Update frmts/vrt/vrtexpression.h

Co-authored-by: Even Rouault <[email protected]>

frmts/vrt/vrtexpression.h

@@ -167,25 +167,34 @@ class MuParserExpression : public MathExpression
 #endif
 
 inline std::unique_ptr<MathExpression>
-MathExpression::Create(const char *pszExpression, const char *pszDialect)
+MathExpression::Create([[maybe_unused]] const char *pszExpression,
+                        const char *pszDialect)
 {
-    CPL_IGNORE_RET_VAL(pszExpression);
-    CPL_IGNORE_RET_VAL(pszDialect);
-
-#if GDAL_VRT_ENABLE_EXPRTK
     if (EQUAL(pszDialect, "exprtk"))
     {
+#if GDAL_VRT_ENABLE_EXPRTK
         return std::make_unique<gdal::ExprtkExpression>(pszExpression);
-    }
+#else
+        CPLError(CE_Failure, CPLE_IllegalArg,
+                 "Dialect '%s' is not supported by this GDAL build. A GDAL "
+                 "build against exprtk would be needed.", pszDialect);
 #endif
-#if GDAL_VRT_ENABLE_MUPARSER
-    if (EQUAL(pszDialect, "muparser"))
+    }
+    else if (EQUAL(pszDialect, "muparser"))
     {
+#if GDAL_VRT_ENABLE_MUPARSER
         return std::make_unique<gdal::MuParserExpression>(pszExpression);
-    }
+#else
+        CPLError(CE_Failure, CPLE_IllegalArg,
+                 "Dialect '%s' is not supported by this GDAL build. A GDAL "
+                 "build against muparser would be needed.", pszDialect);
 #endif
-    CPLError(CE_Failure, CPLE_IllegalArg, "Unknown expression dialect: %s",
-             pszDialect);
+    }
+    else
+    {
+        CPLError(CE_Failure, CPLE_IllegalArg, "Unknown expression dialect: %s",
+                 pszDialect);
+    }
     return nullptr;
 }