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Handle Const.Zero for primitive value types in custom attribute codegen - #19484

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copilot/fix-internal-error-custom-attribute
May 4, 2026
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Handle Const.Zero for primitive value types in custom attribute codegen#19484
T-Gro merged 2 commits into
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copilot/fix-internal-error-custom-attribute

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CopilotAI commented Mar 24, 2026•
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GenAttribArg in IlxGen.fs handles Const.Zero (the default for [<Optional>] params without [<DefaultParameterValue>]) for System.Object, System.String, and System.Type, but not for primitive value types. This causes an internal error for code like:

typeFooAttribute(foo: string,bar: bool)=inherit Attribute()new([<Optional>] bar: bool)= FooAttribute("", bar)[<Foo>]// FS0073: internal error: The type 'System.Boolean' may not be used as a custom attribute valuetypeBar()=classend

Changes

  • Added Const.Zero → default value mappings for all 12 primitive types valid in custom attributes: bool, sbyte, int16, int32, int64, byte, uint16, uint32, uint64, single, double, char
  • Added regression test exercising [<Optional>] attribute constructor params for all primitive value types
Original prompt

This section details on the original issue you should resolve

<issue_title>Internal error when using custom attribute and omitting optional argument</issue_title>
<issue_description>There is an internal compiler error when you use a custom attribute and omit an optional argument tagged with System.Runtime.InteropServices.OptionalAttribute

Repro steps

moduleTestCaseopenSystemopenSystem.Runtime.InteropServicestypeFooAttribute(foo: string,bar: bool)=inherit Attribute()new([<Optional>] bar)= FooAttribute("", bar)[<Foo>]typeBar()=classend

Expected behavior

This should compile

Actual behavior

TestCase.fs(10,3): error FS0073: internal error: The type 'System.Boolean' may not be used as a custom attribute value

Known workarounds

Add a DefaultParameterValue attribute.

Related information

Provide any related information (optional):

  • Mac OS 10.15.1
  • Mono JIT compiler version 6.6.0.155 (2019-08/296a9afdb24 Thu Nov 14 06:12:25 EST 2019)
  • Microsoft (R) F# Compiler version 10.2.3 for F# 4.5
    </issue_description>

<agent_instructions>See the latest comment, a fix is possible.
Careful, the comment has escaped angle brackets - you will need to figure that out.
Make sure to execute all codegen and optional related tests!

</agent_instructions>

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CopilotAI changed the title [WIP] Fix internal error when using custom attribute without optional argumentHandle Const.Zero for primitive value types in custom attribute codegenMar 24, 2026
CopilotAI requested a review from T-GroMarch 24, 2026 13:28
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T-Gro marked this pull request as ready for review March 26, 2026 21:34
@T-Gro
T-Gro requested a review from a team as a code ownerMarch 26, 2026 21:34
@T-Gro
T-Gro requested a review from abonieMarch 26, 2026 21:34
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github-actionsBot commented Mar 26, 2026•
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❗ Release notes required

✅ Found changes and release notes in following paths:

Change pathRelease notes pathDescription
src/Compilerdocs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@github-actionsgithub-actionsBot mentioned this pull request Mar 30, 2026
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T-Gro enabled auto-merge (squash) April 1, 2026 10:17
@T-Gro
T-Groforce-pushed the copilot/fix-internal-error-custom-attribute branch 5 times, most recently from 78029a5 to 9cac358CompareApril 16, 2026 06:03
@T-Gro
T-Groforce-pushed the copilot/fix-internal-error-custom-attribute branch from 9cac358 to 61d2b72CompareApril 21, 2026 06:02
abonie
abonie reviewed Apr 24, 2026
View reviewed changes
Comment threadsrc/Compiler/CodeGen/IlxGen.fs
@T-Gro
T-Groforce-pushed the copilot/fix-internal-error-custom-attribute branch 2 times, most recently from b451e8c to 1f64db5CompareApril 28, 2026 09:25
@T-Gro
T-Gro requested a review from abonieApril 28, 2026 10:36
@github-project-automationgithub-project-automationBot moved this from New to In Progress in F# Compiler and ToolingApr 28, 2026
@T-Gro
T-Groforce-pushed the copilot/fix-internal-error-custom-attribute branch from 1f64db5 to 38da75cCompareMay 4, 2026 07:17
@T-Gro
Fix internal error when using custom attribute with optional value ty…
7e20ed7
…pe argument (#8353)
GenAttribArg in IlxGen.fs handles Const.Zero (the default for [<Optional>] params
without [<DefaultParameterValue>]) for System.Object, System.String, and System.Type,
but not for primitive value types. This causes an internal error (FS0073) when
applying an attribute with an [<Optional>] value type parameter and no default.
Add Const.Zero -> default value mappings for all 12 primitive types valid in custom
attributes: bool, sbyte, int16, int32, int64, byte, uint16, uint32, uint64, single,
double, char.
Fixes#8353
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
Handle Const.Zero for enum types in custom attribute codegen
39eeb14
Address review feedback: when a custom attribute has an [<Optional>] parameter
of an enum type (without [<DefaultParameterValue>]), Const.Zero is now resolved
by looking up the enum's underlying primitive type and recursing into GenAttribArg.
Also adds test coverage for enum types (int-based and byte-based).
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
@T-Gro
T-Groforce-pushed the copilot/fix-internal-error-custom-attribute branch from 38da75c to 39eeb14CompareMay 4, 2026 07:27
@T-Gro
T-Gro merged commit c91ff9c into mainMay 4, 2026
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Internal error when using custom attribute and omitting optional argument

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