Adds the `default` directive on an indexed property, enabling subscript
sugar on the object itself:
property Items[I: Integer]: T read Get write Put; default;
...
V[0] := 10; // lowers to V.Put(0, 10)
WriteLn(V[0]); // lowers to V.Get(0)
This is the mechanism behind the familiar List[i] syntax and was a real
foundational gap (the directive did not even parse — "Expected ':'").
Implementation:
- Parser: accept the trailing `default;` directive after a property
declaration; set TPropertyDecl.IsDefault.
- AST / symbol table: IsDefault on TPropertyDecl and TPropertyInfo;
TRecordTypeDesc.FindDefaultProperty walks the inheritance chain.
- Semantic: Obj[I] read (AnalyseStringSubscriptExpr) synthesises the
default property's field access and reuses the indexed-property read
path; Obj[I] := V write (AnalyseStaticSubscriptAssign) records the
setter on the TStaticSubscriptAssign node.
- Codegen (both backends): the write path emits the setter call via the
existing PropAccessorTarget / EmitPropAccessorCallNative helpers, so it
honours virtual/override on the accessor; the read path delegates the
TStringSubscriptExpr to its folded property-read field access.
- Cross-unit: IsDefault is serialised in the .bif interface so a default
property declared in one unit keeps its subscript sugar elsewhere.
Verified read, write, string-element, inherited, and cross-unit cases on
both backends. Adds IR tests (TPropertyTests.TestCodegen_DefaultProperty_
{Read,Write}) and dual-backend e2e tests (TE2EPropertyTests.TestRun_
DefaultProperty_{ReadWrite,StringElement,Inherited}). Grammar and
language-rationale updated; the two new TStaticSubscriptAssign fields are
marked safe in bif-coverage.status.
798df8eb15