[ty] improve bad specialization results & error messages

crates/ty_python_semantic/resources/corpus/cyclic_pep695_typevars.py

@@ -3,3 +3,5 @@ def name_1[name_0: name_0](name_2: name_0):
         pass
     except name_2:
         pass
+
+def _[T: (T if cond else U)[0], U](): pass

crates/ty_python_semantic/resources/mdtest/generics/legacy/paramspec.md

@@ -250,7 +250,7 @@ reveal_type(OnlyParamSpec[...]().attr)  # revealed: (...) -> None
 def func(c: Callable[P2, None]):
     reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (**P2@func) -> None
 
-# TODO: error: paramspec is unbound
+# error: [invalid-type-form] "ParamSpec `P2` is unbound"
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
 ```
 
@@ -273,11 +273,10 @@ reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str,
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 
-# TODO: We could still specialize for `T1` as the type is valid which would reveal `(...) -> int`
+# error: [invalid-type-form] "ParamSpec `P2` is unbound"
-# TODO: error: paramspec is unbound
+reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> int
-reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> Unknown
 # error: [invalid-type-arguments] "Type argument for `ParamSpec` must be either a list of types, `ParamSpec`, `Concatenate`, or `...`"
-reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> int
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`s.

crates/ty_python_semantic/resources/mdtest/generics/pep695/aliases.md

@@ -74,7 +74,18 @@ type B = ...
 reveal_type(B[int])  # revealed: Unknown
 
 # error: [non-subscriptable] "Cannot subscript non-generic type alias"
-def _(b: B[int]): ...
+def _(b: B[int]):
+    reveal_type(b)  # revealed: Unknown
+
+type IntOrStr = int | str
+
+# error: [non-subscriptable] "Cannot subscript non-generic type alias"
+def _(c: IntOrStr[int]): ...
+
+type ListOfInts = list[int]
+
+# error: [non-subscriptable] "Cannot subscript non-generic type alias"
+def _(l: ListOfInts[int]): ...
 ```
 
 If the type variable has an upper bound, the specialized type must satisfy that bound:
@@ -98,6 +109,15 @@ reveal_type(BoundedByUnion[int])  # revealed: <type alias 'BoundedByUnion[int]'>
 reveal_type(BoundedByUnion[IntSubclass])  # revealed: <type alias 'BoundedByUnion[IntSubclass]'>
 reveal_type(BoundedByUnion[str])  # revealed: <type alias 'BoundedByUnion[str]'>
 reveal_type(BoundedByUnion[int | str])  # revealed: <type alias 'BoundedByUnion[int | str]'>
+
+type TupleOfIntAndStr[T: int, U: str] = tuple[T, U]
+
+def _(x: TupleOfIntAndStr[int, str]):
+    reveal_type(x)  # revealed: tuple[int, str]
+
+# error: [invalid-type-arguments] "Type `int` is not assignable to upper bound `str` of type variable `U@TupleOfIntAndStr`"
+def _(x: TupleOfIntAndStr[int, int]):
+    reveal_type(x)  # revealed: tuple[int, Unknown]
 ```
 
 If the type variable is constrained, the specialized type must satisfy those constraints:
@@ -119,6 +139,15 @@ reveal_type(Constrained[int | str])  # revealed: <type alias 'Constrained[int |
 
 # error: [invalid-type-arguments] "Type `object` does not satisfy constraints `int`, `str` of type variable `T@Constrained`"
 reveal_type(Constrained[object])  # revealed: <type alias 'Constrained[Unknown]'>
+
+type TupleOfIntOrStr[T: (int, str), U: (int, str)] = tuple[T, U]
+
+def _(x: TupleOfIntOrStr[int, str]):
+    reveal_type(x)  # revealed: tuple[int, str]
+
+# error: [invalid-type-arguments] "Type `object` does not satisfy constraints `int`, `str` of type variable `U@TupleOfIntOrStr`"
+def _(x: TupleOfIntOrStr[int, object]):
+    reveal_type(x)  # revealed: tuple[int, Unknown]
 ```
 
 If the type variable has a default, it can be omitted:

crates/ty_python_semantic/resources/mdtest/generics/pep695/paramspec.md

@@ -236,7 +236,7 @@ def func[**P2](c: Callable[P2, None]):
 
