Chapter 30: Text Formatting and Diagnosticslinear-chapter30

Co-authored-by: Hsiang-Ying Fu <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: fuhsnn <66062782+fuhsnn@users.noreply.github.com>
Co-authored-by: Symious <14933944+symious@users.noreply.github.com>
Co-authored-by: Jun Kawasaki <jun784@gmail.com>
Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

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+# Construction: field types and incomplete references
+
+Instance constructors use ordinary field Loads and Stores. There are two checks:
+whether a field has a value, and whether an object reference is usable outside
+construction. Neither requires a collection of constructor-group identities.
+
+## Field initialization
+
+New produces private `TypeMem` containing the object's `TypeStruct`, with
+`TypeScalar.BOT` for fresh fields. Defaults emit ordinary Stores: zero for an
+ordinary integer, null for a nullable pointer, no Store for a required field.
+An assignment strongly updates its field in private memory. Phi meets the
+incoming memories; a missing live arm survives because `BOT.meet(7) == BOT`.
+An initialized unknown integer has `TypeInteger.BOT`, not `TypeScalar.BOT`.
+Arrays retain their language-level zero filling.
+
+One `#selfMem` binding carries this memory through scopes and returns. A Load
+checks its incoming field type. Constructor completion checks the merged Return
+memory; the explicit constructor receives the declaration initializer's merged
+result, including early exits. There is no backwards initialization query,
+per-field parser state, or store-all instance exit.
+
+The existing Iter/Opto passes propagate these types; TypeCheck reports errors.
+There is no extra construction type solve or retained source-use list. An
+initialized field carried unchanged around a loop needs the optimistic SCCP
+solution. Numeric fields in loop memory widen as scalar loop values do.
+Arithmetic, conversions, and numeric Stores preserve Scalar.BOT, so a live bad
+read cannot disappear merely because it is multiplied by zero. Genuinely unused
+reads may be removed by DCE.
+
+The allocator's ConstructNode retains the constructor's final private memory
+for its completion check. TypeCheck gathers these nodes during its existing
+error walk and removes checked boundaries before lowering. The pass removal
+exposes known failures; see [the late-check notes](deferred-construction.md).
+
+Internal construction memory parameters use actual allocation/initializer call
+edges; the hypothetical external caller does not supply their memory. They
+retain their memory family when there are no live callers. Function-valued
+fields retain their declared signatures in private summaries, avoiding repeated
+unfolding of a constructor whose returned memory contains its own function field.
+Class singleton initialization keeps its older store-at-exit implementation.
+
+## Partial references and readable fields
+
+`self` and incoming `C ^arg` parameters are incomplete references. `C? ^arg`
+also accepts null. Ordinary functions and constructors accept the qualifier on
+struct parameters; arrays cannot be qualified this way.
+
+- An allocation result is incomplete if any actual argument is incomplete.
+- With only completed arguments, including null, the result is complete.
+- An incomplete reference may be stored into self or passed to a `^` parameter.
+- Ordinary calls and stores into other objects require completed references.
+- A borrowed partial can read fields its incoming type proves initialized.
+  It cannot write fields, even non-finals. Exact local aliases of self retain
+  the original constructor's access to its current private memory.
+- An object-reference field read through a partial is conservatively
+  incomplete, with no initialized-field guarantees for its referent. There is
+  no inference of initialized-field requirements on `^` arguments.
+  Arrays are already initialized and cannot receive
+  incomplete elements, so an initialized array field can be used normally.
+
+For example:
+
+```simple
+struct Child {
+    Test.Parent? parent;
+    new Child = { Test.Parent? ^par -> parent = par; };
+};
+struct Parent {
+    Child child;
+    int~ payload;
+    new Parent = { int n ->
+        child = new Child(self); // Incomplete result: self is incomplete.
+        // Retention does not authorize reading parent.payload through child.
+        payload = n;
+    };
+};
+val parent = new Parent(arg);
+val local = new Child(parent);  // Complete argument, complete result.
+return local.parent==parent;    // Lookup is allowed.
+```
+
+Here `Test` names the source unit. After the root Parent constructor completes,
+the entire cycle is usable: all members have initialized their own fields, and
+none could expose an incomplete reference to ordinary code during construction.
