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A generic specialized before its declaration is unresolvable in DELPHI mode

Found 2026-08-28 by frankB (Track P) driving feature-pascal-corpus-generics rung 3, after the typinfo/PTypeData blocker cleared and the wall moved from 2082 to 3250.

The repro — one line of difference, and an oracle on both sides

program y_after;
{$MODE DELPHI}{$H+}
type
  TBase<T> = class
    class function Ordinal: TObject;
  end;
  TDeriv<T> = class            { declared AFTER the class that uses it }
  end;
class function TBase<T>.Ordinal: TObject;
begin
  Result := TDeriv<T>.Create;
end;
type TInst = TBase<UnicodeString>;
begin WriteLn(Assigned(TInst.Ordinal)); end.



pascal26:11: error: undefined variable (specialize)
  near: TObject  begin Result  specialize >>> TDeriv  UnicodeString
pascal26:11: error: undefined variable (TDeriv)

Move the TDeriv<T> declaration above TBase<T> — changing nothing else — and pxx compiles and runs it. fpc 3.2.2 accepts both orderings and prints TRUE for each, so there is a live oracle on both arms, not just on the failing one.

What is NOT the trigger

Each of these was varied independently and none of them matters; recorded so the next reader does not re-walk the ladder:

variation result
{$mode objfpc} with explicit generic / specialize compiles — the defect is DELPHI-mode only
specializing a DESCENDANT vs an unrelated SIBLING fails either way
one type parameter vs two fails either way
generic name overloaded by arity (TBox<T,F> and TBox<T>) compiles — not this
class var present or absent irrelevant on its own
class function vs plain method irrelevant on its own
the specialization inside if ... then vs a direct Result := irrelevant on its own

The only thing that moves the verdict is which of the two types is written first. Every "combination" that looked load-bearing at first was an artifact of my repros happening to also reorder the declarations.

Two more measurements that narrow it to instantiation time

variant of the failing program result
nothing instantiates TBase (drop TInst, empty main) compiles
the body writes TDeriv<UnicodeString> — a CONCRETE argument — instead of TDeriv<T> compiles

So the defect needs all three of: DELPHI mode, the argument being the enclosing template's own PARAMETER, and the enclosing template actually being instantiated. Neither the declaration nor the parameterised body is enough on its own — the failure is at instantiation time, on a token group that does not exist in the original source.

That is the useful part for whoever fixes it: with a concrete argument the group TDeriv<UnicodeString> is present in the token stream from the start and is rewritten normally. With TDeriv<T> the concrete group only comes into existence when TBase is instantiated and T is substituted — and by then the rewriting for TDeriv has already run.

Why it is DELPHI-mode only, and where to look

In {$mode objfpc} the programmer writes specialize and the token is in the source. In DELPHI mode there is no such keyword — pxx inserts it while normalising X<T>, and the near: window shows the inserted token: begin Result specialize >>> TDeriv UnicodeString. The failing arm therefore has the parser meeting its own inserted specialize in expression position and resolving it as an identifier, which is why the diagnostic says undefined variable (specialize) — a message that names a keyword as a variable, which is itself the tell.

The two arms differ only in whether the generic's symbol exists at the moment that normalisation runs, so the likely cause is that the DELPHI <...> rewrite is driven off a lookup that happens during a single forward pass rather than after the type section is complete.

Do not fix it by making the resolver tolerate the token. This is the double-case shape from devdocs/dev/normalise-dont-special-case.md: the same construct is reachable through declared-before and declared-after, one path works, and the second path is the one that stays broken. The declared-before arm is the normal form; the fix is to make declared-after reach it, not to grow a second arm.

Gate

make compiler/pascal26 (self-host fixedpoint) plus both repro arms compiling and printing TRUE, plus the corpus wall at generics.defaults.pas:3250 moving. Add both orderings as an fpc-derived regression pair in test-core — both arms, since the passing one is what stops a fix from being written as a special case.

Scope

Track P (Pascal frontend, generic specialization in DELPHI mode). If the fix turns out to need a change below the frontend — IR ops, backends, ABI — that is a Track A ticket to file, not to edit under P.

Localisation — measured vs inferred, kept apart

Measured: every specialize handler is in compiler/pasparser_*.inc (_generic, _expr, _decl, _proc, _name, _prog) and nothing in lexer.inc participates. So this is entirely inside Track P's carved-out files and needs no Track A change — which also means it does not collide with an agent holding A or O.

Inferred, NOT verified — treat as a lead, not a finding. DelphiRewriteGenericUses (pasparser_generic.inc:445) sweeps the token stream per template from a var insertAt cursor, run to a fixed point by its caller at line 905, and its own header comment says the ordering requirement is that each round's aliases land after the previous round's "since an outer alias refers to an inner one". A specialization group that only comes into existence during a later instantiation is exactly the case that ordering has to cover. I did not instrument this and I am not asserting it: the repo's own history is that a plausible story recorded as a cause is the expensive failure here, and the last entry in the rung ticket above is itself a corrected wrong root cause. Whoever takes it should instrument the sweep (PXXDBG) rather than build on this paragraph.

Lead (UNMEASURED) — possibly the same defect as the parked ordering ticket

Recorded by the coordinator on frankA's suggestion, labelled as a lead because nobody has measured it. frankA, which parked the earlier generics-ordering ticket on this rung, reads this one as possibly the same defect family seen from a cleaner angle.

Worth one check before anyone opens a second diagnosis: if they are the same mechanism, this is the ticket to work, because its repro is 14 lines with fpc 3.2.2 accepting both orderings — an oracle on the passing arm as well as the failing one — where the parked ticket's starting position is worse. frankA said so itself.

If they are not the same, say so here explicitly rather than leaving the lead open. An unresolved "these might be one thing" note is the shape that gets believed later: it reads as prior investigation and is not.

Resolved 2026-08-28 (frankA)

This ticket and [[bug-p-a-generic-prerequisite-is-emitted-before-the-referenced-template-exists]] are one defect — settled affirmatively, so one fix closes both. The evidence is in that ticket: the parked repro reduces to this one (strip its incidental inheritance and it fails identically), and both respond the same way to frankB's two perturbations. Reduction, not resemblance — a symptom match alone would not have been enough.

Ruled out first, so nobody re-derives them: frankB's seven variations, all carried over rather than re-run.

Root cause

NestedSpecKnown asks whether a prerequisite is registered — FindSpecialization or FindUClass — not whether a token naming it has been inserted somewhere. So a prerequisite discovered while a method body is being streamed can no longer be declared: emitting before the collapse is too early (nothing registered yet), emitting after it is too late (already collapsed). The scan that finds these prerequisites runs at ParseSpecialization time, when the Delphi rewrite has not yet swept the method impls that name them, so in the failing shape it finds nested=0 and nothing is emitted at all.

The fix does not fight that window — it uses an earlier one. At the end of a top-level type section the rewrite has swept those bodies, and the method impls themselves are still ahead of the parser. EmitLateNestedSpecDecls re-runs the same GenMethImplSOff-bounded scan there and emits declarations for whatever is still unknown, so the alias is registered before the method is streamed at all.

Two details that are measured, not reasoned — both broke first:

Verification (binary 5c9d52bdd0bf)

FPC seed drift, twice in this session. make compiler/pascal26 was green throughout; gate.sh quick went RED because three helpers were called from an include above their definitions. The second occurrence is the interesting one: the forwards existed, but sat below a caller that a later refactor had lifted above them, so "I already added forwards" was true and useless.

Log