Add resolution diagnostic for multi-output procedure in expression position

Strata/Languages/Laurel/Resolution.lean

@@ -213,6 +213,10 @@ structure ResolveState where
   /-- When resolving inside an instance procedure, the owning composite type name.
       Used by `resolveFieldRef` to resolve `self.field` when `self` has type `Any`. -/
   instanceTypeName : Option String := none
+  /-- True when resolving inside an expression where the value is used (e.g., as an
+      argument to another call or operator). Multi-output calls are only diagnosed
+      in value context, not in statement position or direct assignment RHS. -/
+  inValueContext : Bool := false
 
 @[expose] abbrev ResolveM := StateM ResolveState
 
@@ -437,10 +441,30 @@ def resolveStmtExpr (exprMd : StmtExprMd) : ResolveM StmtExprMd := do
   | .StaticCall callee args =>
     let callee' ← resolveRef callee source
       (expected := #[.parameter, .staticProcedure, .datatypeConstructor, .constant])
+    -- Resolve arguments in value context (their results are used as values)
+    let saved := (← get).inValueContext
+    modify fun s => { s with inValueContext := true }
     let args' ← args.mapM resolveStmtExpr
+    modify fun s => { s with inValueContext := saved }
+    -- Multi-output procedures must not appear in value context: the extra
+    -- outputs (e.g. error channels) would be silently discarded.
+    let s ← get
+    if s.inValueContext then
+      let outputCount := match s.scope.get? callee'.text with
+        | some (_, .staticProcedure proc) => proc.outputs.length
+        | some (_, .instanceProcedure _ proc) => proc.outputs.length
+        | _ => 0
+      if outputCount > 1 then
+        let diag := diagnosticFromSource source
+          s!"Multi-output procedure '{callee'.text}' used in expression position; it returns {outputCount} values but only one can be used here. Use a multi-target assignment instead."
+        modify fun s => { s with errors := s.errors.push diag }
     pure (.StaticCall callee' args')
   | .PrimitiveOp op args =>
+    -- Resolve arguments in value context
+    let saved := (← get).inValueContext
+    modify fun s => { s with inValueContext := true }
     let args' ← args.mapM resolveStmtExpr
+    modify fun s => { s with inValueContext := saved }
     pure (.PrimitiveOp op args')
   | .New ref =>
     let ref' ← resolveRef ref source

StrataTest/Languages/Laurel/ResolutionKindTests.lean

@@ -97,4 +97,17 @@ composite Foo extends nat { }
 #guard_msgs (error, drop all) in
 #eval testInputWithOffset "ExtendConstrained" extendConstrained 90 processResolution
 
+/-! ## Multi-output procedure used in expression position -/
+
+def multiOutputInExpr := r"
+procedure multi(x: int) returns (a: int, b: int) opaque;
+procedure test() opaque {
+  assert multi(1) == 1
+//       ^^^^^^^^ error: Multi-output procedure 'multi' used in expression position
+};
+"
+
+#guard_msgs (error, drop all) in
+#eval testInputWithOffset "MultiOutputInExpr" multiOutputInExpr 100 processResolution
+
 end Laurel