A field of a union of same-layout object types is read from the base type

[ASDAlexander77]

Reading a field through a union of object types that share one layout failed verification:

type Shape = { kind: "circle", r: number } | { kind: "square", r: number };
function size(s: Shape) { return s.r; }
// error: 'ts.PropertyRef' op operand #0 must be any const tuple reference type or any tuple
//        reference type ..., but got '!ts.ref<!ts.union<!ts.tuple<…>, !ts.tuple<…>>>'

It also broke s.kind === "circle" and s.r !== null on such a union. Unions of interfaces were not affected. Found while reviewing #231.

Cause

A union whose members share one layout needs no tag (isUnionTypeNeedsTag returns false) and is stored as its base type. mlirGenPropertyAccessExpressionBaseLogic switched to that type (actualType = baseType), but two things still referred to the union:

• the value being accessed was left as the union;
• MLIRPropertyAccessCodeLogic still held the original expression, which was the Load of the union variable.

The tuple accessor took the reference behind that load (getExprLoadRefValue) and built a PropertyRef on ref<union>.

Fix

When the union collapses to its base type:

• the value is cast to that base type;
• the access logic is given the cast value through a new MLIRPropertyAccessCodeLogic::setExpression.

The field is then extracted from a value of the base type.

Tests

• New 00union_object_types_access.ts (compile, jit, and the rc/none corpus). It covers a discriminant test, a field read, and a !== null check on a nullable field of such a union.
• Full release suite: 2993/2993 passed.
• TypeScriptCompilerDefaultLib tests, release jit and compile: 156/156 each. The cast applies to every untagged union collapsed here, not only object types.

Not in this PR

With members of different layouts, if (s.kind === "circle") return …; return s.w; still fails. That needs the early-exit narrowing in #383. Narrowing inside the if already works.

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