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self.p / 2 on an int auto-property returns 3.5 in Python and JS, and self.p + 1L fails to compile in Rust: property reads have no static type #466
Description
What's wrong
#433 made a member read such as self.total carry its field's declared type, so integer arithmetic on it is lowered correctly. The lookup behind that fix does not consider properties:
DeclaredFieldType(Coder/Languages/StandardLanguageGenerator.cs:625-650, reached throughDottedMemberTypeat :574) matches onlyFieldDeclarationandVariableDeclarationmembers of the class.DeclaredMemberType(:807-830) has the same gap.
An automatic PropertyDeclaration is lowered to a plain field in every target. Separated rewrites it only in the member list it writes out, while these lookups read the original classDecl.Members, where the member is still a PropertyDeclaration. Its reads therefore have no known type, and every StaticType-driven lowering is skipped for them.
Reproduction (HEAD 60ace00)
class Acc has three int members: a FieldDeclaration f, an automatic PropertyDeclaration p (HasSetter = true, no accessor bodies) and a VariableDeclaration v. Each has a method int half_X() returning BinaryExpression(VariableReference("self.X"), Divide, Literal<int>(2)), written this.X for JS.
| Target | f and v |
p |
|---|---|---|
| Python | self.f // 2 if ... else -(-self.f // 2), so 7 gives 3 |
return (self.p / 2), so 7 gives 3.5 (executed) |
| JavaScript | Math.trunc((this.f / 2)), so 7 gives 3 |
return (this.p / 2), so 7 gives 3.5 (executed) |
Rust, long wide_X() => self.X + 1L |
(self.f as i64) + 1, which compiles |
(self.p + 1): rustc E0308 mismatched types, i32 vs i64 |
% on p is also wrong in Python, which floors it.
Why it matters
An AST that declares state as an automatic property, which is the normal C# shape, gets silently wrong arithmetic in Python and JS and a compile error in Rust. The same AST with a field is correct. That is exactly the class of bug #433 closed, still open for properties.
Suggested fix
- In
DeclaredFieldTypeandDeclaredMemberType, also matchPropertyDeclarationmembers byNameand use theirType, at least for automatic properties. A property with accessor bodies is still read through its getter, whose return type is the same. - Add tests next to Integer
/and%on an instance field (self.total / 2,p.total % 2) use Python/JS native division: an int-typed method returns -3.5 instead of -3 #433's covering Python/and%, JS/, and Rust widening for an automatic-property read.
Activity
matt-edmondson commentedon Nov 7, 2026
Triage
intautomatic property divided by 2 gives3.5where a field gives3, and nothing fails at generation or run time. The usual C# way of declaring state (an automatic property) is enough to trigger it. Rust only gets a compile error, which is at least visible.StandardLanguageGenerator.DeclaredFieldType(:625-650) andDeclaredMemberType(:807-830), the static-type lookup added by Integer/and%on an instance field (self.total / 2,p.total % 2) use Python/JS native division: an int-typed method returns -3.5 instead of -3 #433. Owner: @matt-edmondsonvoid* b = (-a);, Go types-(1.5f)as float64, and Python/JS do true division on-a / 2#267 (a unary expression loses its static type) and the closed Integer/and%on an instance field (self.total / 2,p.total % 2) use Python/JS native division: an int-typed method returns -3.5 instead of -3 #433, and Rust: a record holding a struct through an auto-property fails to compile (E0277), because the held struct never derives Clone/Debug/PartialEq #353 also involves auto-properties. No open PR changes these lookups: Type a shift as its left operand [patch] #460 (shift typing) and Say an inferred member's type in C# and C++ instead of var/auto [patch] #454 (inferred member types) are nearby but different.PropertyDeclarationbyNamein both lookups and use itsType. Add the Python/and%, JS/and Rust widening tests next to Integer/and%on an instance field (self.total / 2,p.total % 2) use Python/JS native division: an int-typed method returns -3.5 instead of -3 #433's.Generated by Claude Code