Built-in / RTL Exception base class

• Type: feature (compiler / RTL)
• Status: DONE 2026-06-21 (RTL Exception base in lib/rtl/sysutils.pas; chess compiles past class(Exception), now blocks further down on feature-local-typed-constant at chess.pas:846)
• Owner: Track B
• Resolved-in: ee5e568 (RTL Exception in lib/rtl/sysutils.pas) + feature-empty-class-shorthand; closed 0ceee3c.
• Opened: 2026-06-20 (demo dashboard against pinned v14)
• Relation: blocks feature-demo-chess and any FPC-style code that declares exception classes as descendants of Exception.

Symptom

examples/chess/chess.pas fails to compile against pinned stable v14:

pascal26:85: error: base type not found: Exception ()

The source declares:

EChess = class(Exception);

PXX already supports raising class instances and typed on E: TClass do handlers (test/test_exception_typed.pas), but there is no canonical Exception class in the default scope / RTL.

Scope

• Provide a minimal FPC-style Exception base class usable as class(Exception).
• Keep existing class-object exception behavior working.
• Decide whether the class lives in the builtin namespace, sysutils, or both via compiler prelude / default import rules.

Acceptance

• examples/chess/chess.pas gets past EChess = class(Exception).
• Existing exception tests still pass, especially test/test_exception_typed.pas.
• A small regression test can declare, raise, and catch class(Exception).

Log

• 2026-06-20 — Opened after make demos on pinned v14 failed on chess.
• 2026-06-20 — Added to make library-suite-discovery as demo_chess. Current pinned v18 output remains base type not found: Exception; the suite tags it as Track B if a minimal RTL Exception class is sufficient, Track A if default-scope / exception-runtime behavior needs compiler support.
• 2026-06-20 — Track B slice implemented in lib/rtl/sysutils.pas, matching FPC's home for the base class: Exception = class with Message, HelpContext, and Create(const msg). test/lib_sysutils.pas now verifies inherited construction, property access, raise, and typed catch via a local descendant.
• 2026-06-20 — Verified make library-suite green with the SysUtils base class. demo_chess now gets past the missing base class and fails on the next FPC syntax gap, EChess = class(Exception); as an empty descendant shorthand. Tracked separately in feature-empty-class-shorthand.
• 2026-06-21 — CLOSED. feature-empty-class-shorthand landed (in done/), so EChess = class(Exception); now parses. Reconfirmed: ./compiler/pascal26 -Fulib/rtl -Fulib/pcl examples/chess/chess.pas no longer errors on Exception — it advances to chess.pas:846 (local typed constant not supported), tracked in feature-local-typed-constant. Acceptance met: chess past the base class; raise/catch class(Exception) covered by test/lib_sysutils.pas; make test green. → done/.