AstSchema refuses a MemberInitialiser in ConstructionExpression.Arguments, which the AST, serializer and all four generators support

[matt-edmondson]

What happens

The Arguments slot is declared as AstSlotKind.Expression:

// Coder.Graph/AstSchema.cs:41
private static readonly AstSlot ArgumentsSlot = new(ArgumentsSlotName, AstSlotCardinality.Many, AstSlotKind.Expression);

and Accepts resolves that kind through IsExpression, which does not admit MemberInitialiser:

// Coder.Graph/AstSchema.cs:430
AstSlotKind.Expression => IsExpression(candidate),

// Coder.Graph/AstSchema.cs:449-454
public static bool IsExpression(AstNode node) =>
    node is Expression
        or AstLeafNode<string>
        or AstLeafNode<int>
        or AstLeafNode<bool>
        or AstLeafNode<double>;

MemberInitialiser derives from AstNode, not Expression (Coder/Ast/MemberInitialiser.cs:17).

Attachment is gated on that check:

// Coder.Graph/AstSchema.cs:133
if (!Accepts(slot, child))
{
    return false;
}

which makes the handling two cases further down unreachable for a MemberInitialiser:

// Coder.Graph/AstSchema.cs:175
case (ConstructionExpression construction, ArgumentsSlotName):
    construction.Arguments.Add(child);
    return true;

Note that this case deliberately carries no when child is Expression guard, unlike all five neighbouring cases — and ConstructionExpression.Arguments is typed Collection<AstNode> rather than Collection<Expression> (Coder/Ast/ConstructionExpression.cs:38). Both say the intent was to accept it.

Failure scenario

AstSchema.TryAttachAt(
    constructionExpr,
    ArgumentsSlot,
    0,
    new MemberInitialiser("x") { Value = Literal.Number(1) });

returns false and attaches nothing. In the editor, AstGraph.TryConnect refuses the link (Coder.Graph/AstGraph.cs:561), so a designated or named initialiser can never be built in the graph — and one loaded from YAML cannot be detached and reattached.

Why it matters

All four generators implement this node in exactly this position — CSharpGenerator.cs:252,343, CppGenerator.cs:544,567, PythonGenerator.cs:126,137, JavaScriptGenerator.cs:125,172 — and YamlSerializer/YamlDeserializer round-trip it. CLAUDE.md states it as a design rule: "a MemberInitialiser among its arguments is an element that names the member it is for — a designated initialiser in C++, an object initialiser in C#, a keyword argument in Python, an object literal in JavaScript."

The graph is documented as the uniform view of the AST. Here it cannot express a shape that the AST, the serializer and every generator all handle.

Missing coverage

Coder.Test/Graph/DeclarationSlotsTests.cs:126 covers MemberInitialiser's own Value slot only. Nothing tests attaching one into a ConstructionExpression.

Suggested fix

Either give construction arguments a dedicated slot kind accepting IsExpression(candidate) || candidate is MemberInitialiser, or widen the Expression arm for this slot specifically. Add a DeclarationSlotsTests case attaching a MemberInitialiser to ConstructionExpression.Arguments.

Related: #48 covers MemberInitialiser and ConstructionExpression having no editable fields; this is the separate problem of the node not being attachable at all.

[matt-edmondson]

Triage

Priority: Medium Effort: Small

What needs to be done: AstSchema's ArgumentsSlot on ConstructionExpression is typed AstSlotKind.Expression, whose IsExpression check does not admit MemberInitialiser — even though the AST (ConstructionExpression.Arguments is Collection<AstNode>, not Collection<Expression>), the YAML serializer, and all four language generators already support a MemberInitialiser there. This makes the graph/editor unable to build or round-trip a shape every other layer already handles.

Suggested next steps / acceptance criteria:

• Widen the arguments slot's acceptance to IsExpression(candidate) || candidate is MemberInitialiser (either via a dedicated slot kind or a special-cased check for this slot).
• Add a DeclarationSlotsTests case attaching a MemberInitialiser into ConstructionExpression.Arguments via AstSchema.TryAttachAt, and a graph-level test via AstGraph.TryConnect.

Blockers / dependencies: None for this issue itself. The issue notes it's related to but distinct from #48 (which covers MemberInitialiser/ConstructionExpression having no editable fields) — the two can be fixed independently and in either order.

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