feat(queryexec): Project and WhereFeature tell a feature the element lacks from one that is empty — a missing-feature path error with the QVTS record, tolerate to keep the empty cell

changes/unreleased/query-feature-outcomes.changed.md

@@ -0,0 +1 @@
+- `Project` and `WhereFeature` reading a feature an element row lacks now fail with a `missing-feature` error naming the element and the features it does have (`Target`, `Property`, `Candidates`), instead of leaving the cell empty, and carry the QVTS outcome record (`Feature(<name>)` step, the element and its features as frontier); a property no row carries stays `unknown-property`. The new `tolerate = true` argument on both operations keeps the empty cell for sources that mix element kinds (the cookbook's `MassTable` uses it). Metadata paths (`Tag::key`) are unaffected.

docs/internals/design/query-outcomes.md

@@ -28,7 +28,8 @@ to `∅` is the ambiguity QVTS exists to remove.
 | `Named` | the element | `unknown-element`: the longest prefix that resolves, the segment it lacks, the nearest spellings; an alias that denotes nothing | `ambiguous-element`: the elements, by qualified name |
 | `WhereType`, `WhereMetadata` | the selected rows | `unknown-classification` | — |
 | `RelatedElements` | the neighbours, possibly none | `unknown-relationship` | — |
-| `Project`, `WhereFeature` on a feature the element lacks | *today an empty cell* `(none)` | *to do:* a path error per element, with the features reachable on it | — |
+| `Project`, `WhereFeature` on a feature the element lacks | an empty cell, with `tolerate = true` | `missing-feature`: the element (`Target`), the feature (`Property`), the features it does have (`Candidates`); `unknown-property` when no row carries it | — |
+| `Project`, `WhereFeature` on a metadata path (`Tag::key`) the element lacks | *today an empty cell* | *to do:* decide whether an absent annotation is a path error | — |
 | `Project` on a `[1]` feature with two values | *today the first value* | — | *to do:* `multiplicity` |
 
 `Error` carries the frontier in `Target` (the namespace that resolved), `Expected` (the segment it
@@ -38,7 +39,7 @@ QVTS record itself (`internal/doc/queryexec/outcome.go`), field for field with
 `lean/QVTS/Outcome.lean`: a `PathError` is a list of `Failure{Step, Expected, Frontier}`, a
 `ModelError` a list of `Finding{Step, Element, Constraint, Observed}` plus the path failures of the
 same fan-out; `Frontier` carries `At`, `Members (name, metaclass)`, `Features (name, type)`, `Kinds`
-and `Nearest`. `Named` fills it today. The surfaces that show it: `-run-query` prints it under the
+and `Nearest`. `Named` and the `missing-feature` outcome of `Project`/`WhereFeature` fill it today. The surfaces that show it: `-run-query` prints it under the
 error line and `-json` writes it as `checks[].outcome` in the shape `qvts/sparql/expected/*.json`
 records (`outcome`, `step`, `failures` | `findings` + `pathFailures`); `%run-query` prints the same
 lines; the gRPC service attaches it to the failed call as a `QueryOutcome` error detail. The

docs/manual/examples/cookbook.sysml

@@ -164,7 +164,8 @@ package Cookbook {
 		in root : Element;
 		Project(
 			source = PartsByMass(root = root),
-			properties = ("name", "mass", "qualifiedName")
+			properties = ("name", "mass", "qualifiedName"),
+			tolerate = true  // opticalPath and dataPath declare no mass
 		)
 	}
 
@@ -173,6 +174,7 @@ package Cookbook {
 		Project(
 			source = PartsByMass(root = root),
 			properties = ("name", "mass"),
+			tolerate = true,
 			columns = (
 				Column(name = "massLbs", cell = { in row : Subsystem; (row.mass ?? 0.0) * 2.2 }),
 				Column(name = "label", expression = "part: " + Element::name)

docs/manual/query-cookbook.md

@@ -101,7 +101,8 @@ calc def MassTable :> Query {
 	in root : Element;                 // entry parameters, bound by the caller
 	Project(                            // operations compose inside-out
 		source = PartsByMass(root = root),  // ... and queries invoke queries
-		properties = ("name", "mass", "qualifiedName")
+		properties = ("name", "mass", "qualifiedName"),
+		tolerate = true                 // an element lacking a property gets an empty cell
 	)
 }
 ```
@@ -550,7 +551,8 @@ calc def MassTable :> Query {
 	in root : Element;
 	Project(
 		source = PartsByMass(root = root),
-		properties = ("name", "mass", "qualifiedName")
+		properties = ("name", "mass", "qualifiedName"),
+		tolerate = true  // opticalPath and dataPath declare no mass
 	)
 }
 ```
@@ -581,8 +583,22 @@ $ sysml cookbook.sysml -run-query "Cookbook::MassTable root=Cookbook::telescope"
     qualifiedName = "Cookbook::telescope::dataPath"
 ```
 
-A cell for a property the element lacks is empty (`(none)` in the CLI's row
-listing, an empty table cell in a document). A feature whose declared value
+`opticalPath` and `dataPath` are connections: they declare no `mass`. Reading
+a feature of an element that has none so named is by default an error that
+names the element and the features it does have — a path error, telling a
+query that left the model apart from a feature that is present and empty:
+
+```console
+$ sysml cookbook.sysml -run-query "Cookbook::MassTable root=Cookbook::telescope"   # without tolerate = true
+error: query Cookbook::MassTable reads feature mass of Cookbook::telescope::opticalPath, which has no such feature
+```
+
+`tolerate = true` on `Project` (and on `WhereFeature`) accepts the mixed rows
+and leaves the cell empty (`(none)` in the CLI's row listing, an empty table
+cell in a document). A property no row carries at all is an unknown property
+whichever way — that is a misspelling, not a mixed source. A metadata path
+(`Properties::key`) is always tolerated: an annotation is optional by nature.
+A feature whose declared value
 names an element of the model — an enumeration literal, a part, a unit — holds
 that element, printed by name like `general`, whether the value is written on
 the feature or bound by a redefinition (`attribute :>> beam = Beam::'650mm';`);
@@ -785,6 +801,7 @@ calc def MassBudget :> Query {
 	Project(
 		source = PartsByMass(root = root),
 		properties = ("name", "mass"),
+		tolerate = true,
 		columns = (
 			Column(name = "massLbs", cell = { in row : Subsystem; (row.mass ?? 0.0) * 2.2 }),
 			Column(name = "label", expression = "part: " + Element::name)