationScheme).AddFacebook(),
+                services => services.AddAuthentication(o => o.DefaultScheme = FacebookDefaults.AuthenticationScheme).AddFacebook(o => {
+                    o.AppId = "whatever";
+                    o.AppSecret = "whatever";
+                }),
                 context =>
                 {
                     // Gross
@@ -61,7 +68,10 @@ namespace Microsoft.AspNetCore.Authentication.Facebook
         {
             var server = CreateServer(
                 app => { },
-                services => services.AddAuthentication(o => o.DefaultSignInScheme = FacebookDefaults.AuthenticationScheme).AddFacebook(),
+                services => services.AddAuthentication(o => o.DefaultSignInScheme = FacebookDefaults.AuthenticationScheme).AddFacebook(o => {
+                    o.AppId = "whatever";
+                    o.AppSecret = "whatever";
+                }),
                 context =>
                 {
                     // Gross

test/Microsoft.AspNetCore.Authentication.Test/OAuthTests.cs

@@ -27,6 +27,9 @@ namespace Microsoft.AspNetCore.Authentication.OAuth
                     o.SignInScheme = "weeblie";
                     o.ClientId = "whatever";
                     o.ClientSecret = "whatever";
+                    o.CallbackPath = "/whatever";
+                    o.AuthorizationEndpoint = "/whatever";
+                    o.TokenEndpoint = "/whatever";
                 }));
             var error = await Assert.ThrowsAsync<InvalidOperationException>(() => server.SendAsync("https://example.com/"));
             Assert.Contains("cannot be set to itself", error.Message);