fix panic with unknown function inside user function

compiler/semantic/dagen.go

@@ -396,6 +396,13 @@ func (d *dagen) expr(e sem.Expr) dag.Expr {
 	case *sem.AggFunc:
 		d.error(e, errors.New("call to aggregate function in non-aggregate context"))
 		return &dag.BadExpr{Kind: "BadExpr"}
+	case *sem.BadExpr:
+		// The translator reports an error for each bad expression so
+		// a DAG with bad expressions is never run.  But bad expressions
+		// can appear in the functions assembled to evaluate a constant
+		// expression at compile time (see evaluator.maybeEval).  Since
+		// functions are compiled only when called, these are harmless.
+		return &dag.BadExpr{Kind: "BadExpr"}
 	case *sem.ArrayExpr:
 		return &dag.ArrayExpr{
 			Kind:  "ArrayExpr",

compiler/semantic/ztests/const-eval-bad-func.yaml

@@ -0,0 +1,14 @@
+# An error in a function body is reported (not a panic) when a later
+# expression is evaluated at compile time to check for a constant (here,
+# the index this[k]).
+spq: |
+  fn bad(x): nosuch(x)
+  values bad(this), this[k]
+
+input: |
+  {a:1,k:"a"}
+
+error: |
+  no such function at line 1, column 12:
+  fn bad(x): nosuch(x)
+             ~~~~~~~~~