Patch for custom binary operators that use standard comparison symbols

When a custom binary operator uses a standard comparison operator (=, <>, <, >, <=, >=)
EF Core will generate a pattern that causes a syntax error in PostgreSQL.

Example SQL:
  WHERE a < b = TRUE

This patch adds logic to check if the custom binary operator is one of the standard
comparison operators. If so, it wraps the expression in parenthesis.

While it seems like this could be avoided by having the custom translator construct
a standard Expression.LessThan(Expression,Expression), this causes an error to be
thrown...because the binary operator isn't defined.

See the related links for more information.

Example stack trace:

  System.InvalidOperationException : The binary operator LessThanOrEqual is not defined
     for the types 'System.Net.IPAddress' and 'System.Net.IPAddress'.
     at System.Linq.Expressions.Expression.GetUserDefinedBinaryOperatorOrThrow(...)
     at System.Linq.Expressions.Expression.GetComparisonOperator(...)
     at System.Linq.Expressions.Expression.LessThanOrEqual(...)

Related:

- dotnet/efcore#9143
- npgsql#323 (review)

src/EFCore.PG/Query/Sql/Internal/NpgsqlQuerySqlGenerator.cs

@@ -24,6 +24,7 @@
 #endregion
 
 using System;
+using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq.Expressions;
 using System.Text.RegularExpressions;
@@ -41,6 +42,15 @@ public class NpgsqlQuerySqlGenerator : DefaultQuerySqlGenerator
     {
         readonly bool _reverseNullOrderingEnabled;
 
+        /// <summary>
+        /// The collection of standard comparison operators.
+        /// </summary>
+        /// <remarks>
+        /// See: https://github.com/aspnet/EntityFrameworkCore/issues/9143
+        /// </remarks>
+        [NotNull] static readonly HashSet<string> StandardComparisonSymbols =
+            new HashSet<string>(new[] { "=", "<>", "<", ">", "<=", ">=" });
+
         protected override string TypedTrueLiteral => "TRUE::bool";
         protected override string TypedFalseLiteral => "FALSE::bool";
 
@@ -322,12 +332,28 @@ public virtual Expression VisitCustomBinary(CustomBinaryExpression expression)
         {
             Check.NotNull(expression, nameof(expression));
 
+            //--------------------------------------------------------------------------------------------
+            // BUG: custom relational operators cause Postgres to throw 42601: syntax error at or near "="
+            // Example:
+            //   source.Where(x => EF.Functions.LessThan(x.Inet, inet));
+            //   WHERE x."Inet" < @__inet_1 = TRUE
+            //
+            // See: https://github.com/aspnet/EntityFrameworkCore/issues/9143
+            //--------------------------------------------------------------------------------------------
+            var operatorIsComparisonSymbol = StandardComparisonSymbols.Contains(expression.Operator);
+
+            if (operatorIsComparisonSymbol)
+                Sql.Append('(');
+
             Visit(expression.Left);
             Sql.Append(" ");
             Sql.Append(expression.Operator);
             Sql.Append(" ");
             Visit(expression.Right);
 
+            if (operatorIsComparisonSymbol)
+                Sql.Append(')');
+
             return expression;
         }