Fix Statement Where clone array corruption in v2

Method-chaining in gorm is predicated on a `Clause`'s `MergeClause`
method ensuring that the two clauses are disconnected in terms of
pointers (at least in the Wherec case).

However, the original Where implementation used `append`, which
only returns a new instance if the backing array needs to be resized.
In some cases, this is true.  Practically, go doubles the size of the
slice once it gets full, so the following slice `append` calls would
result in a new slice:

* 0 -> 1
* 1 -> 2
* 2 -> 4
* 4 -> 8
* and so on.

So, when the number of "where" conditions was 0, 1, 2, or 4, method-chaining
would work as expected.  However, when it was 3, 5, 6, or 7, modifying the
copy would modify the original.

This also updates the "order by", "group by" and "set" clauses.

clause/group_by.go

@@ -30,8 +30,13 @@ func (groupBy GroupBy) Build(builder Builder) {
 // MergeClause merge group by clause
 func (groupBy GroupBy) MergeClause(clause *Clause) {
 	if v, ok := clause.Expression.(GroupBy); ok {
-		groupBy.Columns = append(v.Columns, groupBy.Columns...)
-		groupBy.Having = append(v.Having, groupBy.Having...)
+		copiedColumns := make([]Column, len(v.Columns))
+		copy(copiedColumns, v.Columns)
+		groupBy.Columns = append(copiedColumns, groupBy.Columns...)
+
+		copiedHaving := make([]Expression, len(v.Having))
+		copy(copiedHaving, v.Having)
+		groupBy.Having = append(copiedHaving, groupBy.Having...)
 	}
 	clause.Expression = groupBy
 }

clause/order_by.go

@@ -40,7 +40,9 @@ func (orderBy OrderBy) MergeClause(clause *Clause) {
 			}
 		}
 
-		orderBy.Columns = append(v.Columns, orderBy.Columns...)
+		copiedColumns := make([]OrderByColumn, len(v.Columns))
+		copy(copiedColumns, v.Columns)
+		orderBy.Columns = append(copiedColumns, orderBy.Columns...)
 	}
 
 	clause.Expression = orderBy

clause/set.go

@@ -32,7 +32,9 @@ func (set Set) Build(builder Builder) {
 
 // MergeClause merge assignments clauses
 func (set Set) MergeClause(clause *Clause) {
-	clause.Expression = set
+	copiedAssignments := make([]Assignment, len(set))
+	copy(copiedAssignments, set)
+	clause.Expression = Set(copiedAssignments)
 }
 
 func Assignments(values map[string]interface{}) Set {

clause/where.go

@@ -40,7 +40,9 @@ func (where Where) Build(builder Builder) {
 // MergeClause merge where clauses
 func (where Where) MergeClause(clause *Clause) {
 	if w, ok := clause.Expression.(Where); ok {
-		where.Exprs = append(w.Exprs, where.Exprs...)
+		copiedExpressions := make([]Expression, len(w.Exprs))
+		copy(copiedExpressions, w.Exprs)
+		where.Exprs = append(copiedExpressions, where.Exprs...)
 	}
 
 	clause.Expression = where

statement_test.go

@@ -0,0 +1,37 @@
+package gorm
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+	
+	"gorm.io/gorm/clause"
+)
+
+func TestWhereCloneCorruption(t *testing.T) {
+	for whereCount := 1; whereCount <= 8; whereCount++ {
+		t.Run(fmt.Sprintf("w=%d", whereCount), func(t *testing.T) {
+			s := new(Statement)
+			for w := 0; w < whereCount; w++ {
+				s = s.clone()
+				s.AddClause(clause.Where{
+					Exprs: s.BuildCondtion(fmt.Sprintf("where%d", w)),
+				})
+			}
+
+			s1 := s.clone()
+			s1.AddClause(clause.Where{
+				Exprs: s.BuildCondtion("FINAL1"),
+			})
+			s2 := s.clone()
+			s2.AddClause(clause.Where{
+				Exprs: s.BuildCondtion("FINAL2"),
+			})
+
+			if reflect.DeepEqual(s1.Clauses["WHERE"], s2.Clauses["WHERE"]) {
+				t.Errorf("Where conditions should be different")
+			}
+		})
+	}
+}
+