 P2 = ParamSpec("P2")
 
-# TODO: error: paramspec is unbound
+# error: [invalid-type-form] "ParamSpec `P2` is unbound"
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
 ```
 
@@ -259,10 +259,10 @@ reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str,
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 
-# TODO: error: paramspec is unbound
+# error: [invalid-type-form] "ParamSpec `P2` is unbound"
-reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> Unknown
+reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> int
 # error: [invalid-type-arguments]
-reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> int
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`.

crates/ty_python_semantic/resources/mdtest/implicit_type_aliases.md

@@ -656,10 +656,9 @@ A generic alias that is already fully specialized cannot be specialized again:
 ```py
 ListOfInts = list[int]
 
-# error: [invalid-type-arguments] "Too many type arguments: expected 0, got 1"
+# error: [non-subscriptable] "Cannot subscript non-generic type alias: `<class 'list[int]'>` is already specialized"
 def _(doubly_specialized: ListOfInts[int]):
-    # TODO: This should ideally be `list[Unknown]` or `Unknown`
+    reveal_type(doubly_specialized)  # revealed: Unknown
-    reveal_type(doubly_specialized)  # revealed: list[int]
 ```
 
 Specializing a generic implicit type alias with an incorrect number of type arguments also results
@@ -695,23 +694,21 @@ def this_does_not_work() -> TypeOf[IntOrStr]:
     raise NotImplementedError()
 
 def _(
-    # TODO: Better error message (of kind `invalid-type-form`)?
+    # error: [non-subscriptable] "Cannot subscript non-generic type alias"
-    # error: [invalid-type-arguments] "Too many type arguments: expected 0, got 1"
     specialized: this_does_not_work()[int],
 ):
-    reveal_type(specialized)  # revealed: int | str
+    reveal_type(specialized)  # revealed: Unknown
 ```
 
 Similarly, if you try to specialize a union type without a binding context, we emit an error:
 
 ```py
-# TODO: Better error message (of kind `invalid-type-form`)?
+# error: [non-subscriptable] "Cannot subscript non-generic type alias"
-# error: [invalid-type-arguments] "Too many type arguments: expected 0, got 1"
 x: (list[T] | set[T])[int]
 
 def _():
     # TODO: `list[Unknown] | set[Unknown]` might be better
-    reveal_type(x)  # revealed: list[typing.TypeVar] | set[typing.TypeVar]
+    reveal_type(x)  # revealed: Unknown
 ```
 
 ### Multiple definitions

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -3527,10 +3527,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         return Ok(param_type);
                     }
 
-                    Type::KnownInstance(known_instance)
+                    Type::KnownInstance(known_instance @ KnownInstanceType::TypeVar(typevar))
                         if known_instance.class(self.db()) == KnownClass::ParamSpec =>
                     {
-                        // TODO: Emit diagnostic: "ParamSpec "P" is unbound"
+                        if let Some(diagnostic_builder) =
+                            self.context.report_lint(&INVALID_TYPE_FORM, expr)
+                        {
+                            diagnostic_builder.into_diagnostic(format_args!(
+                                "ParamSpec `{}` is unbound",
+                                typevar.name(self.db())
+                            ));
+                        }
                         return Err(());
                     }
 
@@ -11609,6 +11616,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         generic_context: GenericContext<'db>,
         specialize: impl FnOnce(&[Option<Type<'db>>]) -> Type<'db>,
     ) -> Type<'db> {
+        enum ExplicitSpecializationError {
+            InvalidParamSpec,
+            UnsatisfiedBound,
+            UnsatisfiedConstraints,
+            /// These two errors override the errors above, causing all specializations to be `Unknown`.
+            MissingTypeVars,
+            TooManyArguments,
+            /// This error overrides the errors above, causing the type itself to be `Unknown`.
+            NonGeneric,
+        }
+
         fn add_typevar_definition<'db>(
             db: &'db dyn Db,
             diagnostic: &mut Diagnostic,
@@ -11661,7 +11679,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             }
         };
 
