Why does this violate the type constraint?

Ran into this problem. It was crashing on the startup.cs file.changed

services.AddIdentity<ApplicationUser, ApplicationIdentityRole>()                .AddEntityFrameworkStores<ApplicationDbContext>()                .AddDefaultTokenProviders();

to

services.AddIdentity<ApplicationUser, ApplicationIdentityRole>()                .AddEntityFrameworkStores<ApplicationDbContext,int>()                .AddDefaultTokenProviders();

declaring the key type seemed to get past the crash

Ran into this problem as well. I had to add IdentityRole key type also, because it was still throwing the same error.

        services.AddIdentity<ApplicationUser, IdentityRole<int>>()            .AddEntityFrameworkStores<ApplicationDbContext,int>()            .AddDefaultTokenProviders();

Note for EF Core Users

Just to add to the above, if you are using .Net core 3.0 (not sure about earlier versions), there is no longer a AddEntityFrameworkStores<TContext,TKey> method.

Instead there is a generic variant of IdentityDbContext so instead you derive your DbContext from IdentityDbContext<TUser,TRole,TKey>

e.g. in my case

class ApplicationUser : IdentityUser<int> {...}class ApplicationDbContext : IdentityDbContext<ApplicationUser, IdentityRole<int>, int> {...}

then in your startup you can use services.AddDefaultIdentity<ApplicationUser>

cribbed from the last comment in https://github.com/aspnet/Identity/issues/1082