completion: do not insert extra colon if colon has been typed (#32134)

src/EditorFeatures/Test2/IntelliSense/CSharpCompletionCommandHandlerTests.vb

@@ -4180,6 +4180,29 @@ class C
             End Using
         End Function
 
+        <WorkItem(30097, "https://github.com/dotnet/roslyn/issues/30097")>
+        <MemberData(NameOf(AllCompletionImplementations))>
+        <WpfTheory, Trait(Traits.Feature, Traits.Features.Completion)>
+        Public Async Function TestNamedParameterDoesNotAddExtraColon(completionImplementation As CompletionImplementation) As Task
+            Using state = TestStateFactory.CreateCSharpTestState(completionImplementation,
+                              <Document>
+class C
+{
+    public double M(double some) => m;
+
+    public void Test()
+    {    
+        $$
+    }
+}
+                              </Document>)
+
+                state.SendTypeChars("M(some:M(some:")
+                Await state.AssertNoCompletionSession()
+                Assert.Equal("        M(some:M(some:", state.GetLineTextFromCaretPosition())
+            End Using
+        End Function
+
         Private Class MultipleChangeCompletionProvider
             Inherits CompletionProvider
 

src/Features/CSharp/Portable/Completion/CompletionProviders/NamedParameterCompletionProvider.cs

@@ -258,7 +258,8 @@ int IEqualityComparer<IParameterSymbol>.GetHashCode(IParameterSymbol obj)
                 selectedItem.Span,
                 // Do not insert colon on <Tab> so that user can complete out a variable name that does not currently exist.
                 // ch == null is to support the old completion only.
-                (ch == null || ch == '\t') ? selectedItem.DisplayText : selectedItem.GetEntireDisplayText()));
+                // Do not insert an extra colon if colon has been explicitly typed.
+                (ch == null || ch == '\t' || ch == ':') ? selectedItem.DisplayText : selectedItem.GetEntireDisplayText()));
         }
     }
 }