+The Child constructor still treats its `^par` as incomplete when the actual
+argument is complete. This public constructor also has unknown callers, whose
+partial arguments promise no initialized fields, so `par.payload` is unreadable.
+
+## Forward field views
+
+At a use of self, `InitViewNode` combines the pointer with that program point's
+private memory. Its TypeMemPtr contains declared types for initialized fields
+and Scalar.BOT for missing fields. The node retains the declaration separately
+from the incoming flow shape, so an exact self alias captured before a Store
+can receive a fresh view afterward. Earlier calls retain their earlier views.
+
+For example, a private helper can read `x` while `y` is still unset:
+
+```simple
+struct _C {
+    int~ x;
+    int~ y;
+    new _C = { int n -> x=n; y=_read(self); };
+};
+val _read = { _C ^p -> return p.x; };
+return new _C(arg).y;
+```
+
+Parm meets the incoming shapes using the normal type flow. If any live caller
+lacks `x`, Scalar.BOT survives and the Load reports the missing initialization.
+Unknown callers supply an empty field promise. Parsing must also start borrowed
+parameters at that bound: otherwise early folding of `p.x*0` could hide a bad
+read before SCCP. The hidden constructor receiver retains its declaration;
+its private memory supplies its read checks and precise field updates.
+
+Values still come from normal memory. Before a call borrowing self, EscapeNode
+borrow slices carry the current constructor Stores onto the ordinary call
+memory edge. These slices are aliasable and do not publish the private identity
+or its fields into the global escape sets. No hidden partial-memory arguments,
+field bitsets, additional passes, or recursive flow queries are introduced.
+
+InitView folds before TypeCheck when its input already has the same type and
+retains a structural partial view. Nested partial-only Construct wrappers also
+collapse. A Parm's current partial type alone is insufficient: inlining may
+later substitute a complete actual argument, which must not grant the borrowed
+parameter permission to write or publish that object. The completion form of
+Construct already folds once its memory proves initialization and its result
+type matches its input.
+
+TypeCheck checks the consumers and erases remaining InitViews along with the
+checked Construct boundaries in its existing walk. A surviving field view is
+not necessarily erroneous: a reader may need only the initialized subset.
+Construct remains necessary for
+final-field completion and for the partial qualifier of each source `new`.
+Alias Phis and nested reference loads are conservative; they do not acquire
+the original constructor's write permission. This prototype adds no general
+ban on overlapping allocations of the same class: only the owning receiver
+uses private memory, while borrowed reads use aliasable memory.
+
+## Aliases and loops
+
+Incomplete is a property of a value, not just the current contents of a parser
+Var. Consider `while (n--) { use(p); p = self; }`: the backedge introduces an
+incomplete value after `use(p)` has been parsed. `TypeMemPtr._partial` carries
+that restriction through ordinary forward typing. It is an independent
+two-point lattice: meet ORs the restriction and dual complements it. Copies,
+conversions, Stores, and Phis preserve it; a merge is incomplete if any live
+input is incomplete. A `^` formal stays incomplete even when its known callers
+are complete. The receiver's private memory identifies the own-self exception;
+another incomplete pointer of the same class has no such permission.
+
+A small `ConstructNode` wraps each source `new`. Its runtime value is the
+allocator result; its type adds the OR of that call's actual argument qualifiers.
+This distinguishes `new Child(self)` from `new Child(null)` even when both call
+the same constructor. Looking only at the shared callee Return would lose this
+distinction. The node disappears after TypeCheck, before lowering.
+
+A completed receiver gives a completed view of its pointer fields. Construction
+may retain incomplete references but cannot publish them, so reaching the root
+`new` boundary with complete arguments makes the entire reachable cycle usable.
+There is no group identity or traversal to mark members complete. Conversely,
+object-reference fields read through incomplete self remain conservative, even
+if the particular stored reference was complete.
+
+The existing optimistic SCCP solve also handles qualifier cycles. For
+`p = null; while (n--) p = new Link(p);`, no input introduces incompleteness,
+so `p` stays complete and a subsequent field read is allowed. A backedge carrying
+self instead makes the Phi and its uses incomplete.
