Report ambiguous column names error based on output columns.

This is in constrast to the prior behavior, where as long as you didn't select
an ambiguous column from an outer query, you were considered OK. This let you
produce queries that spat ambiguous queries out to the consuming program source
code, which was bad.

Author: rspeele

src/Rezoom.SQL.Compiler/TypeChecker.fs

@@ -227,6 +227,7 @@ type private TypeChecker(cxt : ITypeInferenceContext, scope : InferredSelectScop
                 checker, Some texpr, None
         let columns = checker.ResultColumns(select.Columns, knownShape)
         let infoColumns =
+            let used = HashSet()
             seq {
                 for column in columns.Columns do
                     match column.Case with
@@ -237,7 +238,9 @@ type private TypeChecker(cxt : ITypeInferenceContext, scope : InferredSelectScop
                                 ColumnName =
                                     match implicitAlias (expr.Value, alias) with
                                     | None -> failAt column.Source Error.expressionRequiresAlias
-                                    | Some alias -> alias
+                                    | Some alias ->
+                                        if used.Add(alias) then alias
+                                        else failAt column.Source (Error.ambiguousColumn alias)
                             }
                      // typechecker should've eliminated alternatives
                      | _ -> bug "All wildcards must be expanded -- this is a typechecker bug"

src/Rezoom.SQL.Test/TestRoundTrip.fs

@@ -28,7 +28,7 @@ let ``select`` () =
 [<Test>]
 let ``fancy select`` () =
     roundtrip """
-        select g.*, u.*
+        select g.Id as GroupId, g.Name as GroupName, u.Id as UserId, u.Name as UserName
         from Users u
         left join UserGroupMaps gm on gm.UserId = u.Id
         left join Groups g on g.Id = gm.GroupId
@@ -38,7 +38,7 @@ let ``fancy select`` () =
 [<Test>]
 let ``fancy select with order by`` () =
     roundtrip """
-        select g.*, u.*
+        select g.Id as GroupId, g.Name as GroupName, u.Id as UserId, u.Name as UserName
         from Users u
         left join UserGroupMaps gm on gm.UserId = u.Id
         left join Groups g on g.Id = gm.GroupId
@@ -187,7 +187,7 @@ let ``date literals`` () =
 [<Test>]
 let ``join subqueries`` () =
     roundtrip """
-        select * from
+        select us.Id as UID, gs.Id as GID from
             (select u.Id from Users u) us
             join
             (select g.Id from Groups g) gs

src/Rezoom.SQL.Test/TestTypeErrors.fs

@@ -120,4 +120,14 @@ let ``cte is not in scope for update`` () =
     expectError (Error.objectNotATable "cte")
         """
             with cte(x) as (select 1) update cte set x = 1;
+        """
+
+[<Test>]
+let ``ambiguous columns`` () =
+    expectError (Error.ambiguousColumn "Id")
+        """
+            select *
+            from users u
+            join usergroupmaps ugm on ugm.userid = u.id
+            join groups g on g.id = ugm.groupid
         """