SP 2013 - Visual Webpart- Error on Build: The type or namespace name 'ASP' could not be found in the global namespace

I'm working on a Sharepoint 2013 project which has "suddenly" started producing an error in visual web parts onBuild or Deploy in Visual Studio 2015:

The files in question are:

EditFolder.ascx

EditFolder.ascx.cs

EditFolder.ascx.g.cs:

        [GeneratedCodeAttribute("Microsoft.VisualStudio.SharePoint.ProjectExtensions.CodeGenerators.SharePointWebP" +"artCodeGenerator", "14.0.0.0")]
        private static System.Reflection.MethodInfo @__PageInspector_SetTraceDataMethod = global::ASP.mycontrol_ascx.@__PageInspector_LoadHelper("SetTraceData");
        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Never)]
        [GeneratedCodeAttribute("Microsoft.VisualStudio.SharePoint.ProjectExtensions.CodeGenerators.SharePointWebP" +
            "artCodeGenerator", "14.0.0.0")]
        private void @__PageInspector_SetTraceData(object[] parameters) {
            if ((global::ASP.mycontrol_ascx.@__PageInspector_SetTraceDataMethod != null)) {
                global::ASP.mycontrol_ascx.@__PageInspector_SetTraceDataMethod.Invoke(null, parameters);
            }
        }
        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Never)]
        [GeneratedCodeAttribute("Microsoft.VisualStudio.SharePoint.ProjectExtensions.CodeGenerators.SharePointWebP" +
            "artCodeGenerator", "14.0.0.0")]
        private void @__PageInspector_SetTraceData(object[] parameters) {
            if ((global::ASP.mycontrol_ascx.@__PageInspector_SetTraceDataMethod != null)) {
                global::ASP.mycontrol_ascx.@__PageInspector_SetTraceDataMethod.Invoke(null, parameters);
            }
        }

The error I get is:

Severity	Code	Description	Project	File	Line	Suppression State
Error	CS0400	The type or namespace name 'ASP' could not be found in the global namespace (are you missing an 
assembly reference?)	IMS SP2013 Folder Web Parts	
C:\WebParts\IMS\IMS\IMS 2013\IMS SP2013 Folder Web Parts\IMS SP2013 Folder Web Parts\Edit_Folder\EditFolder\EditFolder.ascx.g.cs	868	Active

Is there something in the WEB config in IIS that might be causing this, or a setting in Visual Studio that has been changed somehow?