fix(compiler/mysql): prevent panic in convertSetOprSelectList() (#4042)

Resolves #2453

Author: andrewmbenton

internal/endtoend/testdata/select_union/mysql/go/query.sql.go

@@ -22,4 +22,9 @@ SELECT * FROM foo;
 -- name: SelectUnionOther :many
 SELECT * FROM foo
 UNION
-SELECT * FROM bar;
+SELECT * FROM bar;
+
+-- name: SelectUnionAliased :many
+(SELECT * FROM foo)
+UNION
+SELECT * FROM bar;

internal/endtoend/testdata/select_union/mysql/query.sql

@@ -22,4 +22,9 @@ SELECT * FROM foo;
 -- name: SelectUnionOther :many
 SELECT * FROM foo
 UNION
-SELECT * FROM bar;
+SELECT * FROM bar;
+
+-- name: SelectUnionAliased :many
+(SELECT * FROM foo)
+UNION
+SELECT * FROM foo;

internal/endtoend/testdata/select_union/postgres/pgx/v4/go/query.sql.go

@@ -22,4 +22,9 @@ SELECT * FROM foo;
 -- name: SelectUnionOther :many
 SELECT * FROM foo
 UNION
-SELECT * FROM bar;
+SELECT * FROM bar;
+
+-- name: SelectUnionAliased :many
+(SELECT * FROM foo)
+UNION
+SELECT * FROM foo;

internal/endtoend/testdata/select_union/postgres/pgx/v4/query.sql

@@ -22,4 +22,9 @@ SELECT * FROM foo;
 -- name: SelectUnionOther :many
 SELECT * FROM foo
 UNION
-SELECT * FROM bar;
+SELECT * FROM bar;
+
+-- name: SelectUnionAliased :many
+(SELECT * FROM foo)
+UNION
+SELECT * FROM foo;

internal/endtoend/testdata/select_union/postgres/pgx/v5/go/query.sql.go

@@ -22,4 +22,4 @@ SELECT * FROM foo;
 -- name: SelectUnionOther :many
 SELECT * FROM foo
 UNION
-SELECT * FROM bar;
+SELECT * FROM bar;

internal/endtoend/testdata/select_union/postgres/pgx/v5/query.sql

@@ -1182,7 +1182,12 @@ func (c *cc) convertSetOprType(n *pcast.SetOprType) (op ast.SetOperation, all bo
 func (c *cc) convertSetOprSelectList(n *pcast.SetOprSelectList) ast.Node {
 	selectStmts := make([]*ast.SelectStmt, len(n.Selects))
 	for i, node := range n.Selects {
-		selectStmts[i] = c.convertSelectStmt(node.(*pcast.SelectStmt))
+		switch node := node.(type) {
+		case *pcast.SelectStmt:
+			selectStmts[i] = c.convertSelectStmt(node)
+		case *pcast.SetOprSelectList:
+			selectStmts[i] = c.convertSetOprSelectList(node).(*ast.SelectStmt)
+		}
 	}
 
 	op, all := c.convertSetOprType(n.AfterSetOperator)