Fix crash decompiling reference assemblies with stripped method bodies

[siegfriedpammer]

Problem

CSharpDecompiler.IsAccessorInterfaceImplementationRuntimeHelper bounds the IL body of a candidate accessor from above, but never from below. Reference assemblies keep the method RVA while stripping the body to zero bytes, so an explicit interface accessor there passes the size check and the very first opcode read runs off the end of the blob. The BadImageFormatException escapes MemberIsHidden and aborts decompilation of the entire type.

Repro with any Microsoft.NETFramework.ReferenceAssemblies pack:

ilspycmd .../microsoft.netframework.referenceassemblies.net48/1.0.3/build/.NETFramework/v4.8/System.Web.dll -t System.Web.HttpApplication

---> System.BadImageFormatException: Read out of bounds.
   at ICSharpCode.Decompiler.Disassembler.ILParser.DecodeOpCode(BlobReader& blob)
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.IsAccessorInterfaceImplementationRuntimeHelper(...)
   at ICSharpCode.Decompiler.CSharp.CSharpDecompiler.MemberIsHidden(...)

Fix

A minimumMethodSize counterpart to the existing maximumMethodSize, plus three guards the bound cannot cover on its own: minimumMethodSize assumes the one-byte ldarg.N encoding, but ldarg encodes as four bytes, so a body that passes the bound can still be consumed by the argument loop before the trailing call/ret.

The sibling fixed-sequence matcher, TransformDisplayClassUsage.IsCallToObjectCtor, already guards its lower bound (reader.Length < 7) and catches BadImageFormatException; this one did not. I audited every DecodeOpCode call site: all others are while (blob.RemainingBytes > 0) scanners whose opcode read is guarded by the loop condition, so no change is needed there and no bounds-checking overhead is added to normal IL blobs.

Testing

• New ILPretty fixture TruncatedAccessorBody, red before the fix with the same stack trace, green after. ilasm cannot emit a zero-byte body, so the fixture uses the truncated-tail variant of the same defect.
• System.Web, System.Web.Extensions, mscorlib, System and System.Core from the net48 reference pack now decompile in full with zero errors.
• Full ILSpy.XPlat.slnf suite: 3367 tests, 0 failed, 44 skipped (Windows-only).

How it was found

Fuzzing the nuget.org catalog through the decompiler. Every package whose dependency closure resolved to the Microsoft.NETFramework.ReferenceAssemblies packs hit this.

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Written by an AI agent (Claude) on Siegfried's behalf.