Primary-constructor conversion emits `this` in a field initializer (CS0027)

[siegfriedpammer]

Sibling of #3979: the same transform produces an uncompilable field initializer when the constructor reads the field through an explicit this..

Repro

public struct S2
{
    public int value;
    public int b;

    // The parameter shadows the field, so the qualifier is mandatory.
    public S2(int value)
    {
        this.value = value;
        b = this.value + 1;
    }
}

Decompiled with master (d60a8662a):

public struct S2(int value)
{
    public int value = value;

    public int b = this.value + 1;
}

Recompiling that output:

error CS0027: Keyword 'this' is not available in the current context

Notes

• Pre-existing, and not addressed by Keep instance-dependent assignments in constructors #3980. That PR fixes the unqualified spelling from Primary-constructor conversion creates an invalid field initializer (CS0236) #3979 (CS0236) — verified: with Keep instance-dependent assignments in constructors #3980 the B = A + 1 case keeps its real constructor. For the this.-qualified case above, Keep instance-dependent assignments in constructors #3980's output is byte-identical to master's.
• ReferencesInstanceMember (ICSharpCode.Decompiler/CSharp/Transforms/TransformFieldAndConstructorInitializers.cs:407 as of Keep instance-dependent assignments in constructors #3980) matches ThisResolveResult, both as a bare expression and as a MemberResolveResult.TargetResult. The decompiler produces that resolve result only for implicit-this simple names and base references; an explicit this.X read goes through ExpressionBuilder.ConvertField -> TranslateTarget -> ConvertVariable, so the ThisReferenceExpression carries an ILVariableResolveResult instead and neither case fires.
• Two everyday triggers: a constructor parameter shadowing the field (the usual this.value = value convention), and the AlwaysQualifyMemberReferences setting, which qualifies every reference and therefore covers the Primary-constructor conversion creates an invalid field initializer (CS0236) #3979 shape as well.

Possible direction: also treat an ILVariableResolveResult whose Variable.IsThis() — plus the ByReferenceResolveResult that wraps it for structs — as an instance-member reference. The existing generated-backing-field (<x>P) carve-out would then need to skip the target subtree, since under AlwaysQualifyMemberReferences that access is spelled this.<x>P.

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Filed by an AI agent (Claude) on behalf of @siegfriedpammer. Repro built and both outputs recompiled to confirm the diagnostics.