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[syntax-error]: default type parameter followed by non-default type parameter #21657

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[syntax-error]: default type parameter followed by non-default type parameter #21657

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b5e3768
feat: implement syntax error Typeparameterdefaultorder
11happy Nov 27, 2025
e666761
apply for typealias as well, add more tests
11happy Nov 30, 2025
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1 change: 1 addition & 0 deletions crates/ruff_linter/src/checkers/ast/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -746,6 +746,7 @@ impl SemanticSyntaxContext for Checker<'_> {
| SemanticSyntaxErrorKind::LoadBeforeNonlocalDeclaration { .. }
| SemanticSyntaxErrorKind::NonlocalAndGlobal(_)
| SemanticSyntaxErrorKind::AnnotatedGlobal(_)
| SemanticSyntaxErrorKind::TypeParameterDefaultOrder(_)
| SemanticSyntaxErrorKind::AnnotatedNonlocal(_) => {
self.semantic_errors.borrow_mut().push(error);
}
Expand Down
1 change: 1 addition & 0 deletions crates/ruff_python_parser/resources/inline/err/type_parameter_default_order.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
class C[T = int, U]: ...
57 changes: 52 additions & 5 deletions crates/ruff_python_parser/src/semantic_errors.rs
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Expand Up @@ -144,11 +144,18 @@ impl SemanticSyntaxChecker {
}
}
}
Stmt::ClassDef(ast::StmtClassDef { type_params, .. })
| Stmt::TypeAlias(ast::StmtTypeAlias { type_params, .. }) => {
if let Some(type_params) = type_params {
Self::duplicate_type_parameter_name(type_params, ctx);
}
Stmt::ClassDef(ast::StmtClassDef {
type_params: Some(type_params),
..
}) => {
Self::duplicate_type_parameter_name(type_params, ctx);
Self::type_parameter_default_order(type_params, ctx);
}
Stmt::TypeAlias(ast::StmtTypeAlias {
type_params: Some(type_params),
..
}) => {
Self::duplicate_type_parameter_name(type_params, ctx);
Comment on lines 147 to 155
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@dhruvmanila dhruvmanila Nov 27, 2025

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This check needs to be applied to type aliases as well:

In [1]: type Int[T = int, U] = int
  Cell In[1], line 1
    type Int[T = int, U] = int
                      ^
SyntaxError: non-default type parameter 'U' follows default type parameter

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@11happy 11happy Nov 30, 2025

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done

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@11happy 11happy Nov 30, 2025

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however some additional detection's are there for example -

type X[*Ts = x := int] = int

in type params:

type_params: TypeParams { range: 6..22, node_index: NodeIndex(None), type_params: [TypeVarTuple(TypeParamTypeVarTuple { node_index: NodeIndex(None), range: 7..14, name: Identifier { id: Name("Ts"), range: 8..10, node_index: NodeIndex(None) }, default: Some(Name(ExprName { node_index: NodeIndex(None), range: 13..14, id: Name("x"), ctx: Load })) }), TypeVar(TypeParamTypeVar { node_index: NodeIndex(None), range: 18..21, name: Identifier { id: Name("int"), range: 18..21, node_index: NodeIndex(None) }, bound: None, default: None })]

int is getting detected as seperate non default typevar, whereas I think should it be parsed as single expression ?

}
Stmt::Assign(ast::StmtAssign { targets, value, .. }) => {
if let [Expr::Starred(ast::ExprStarred { range, .. })] = targets.as_slice() {
Expand Down Expand Up @@ -611,6 +618,37 @@ impl SemanticSyntaxChecker {
}
}

fn type_parameter_default_order<Ctx: SemanticSyntaxContext>(
type_params: &ast::TypeParams,
ctx: &Ctx,
) {
let mut seen_default = false;
for type_param in type_params.iter() {
let has_default = match type_param {
ast::TypeParam::TypeVar(ast::TypeParamTypeVar { default, .. })
| ast::TypeParam::TypeVarTuple(ast::TypeParamTypeVarTuple { default, .. })
| ast::TypeParam::ParamSpec(ast::TypeParamParamSpec { default, .. }) => {
default.is_some()
}
};

if seen_default && !has_default {
// test_err type_parameter_default_order
// class C[T = int, U]: ...
Comment on lines +634 to +635
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@dhruvmanila dhruvmanila Nov 27, 2025

