[red-knot] remove CallOutcome::Cast variant (#15461)

## Summary

Simplification follow-up to #15413.

There's no need to have a dedicated `CallOutcome` variant for every
known function, it's only necessary if the special-cased behavior of the
known function includes emitting extra diagnostics. For `typing.cast`,
there's no such need; we can use the regular `Callable` outcome variant,
and update the return type according to the cast. (This is the same way
we already handle `len`.)

One reason to avoid proliferating unnecessary `CallOutcome` variants is
that currently we have to explicitly add emitting call-binding
diagnostics, for each outcome variant. So we were previously wrongly
silencing any binding diagnostics on calls to `typing.cast`. Fixing this
revealed a separate bug, that we were emitting a bogus error anytime
more than one keyword argument mapped to a `**kwargs` parameter. So this
PR also adds test and fix for that bug.

## Test Plan

Existing `cast` tests pass unchanged, added new test for `**kwargs` bug.

crates/red_knot_python_semantic/src/types.rs

@@ -2021,11 +2021,11 @@ impl<'db> Type<'db> {
                             return CallOutcome::callable(binding);
                         };
 
-                        let Some(casted_ty) = arguments.first_argument() else {
-                            return CallOutcome::callable(binding);
+                        if let Some(casted_ty) = arguments.first_argument() {
+                            binding.set_return_ty(casted_ty);
                         };
 
-                        CallOutcome::casted(binding, casted_ty)
+                        CallOutcome::callable(binding)
                     }
 
                     _ => CallOutcome::callable(binding),
@@ -3877,7 +3877,6 @@ impl<'db> Class<'db> {
                 | CallOutcome::RevealType { binding, .. }
                 | CallOutcome::StaticAssertionError { binding, .. }
                 | CallOutcome::AssertType { binding, .. } => Ok(binding.return_ty()),
-                CallOutcome::Cast { casted_ty, .. } => Ok(casted_ty),
             };
 
             return return_ty_result.map(|ty| ty.to_meta_type(db));

crates/red_knot_python_semantic/src/types/call.rs

@@ -48,14 +48,10 @@ pub(super) enum CallOutcome<'db> {
         binding: CallBinding<'db>,
         asserted_ty: Type<'db>,
     },
-    Cast {
-        binding: CallBinding<'db>,
-        casted_ty: Type<'db>,
-    },
 }
 
 impl<'db> CallOutcome<'db> {
-    /// Create a new `CallOutcome::Callable` with given return type.
+    /// Create a new `CallOutcome::Callable` with given binding.
     pub(super) fn callable(binding: CallBinding<'db>) -> CallOutcome<'db> {
         CallOutcome::Callable { binding }
     }
@@ -92,11 +88,6 @@ impl<'db> CallOutcome<'db> {
         }
     }
 
-    /// Create a new `CallOutcome::Casted` with given casted and return types.
-    pub(super) fn casted(binding: CallBinding<'db>, casted_ty: Type<'db>) -> CallOutcome<'db> {
-        CallOutcome::Cast { binding, casted_ty }
-    }
-
     /// Get the return type of the call, or `None` if not callable.
     pub(super) fn return_ty(&self, db: &'db dyn Db) -> Option<Type<'db>> {
         match self {
@@ -128,10 +119,6 @@ impl<'db> CallOutcome<'db> {
                 binding,
                 asserted_ty: _,
             } => Some(binding.return_ty()),
-            Self::Cast {
-                binding: _,
-                casted_ty,
-            } => Some(*casted_ty),
         }
     }
 
@@ -360,10 +347,6 @@ impl<'db> CallOutcome<'db> {
 
                 Ok(binding.return_ty())
             }
-            Self::Cast {
-                binding: _,
-                casted_ty,
-            } => Ok(*casted_ty),
         }
     }
 }

crates/red_knot_python_semantic/src/types/call/bind.rs

@@ -84,7 +84,7 @@ pub(crate) fn bind_call<'db>(
             }
         }
         if let Some(existing) = parameter_tys[index].replace(*argument_ty) {
-            if parameter.is_variadic() {
+            if parameter.is_variadic() || parameter.is_keyword_variadic() {
                 let union = UnionType::from_elements(db, [existing, *argument_ty]);
                 parameter_tys[index].replace(union);
             } else {