[ty] Handle various invalid explicit specializations for `ParamSpec` (#21821)

## Summary

fixes: https://github.com/astral-sh/ty/issues/1788

## Test Plan

Add new mdtests.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

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

@@ -244,6 +244,7 @@ Explicit specialization of a generic class involving `ParamSpec` is done by prov
 of types, `...`, or another in-scope `ParamSpec`.
 
 ```py
+reveal_type(OnlyParamSpec[[]]().attr)  # revealed: () -> None
 reveal_type(OnlyParamSpec[[int, str]]().attr)  # revealed: (int, str, /) -> None
 reveal_type(OnlyParamSpec[...]().attr)  # revealed: (...) -> None
 
@@ -252,8 +253,28 @@ def func(c: Callable[P2, None]):
 
 # TODO: error: paramspec is unbound
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
+
+# error: [invalid-type-arguments] "No type argument provided for required type variable `P1` of class `OnlyParamSpec`"
+reveal_type(OnlyParamSpec[()]().attr)  # revealed: (...) -> None
 ```
 
+An explicit tuple expression (unlike an implicit one that omits the parentheses) is also accepted
+when the `ParamSpec` is the only type variable. But, this isn't recommended is mainly a fallout of
+it having the same AST as the one without the parentheses. Both mypy and Pyright also allow this.
+
+```py
+reveal_type(OnlyParamSpec[(int, str)]().attr)  # revealed: (int, str, /) -> None
+```
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+reveal_type(OnlyParamSpec[]().attr)  # revealed: (...) -> None
+```
+
+<!-- blacken-docs:on -->
+
 The square brackets can be omitted when `ParamSpec` is the only type variable
 
 ```py
@@ -269,6 +290,7 @@ reveal_type(OnlyParamSpec[int]().attr)  # revealed: (int, /) -> None
 But, they cannot be omitted when there are multiple type variables.
 
 ```py
+reveal_type(TypeVarAndParamSpec[int, []]().attr)  # revealed: () -> int
 reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
@@ -276,8 +298,12 @@ reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 # TODO: We could still specialize for `T1` as the type is valid which would reveal `(...) -> int`
 # TODO: error: paramspec is unbound
 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 `...`"
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
 reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, ()]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, (int, str)]().attr)  # revealed: (...) -> Unknown
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`s.

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

@@ -228,6 +228,7 @@ Explicit specialization of a generic class involving `ParamSpec` is done by prov
 of types, `...`, or another in-scope `ParamSpec`.
 
 ```py
+reveal_type(OnlyParamSpec[[]]().attr)  # revealed: () -> None
 reveal_type(OnlyParamSpec[[int, str]]().attr)  # revealed: (int, str, /) -> None
 reveal_type(OnlyParamSpec[...]().attr)  # revealed: (...) -> None
 
@@ -238,8 +239,28 @@ P2 = ParamSpec("P2")
 
 # TODO: error: paramspec is unbound
 reveal_type(OnlyParamSpec[P2]().attr)  # revealed: (...) -> None
+
+# error: [invalid-type-arguments] "No type argument provided for required type variable `P1` of class `OnlyParamSpec`"
+reveal_type(OnlyParamSpec[()]().attr)  # revealed: (...) -> None
 ```
 
+An explicit tuple expression (unlike an implicit one that omits the parentheses) is also accepted
+when the `ParamSpec` is the only type variable. But, this isn't recommended is mainly a fallout of
+it having the same AST as the one without the parentheses. Both mypy and Pyright also allow this.
+
+```py
+reveal_type(OnlyParamSpec[(int, str)]().attr)  # revealed: (int, str, /) -> None
+```
+
+<!-- blacken-docs:off -->
+
+```py
+# error: [invalid-syntax]
+reveal_type(OnlyParamSpec[]().attr)  # revealed: (...) -> None
+```
+
+<!-- blacken-docs:on -->
+
 The square brackets can be omitted when `ParamSpec` is the only type variable
 
 ```py
@@ -255,14 +276,19 @@ reveal_type(OnlyParamSpec[int]().attr)  # revealed: (int, /) -> None
 But, they cannot be omitted when there are multiple type variables.
 
 ```py
+reveal_type(TypeVarAndParamSpec[int, []]().attr)  # revealed: () -> int
 reveal_type(TypeVarAndParamSpec[int, [int, str]]().attr)  # revealed: (int, str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, [str]]().attr)  # revealed: (str, /) -> int
 reveal_type(TypeVarAndParamSpec[int, ...]().attr)  # revealed: (...) -> int
 
 # TODO: error: paramspec is unbound
 reveal_type(TypeVarAndParamSpec[int, P2]().attr)  # revealed: (...) -> Unknown
-# error: [invalid-type-arguments]
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
 reveal_type(TypeVarAndParamSpec[int, int]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, ()]().attr)  # revealed: (...) -> Unknown
+# error: [invalid-type-arguments] "Type argument for `ParamSpec` must be"
+reveal_type(TypeVarAndParamSpec[int, (int, str)]().attr)  # revealed: (...) -> Unknown
 ```
 
 Nor can they be omitted when there are more than one `ParamSpec`.

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

@@ -3472,17 +3472,13 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 ));
             }
 
+            ast::Expr::Tuple(_) if !exactly_one_paramspec => {
+                // Tuple expression is only allowed when the generic context contains only one
+                // `ParamSpec` type variable and no other type variables.
+            }
+
             ast::Expr::Tuple(ast::ExprTuple { elts, .. })
             | ast::Expr::List(ast::ExprList { elts, .. }) => {
-                // This should be taken care of by the caller.
-                if expr.is_tuple_expr() {
-                    assert!(
-                        exactly_one_paramspec,
-                        "Inferring ParamSpec value during explicit specialization for a \
-                    tuple expression should only happen when it contains exactly one ParamSpec"
-                    );
-                }
-
                 let mut parameter_types = Vec::with_capacity(elts.len());
 
                 // Whether to infer `Todo` for the parameters
@@ -3519,7 +3515,11 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                 return Ok(Type::paramspec_value_callable(db, Parameters::todo()));
             }
 
-            ast::Expr::Name(_) => {
+            ast::Expr::Name(name) => {
+                if name.is_invalid() {
+                    return Err(());
+                }
+
                 let param_type = self.infer_type_expression(expr);
 
                 match param_type {
@@ -11632,7 +11632,7 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
         let exactly_one_paramspec = generic_context.exactly_one_paramspec(db);
         let (type_arguments, store_inferred_type_arguments) = match slice_node {
             ast::Expr::Tuple(tuple) => {
-                if exactly_one_paramspec {
+                if exactly_one_paramspec && !tuple.elts.is_empty() {
                     (std::slice::from_ref(slice_node), false)
                 } else {
                     (tuple.elts.as_slice(), true)