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Let's add some cases where the default is not at the start of the list and it has multiple non-default type parameter following a type parameter with default like:

class C[T1, T2 = int, T3, T4]: ...

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@11happy 11happy Nov 30, 2025

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done

Self::add_error(
ctx,
SemanticSyntaxErrorKind::TypeParameterDefaultOrder(
type_param.name().id.to_string(),
),
type_param.range(),
);
}
if has_default {
seen_default = true;
}
}
}

fn duplicate_parameter_name<Ctx: SemanticSyntaxContext>(
parameters: &ast::Parameters,
ctx: &Ctx,
Expand Down Expand Up @@ -1066,6 +1104,12 @@ impl Display for SemanticSyntaxError {
SemanticSyntaxErrorKind::DuplicateTypeParameter => {
f.write_str("duplicate type parameter")
}
SemanticSyntaxErrorKind::TypeParameterDefaultOrder(name) => {
write!(
f,
"non default type parameter `{name}` follows default type parameter"
)
}
SemanticSyntaxErrorKind::MultipleCaseAssignment(name) => {
write!(f, "multiple assignments to name `{name}` in pattern")
}
Expand Down Expand Up @@ -1572,6 +1616,9 @@ pub enum SemanticSyntaxErrorKind {

/// Represents a nonlocal statement for a name that has no binding in an enclosing scope.
NonlocalWithoutBinding(String),

/// Represents a default type parameter followed by a non-default type parameter.
TypeParameterDefaultOrder(String),
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, get_size2::GetSize)]
Expand Down
92 changes: 92 additions & 0 deletions ...es/ruff_python_parser/tests/snapshots/invalid_syntax@type_parameter_default_order.py.snap
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@@ -0,0 +1,92 @@
---
source: crates/ruff_python_parser/tests/fixtures.rs
input_file: crates/ruff_python_parser/resources/inline/err/type_parameter_default_order.py
---
## AST

```
Module(
ModModule {
node_index: NodeIndex(None),
range: 0..25,
body: [
ClassDef(
StmtClassDef {
node_index: NodeIndex(None),
range: 0..24,
decorator_list: [],
name: Identifier {
id: Name("C"),
range: 6..7,
node_index: NodeIndex(None),
},
type_params: Some(
TypeParams {
range: 7..19,
node_index: NodeIndex(None),
type_params: [
TypeVar(
TypeParamTypeVar {
node_index: NodeIndex(None),
range: 8..15,
name: Identifier {
id: Name("T"),
range: 8..9,
node_index: NodeIndex(None),
},
bound: None,
default: Some(
Name(
ExprName {
node_index: NodeIndex(None),
range: 12..15,
id: Name("int"),
ctx: Load,
},
),
),
},
),
TypeVar(
TypeParamTypeVar {
node_index: NodeIndex(None),
range: 17..18,
name: Identifier {
id: Name("U"),
range: 17..18,
node_index: NodeIndex(None),
},
bound: None,
default: None,
},
),
],
},
),
arguments: None,
body: [
Expr(
StmtExpr {
node_index: NodeIndex(None),
range: 21..24,
value: EllipsisLiteral(
ExprEllipsisLiteral {
node_index: NodeIndex(None),
range: 21..24,
},
),
},
),
],
},
),
],
},
)
```
## Semantic Syntax Errors

|
1 | class C[T = int, U]: ...
| ^ Syntax Error: non default type parameter `U` follows default type parameter
|