+
+`Construction` selects private self memory during parsing; it retains no
+source-use lists or flow facts and performs no recursive Load/Phi/Call query. The old
+root and allocation bitsets are gone. There are no field-requirement sets,
+hidden `^` memory arguments, or runtime group objects.
+
+`PartialReadTest` exercises field reads, missing fields, forwarding, merged
+callers, loops, exact aliases, constructor retention, and borrowed write and
+publication restrictions. `ConstructionGroupTest` exercises cycles, complete/null arguments, prohibited
+lookups/publication, aliases, branch merges, complete and incomplete loop
+backedges, and declared restrictions with only complete callers, including an
+uninlined constructor through machine encoding. `ConstructorCompletionTest`,
+`FinalConstructionTest`, and `BoundaryTest` exercise field-memory semantics.
+The former inferred-requirement litmus tests are preserved, explicitly deferred,
+in `ConstructionRequirementsTest`. Separate compilation of `^` functions is
+still deliberately unsupported. The pointer qualifier has an object-file
+representation, but importing and checking the full construction contract is
+outside this experiment. Rebuild chapter 29 object files after the type-format
+change.
+
+## Deferred field-aware contracts (2026-11-07)
+
+Decision: do not implement inferred contracts now. Cliff considers the required
+infrastructure too large for the feature alone; revisit when a general
+backwards/liveness flow is justified, potentially for memory management.
+The later forward-field-view prototype above does not implement this contract
+discovery proposal. Separate compilation of `^` interfaces remains rejected.
+
+The intended extension would let functions and constructors receive partially
+initialized objects, read initialized fields, retain partial references with
+their restrictions, and write non-final fields. Only the original constructor
+would retain permission to initialize/reassign its own final fields; passing a
+pointer must not pass that permission. Final-field reads during construction
+would not establish value stability against later writes by that constructor.
+
+The proposed contract representation uses normal types, not a separate field
+bitset. For a function reading only `P.pi`, its unknown-caller default would be
+schematically `Partial{pi:declared-pi-type, name:TypeScalar.BOT}`. Scalar.BOT
+means no initialization guarantee, not a requirement that the caller leave the
+field unset. Required fields receive their declared types, covering every legal
+external caller. MEET with actual arguments preserves those guarantees only
+when the callers satisfy them; extra initialized fields supplied by locals do
+not strengthen the body beyond the external contract. TypeStruct/Field already
+meet field contents, and nested/cyclic types can represent nested contracts.
+TOP is not the missing-initialization marker.
+
+Cliff proposed discovering conservative, context-insensitive requirements after
+Iter and before SCCP. Freeze the resulting default argument types on functions;
+SCCP consumes them without refining the contracts. Reads on branches SCCP later
+eliminates may still impose requirements, as may reads following assignments.
+That rejection of some valid programs is acceptable for an opt-in `^` feature.
+Combine compatible requirement shapes with JOIN; combine possible caller states
+with MEET. Keep the contract distinct from the merged Parm type, and check each
+actual argument against it for a call-site diagnostic such as
+"foo requires Partial.pi to be initialized". Serialize contracts with declaration
+metadata so checking imported calls does not require loading bodies.
+
+The complication is discovering and propagating those contracts. Local SSA
+aliases share values; Phi demands reach every input. A demand on a loaded field
+becomes a nested receiver requirement. Calls substitute actual arguments for
+formals, and recursive forwarding needs a convergent propagation rule. Cyclic
+types represent recursive requirements but do not by themselves prevent a
+discovery algorithm from unfolding progressively deeper paths. Indirect calls
+need contracts available through their callable interfaces; returns need usable
+guarantees without retroactively strengthening an exported contract for a client.
+Stored references and helper writes also require a coherent account of private
+construction memory and object identity; that discussion was deliberately left
+unfinished. None of this requires H-M or bidirectional type inference, but it
+does require a substantial backwards demand analysis to handle well.
+
+The ignored ConstructionRequirementsTest retains useful litmus cases, not an
+approved implementation plan. Do not restart this work without a new request.