Beautify enum declarations by referencing previously declared members

[siegfriedpammer]

Problem

Enum declarations are decompiled with every member shown as a bare number, even when the original source referenced other members: All = 7 instead of All = Item1 | Item2 | Item3, Item2B = 2 instead of Item2B = Item2A. (No open issue; this revives and completes the old stash/beautify-enum-member-declarations experiment.)

Solution

Enum member initializers that duplicate an earlier member now reference it, and combined [Flags] values are shown as a union of their earlier single-bit members or as a complement:

[Flags]
public enum SimpleFlagsEnum
{
	None = 0,
	Item1 = 1,
	Item2 = 2,
	Item3 = 4,
	All = Item1 | Item2 | Item3   // was: All = 7
}

public enum EnumDuplicateItemTest
{
	Item0 = 0,
	Item1 = 1,
	Item2A = 2,
	Item2B = Item2A               // was: Item2B = 2
}

Guardrails keep the output faithful to how such enums are written by hand:

• Only previously declared members are referenced (field row order), so forward references never appear and the output always compiles.
• A multi-bit value lying entirely within a larger, earlier-declared member is treated as a field encoding inside that mask rather than a flag union and stays numeric; zero-valued members of [Flags] enums stay numeric too (mask-style enums routinely have several unrelated zero members, e.g. MethodAttributes.PrivateScope/ReuseSlot).
• The ~X complement form is suppressed in byte/ushort enum declarations, where the initializer constant folds in int and would not compile (CS0031); expression contexts keep it for every underlying type.
• Enums with unusual underlying types (bool, native int) keep the plain constant conversion.

With these rules, decompiling System.Private.CoreLib reproduces the hand-written declarations of TypeAttributes, MethodAttributes, FileAttributes and AttributeTargets (including All = Assembly | Module | ... | GenericParameter) almost verbatim: numeric exactly where the original sources are numeric, while unambiguous aliases such as NewSlot = VtableLayoutMask are referenced.

Most in need of attention: the two output-policy heuristics (mask-encoding suppression and numeric zeros in [Flags] declarations) are aesthetic trade-offs; the fixtures pin the chosen behavior.

• At least one test covering the code changed — new Pretty fixtures in EnumTests cover mask-family enums, unsigned/byte/short flags, complements at declaration and usage sites, and zero/non-flags duplicates; full suite green (all non-Correctness tests 2171 passed, Correctness 197 passed).

The code and this description were largely written by an AI agent (Claude Code), reviewed by me.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves enum decompilation output by emitting member references (and [Flags] unions/complements) instead of always printing raw numeric literals, while enforcing ordering/compilability rules for enum member initializers.

Changes:

• Extend TypeSystemAstBuilder.ConvertEnumValue to support enum-member-initializer context (unqualified references, avoid self/forward references, suppress non-compiling complements).
• Update enum field decompilation to use the enhanced enum-value conversion for standard integral underlying types.
• Add/extend pretty-test fixtures to pin the new enum formatting behavior across flags, duplicates, masks, complements, and underlying types.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs	Implements the enum “beautification” logic (member references, flags unions, initializer-specific guardrails).
ICSharpCode.Decompiler/CSharp/CSharpDecompiler.cs	Switches enum member initializer generation to use ConvertEnumValue when safe, falling back for unusual underlying types.
ICSharpCode.Decompiler.Tests/TestCases/Pretty/EnumTests.cs	Adds fixtures covering duplicates, flags unions, complement handling, mask-family heuristics, and underlying-type edge cases.

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