Fix #3804: decompile foreach over inline array

[siegfriedpammer]

Fixes #3804.

Problem

Decompiling a foreach over an inline-array value emitted code referencing the compiler-internal <PrivateImplementationDetails>.InlineArrayElementRef helper, whose type name contains angle brackets and cannot be written in C# — so the output did not compile.

Root cause

The C# compiler lowers foreach over an inline array into a for loop whose body calls <PrivateImplementationDetails>.InlineArrayElementRef(ref b, i) with the loop variable as the index. InlineArrayTransform.MatchInlineArrayElementRef only recognized a constant index (LdcI4), so the call survived into the output verbatim.

Fix

Accept a non-constant index in the matcher. The buffer-length range check only applies to a compile-time-constant index; the compiler emits this helper only where the index is provably in range. Everything downstream (LdElemaInlineArray, indexer rendering) is already index-agnostic, so the body recovers to the inline-array indexer b[i], producing a valid for loop. The decompiler does not reconstruct the original foreach, but the output now compiles.

Test

Adds a Sum(Byte16 b) method to the InlineArrayTests Pretty fixture. Because the fixture is both input and expected output, it uses the EXPECTED_OUTPUT idiom: the foreach is compiled (to trigger the lowering), and the for loop over b[i] is the expected decompilation. Verified against all roslyn4OrNewerOptions configs.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Fixes a C# decompilation bug where foreach over an inline-array value could decompile into a reference to the compiler-internal <PrivateImplementationDetails>.InlineArrayElementRef(...), producing non-compilable C# due to the angle brackets in the type name. The change updates the inline-array helper matcher so the call is rewritten into the inline-array indexer form (e.g. b[i]) even when the index is not a compile-time constant, and adds a regression test.

Changes:

• Relax InlineArrayElementRef pattern matching to accept non-constant index expressions while keeping bounds validation for constant indices.
• Add a Pretty test case that compiles a foreach over an inline array and expects decompilation into a compilable for loop using b[i].

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
ICSharpCode.Decompiler/IL/Transforms/InlineArrayTransform.cs	Allows matching InlineArrayElementRef with non-constant indices (e.g. loop variable from foreach lowering), enabling rewrite to ldelema.inlinearray and ultimately b[i].
ICSharpCode.Decompiler.Tests/TestCases/Pretty/InlineArrayTests.cs	Adds a regression fixture using the EXPECTED_OUTPUT idiom to validate decompilation of foreach over an inline array into a compilable loop.

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[dgrunwald]

This changes the behavior with an out-of-bounds non-constant index.

This transform is only valid if the decompiler can prove the index is in-bounds.