Improve error for higher-order logic (#8207)

Makes the error more informative when a higher-order function variable is encountered.
Fixes cvc5/cvc5-projects#450.

diff --git a/src/theory/uf/theory_uf.cpp b/src/theory/uf/theory_uf.cpp
index 9058a57f3772d65d98c1355425c29d204cfc8407..e22698bc0b31b840d24eea9a85614c1026968bab 100644 (file)
--- a/src/theory/uf/theory_uf.cpp
+++ b/src/theory/uf/theory_uf.cpp
@@ -221,7 +221,15 @@ TrustNode TheoryUF::ppRewrite(TNode node, std::vector<SkolemLemma>& lems)
     if (!isHol)
     {
       std::stringstream ss;
-      ss << "Partial function applications are only supported with "
+      if (k == kind::HO_APPLY)
+      {
+        ss << "Partial function applications";
+      }
+      else
+      {
+        ss << "Function variables";
+      }
+      ss << " are only supported with "
             "higher-order logic. Try adding the logic prefix HO_.";
       throw LogicException(ss.str());
     }