Fix custom Uri parser edge-case when UriKind.Relative is used

src/libraries/System.Private.Uri/src/System/UriExt.cs

@@ -119,12 +119,6 @@ private void CreateThis(string? uri, bool dontEscape, UriKind uriKind, in UriCre
 
                     return GetException(err);
                 }
-
-                if (uriKind == UriKind.Relative)
-                {
-                    // Here we know that we can create an absolute Uri, but the user has requested only a relative one
-                    return GetException(ParsingError.CannotCreateRelative);
-                }
             }
             else
             {
@@ -148,12 +142,12 @@ private void CreateThis(string? uri, bool dontEscape, UriKind uriKind, in UriCre
                     // we use = here to clear all parsing flags for a uri that we think is invalid.
                     _flags = Flags.UserDrivenParsing | (_flags & Flags.UserEscaped);
                 }
-                else if (uriKind == UriKind.Relative)
-                {
-                    // Here we know that custom parser can create an absolute Uri, but the user has requested only a
-                    // relative one
-                    return GetException(ParsingError.CannotCreateRelative);
-                }
+            }
+
+            if (uriKind == UriKind.Relative)
+            {
+                // Here we know that we can create an absolute Uri, but the user has requested only a relative one
+                return GetException(ParsingError.CannotCreateRelative);
             }
 
             if (hasUnicode)

src/libraries/System.Private.Uri/tests/FunctionalTests/UriCreationOptionsTest.cs

@@ -287,8 +287,5 @@ public void DisableCanonicalization_GetComponentsThrowsForPathAndQuery(UriFormat
             Assert.Throws<InvalidOperationException>(() => uri.GetComponents(UriComponents.PathAndQuery, format));
             Assert.Throws<InvalidOperationException>(() => uri.GetComponents(UriComponents.AbsoluteUri, format));
         }
-
-
-        private sealed class CustomUriParser : UriParser { }
     }
 }

src/libraries/System.Private.Uri/tests/FunctionalTests/UriParserTest.cs

@@ -576,5 +576,50 @@ public static void NetTcpStyleUriParser_ctor()
             new NetTcpStyleUriParser();
         }
         #endregion UriParser template tests
+
+        [Theory]
+        [InlineData(UriKind.Absolute, true)]
+        [InlineData(UriKind.Absolute, false)]
+        [InlineData(UriKind.RelativeOrAbsolute, true)]
+        [InlineData(UriKind.RelativeOrAbsolute, false)]
+        [InlineData(UriKind.Relative, true)]
+        [InlineData(UriKind.Relative, false)]
+        public static void CustomParserCanRecoverOnInvalidUri(UriKind uriKind, bool recover)
+        {
+            string scheme = $"custom-recover-on-invalid-{uriKind}-{recover}";
+            UriParser.Register(new CustomParser_RecoversOnInvalidUri(recover), scheme, -1);
+
+            var uriString = recover ? $"{scheme}:not a valid host" : $"{scheme}://host";
+
+            if (uriKind == UriKind.Relative)
+            {
+                Assert.Throws<UriFormatException>(() => new Uri(uriString, uriKind));
+                Assert.False(Uri.TryCreate(uriString, uriKind, out _));
+            }
+            else
+            {
+                var uri1 = new Uri(uriString, uriKind);
+                Assert.True(Uri.TryCreate(uriString, uriKind, out Uri? uri2));
+                Assert.Same(uri1.OriginalString, uri2.OriginalString);
+                Assert.Equal("foo", uri1.Host);
+                Assert.Equal("foo", uri2.Host);
+            }
+        }
+
+        private sealed class CustomParser_RecoversOnInvalidUri(bool recover) : GenericUriParser(GenericUriParserOptions.Default)
+        {
+            protected override void InitializeAndValidate(Uri uri, out UriFormatException? parsingError)
+            {
+                base.InitializeAndValidate(uri, out parsingError);
+
+                if (recover)
+                {
+                    Assert.NotNull(parsingError);
+                    parsingError = null;
+                }
+            }
+
+            protected override string GetComponents(Uri uri, UriComponents components, UriFormat format) => "foo";
+        }
     }
 }