title: efcore 3.1 does not support querying with string concatenation?
body: <p>Is there any way to query with EFCore 3.1 by joining multiple fields together with String.Format, or $"{}" or just the traditional "" + "" + ""?</p>
  
  <p>I have this code:</p>
  
  <pre><code>await this.Db.ACoolDbSet.Where(y =&gt; y.Plums + " " + y.Pears == "LOL").ToListAsync();
  </code></pre>
  
  <p><code>Plums</code> and <code>Pears</code> are integers. </p>
  
  <p>It results in this error:</p>
  
  <pre><code>System.InvalidOperationException: 'Null TypeMapping in Sql Tree'
  </code></pre>
  
  <p>Is this expected?</p>
  
  <pre><code>This exception was originally thrown at this call stack:
  Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.SqlTypeMappingVerifyingExpressionVisitor.VisitExtension(System.Linq.Expressions.Expression)
  System.Linq.Expressions.Expression.Accept(System.Linq.Expressions.ExpressionVisitor)
  System.Linq.Expressions.ExpressionVisitor.Visit(System.Linq.Expressions.Expression)
  Microsoft.EntityFrameworkCore.Query.SqlExpressions.SqlBinaryExpression.VisitChildren(System.Linq.Expressions.ExpressionVisitor)
  System.Linq.Expressions.ExpressionVisitor.VisitExtension(System.Linq.Expressions.Expression)
  Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.SqlTypeMappingVerifyingExpressionVisitor.VisitExtension(System.Linq.Expressions.Expression)
  System.Linq.Expressions.Expression.Accept(System.Linq.Expressions.ExpressionVisitor)
  System.Linq.Expressions.ExpressionVisitor.Visit(System.Linq.Expressions.Expression)
  Microsoft.EntityFrameworkCore.Query.RelationalSqlTranslatingExpressionVisitor.Translate(System.Linq.Expressions.Expression)
  Microsoft.EntityFrameworkCore.Query.RelationalQueryableMethodTranslatingExpressionVisitor.TranslateExpression(System.Linq.Expressions.Expression)
  ...
  [Call Stack Truncated]
  </code></pre>
  
  <p>Adding y.Plums.ToString() and y.Pears.ToString() resolves the issue. String.Format and $"{}" are still not working unfortunately</p>