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fix: add Union branch to check_generic_type_compact to prevent param-type-mismatch false positives #1107

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fix: add Union branch to check_generic_type_compact to prevent param-type-mismatch false positives #1107

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13 changes: 13 additions & 0 deletions crates/emmylua_code_analysis/src/semantic/type_check/generic_type.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -99,6 +99,19 @@ pub fn check_generic_type_compact(
check_guard.next_level()?,
)
}
LuaType::Union(union_type) => {
// When compact_type is a Union, every member of the union must be
// assignable to the source generic type for the assignment to be safe.
for member_type in union_type.into_vec() {
check_generic_type_compact(
context,
source_generic,
&member_type,
check_guard.next_level()?,
)?;
}
Ok(())
}
Comment on lines +102 to +114

@gemini-code-assist gemini-code-assist Bot Jun 11, 2026

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high

Calling check_generic_type_compact directly on union members bypasses critical type-checking logic defined in check_general_type_compact, such as is_like_any (which handles any and unknown), escape_type (which resolves aliases), and fast_eq_check.

If a union contains unknown or an alias type, calling check_generic_type_compact directly will result in a false positive TypeNotMatch error because those types are not explicitly handled in the match statement of check_generic_type_compact.

To fix this, we should construct a LuaType::Generic from source_generic and call check_general_type_compact for each member of the union.

Additionally, it is highly recommended to add a test case to crates/emmylua_code_analysis/src/semantic/type_check/test.rs to verify this behavior:

#[test]
fn test_issue_1106() {
    let mut ws = VirtualWorkspace::new();
    ws.def(
        r#"
        ---@class Item
        ---@field id integer
        ---@class Box<T>
        ---@field value T
        ---@class Service
        ---@field execute fun(self:Service, box:Box<Item>)
        ---@field fetch fun(self:Service):Box<Item>
        "#,
    );

    assert!(ws.has_no_diagnostic(
        DiagnosticCode::ParamTypeMismatch,
        r#"
        local Service = {}
        function Service:execute(_box) end
        function Service:fetch() return {} --[[@as Box<Item>]] end
        function Service:run()
            local b = self:fetch()
            self:execute(b)
        end
        "#
    ));
}
Suggested change
LuaType::Union(union_type) => {
// When compact_type is a Union, every member of the union must be
// assignable to the source generic type for the assignment to be safe.
for member_type in union_type.into_vec() {
check_generic_type_compact(
context,
source_generic,
&member_type,
check_guard.next_level()?,
)?;
}
Ok(())
}
LuaType::Union(union_type) => {
// When compact_type is a Union, every member of the union must be
// assignable to the source generic type for the assignment to be safe.
let source_type = LuaType::Generic(source_generic.clone().into());
for member_type in union_type.into_vec() {
check_general_type_compact(
context,
&source_type,
&member_type,
check_guard.next_level()?,
)?;
}
Ok(())
}

_ => Err(TypeCheckFailReason::TypeNotMatch),
}
}
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