-        let mut has_error = false;
+        let mut error: Option<ExplicitSpecializationError> = None;
 
         for (index, item) in typevars.zip_longest(type_arguments.iter()).enumerate() {
             match item {
@@ -11677,8 +11695,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         ) {
                             Ok(paramspec_value) => paramspec_value,
                             Err(()) => {
-                                has_error = true;
+                                error = Some(ExplicitSpecializationError::InvalidParamSpec);
-                                Type::unknown()
+                                Type::paramspec_value_callable(db, Parameters::unknown())
                             }
                         }
                     } else {
@@ -11710,8 +11728,10 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                                     ));
                                     add_typevar_definition(db, &mut diagnostic, typevar);
                                 }
-                                has_error = true;
+                                error = Some(ExplicitSpecializationError::UnsatisfiedBound);
-                                continue;
+                                specialization_types.push(Some(Type::unknown()));
+                            } else {
+                                specialization_types.push(Some(provided_type));
                             }
                         }
                         Some(TypeVarBoundOrConstraints::Constraints(constraints)) => {
@@ -11744,15 +11764,17 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                                     ));
                                     add_typevar_definition(db, &mut diagnostic, typevar);
                                 }
-                                has_error = true;
+                                error = Some(ExplicitSpecializationError::UnsatisfiedConstraints);
-                                continue;
+                                specialization_types.push(Some(Type::unknown()));
-                            }
+                            } else {
-                        }
-                        None => {}
-                    }
-
                                 specialization_types.push(Some(provided_type));
                             }
+                        }
+                        None => {
+                            specialization_types.push(Some(provided_type));
+                        }
+                    }
+                }
                 EitherOrBoth::Left(typevar) => {
                     if typevar.default_type(db).is_none() {
                         // This is an error case, so no need to push into the specialization types.
@@ -11786,10 +11808,29 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     }
                 ));
             }
-            has_error = true;
+            error = Some(ExplicitSpecializationError::MissingTypeVars);
         }
 
         if let Some(first_excess_type_argument_index) = first_excess_type_argument_index {
+            if typevars_len == 0 {
+                // Type parameter list cannot be empty, so if we reach here, `value_ty` is not a generic type.
+                if let Some(builder) = self
+                    .context
+                    .report_lint(&NON_SUBSCRIPTABLE, &*subscript.value)
+                {
+                    if value_ty.is_generic_alias() {
+                        builder.into_diagnostic(format_args!(
+                            "Cannot subscript non-generic type alias: `{}` is already specialized",
+                            value_ty.display(db),
+                        ));
+                    } else {
+                        builder.into_diagnostic(format_args!(
+                            "Cannot subscript non-generic type alias"
+                        ));
+                    }
+                }
+                error = Some(ExplicitSpecializationError::NonGeneric);
+            } else {
                 let node = get_node(first_excess_type_argument_index);
                 if let Some(builder) = self.context.report_lint(&INVALID_TYPE_ARGUMENTS, node) {
                     let description = CallableDescription::new(db, value_ty);
@@ -11812,7 +11853,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         type_arguments.len(),
                     ));
                 }
-            has_error = true;
+                error = Some(ExplicitSpecializationError::TooManyArguments);
+            }
         }
 
         if store_inferred_type_arguments {
@@ -11822,7 +11864,12 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             );
         }
 
-        if has_error {
+        match error {
+            Some(ExplicitSpecializationError::NonGeneric) => Type::unknown(),
+            Some(
+                ExplicitSpecializationError::MissingTypeVars
+                | ExplicitSpecializationError::TooManyArguments,
+            ) => {
                 let unknowns = generic_context
                     .variables(self.db())
                     .map(|typevar| {
@@ -11833,10 +11880,15 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                         })
                     })
                     .collect::<Vec<_>>();
-            return specialize(&unknowns);
+                specialize(&unknowns)
+            }
+            Some(
+                ExplicitSpecializationError::UnsatisfiedBound
+                | ExplicitSpecializationError::UnsatisfiedConstraints
+                | ExplicitSpecializationError::InvalidParamSpec,
+            )
+            | None => specialize(&specialization_types),
         }
-
-        specialize(&specialization_types)
     }
 
     fn infer_subscript_expression_types(