fix: Prevent variable redeclaration in single param conflict for pgx (sqlc-dev#2058)

Author: zaneli

internal/codegen/golang/templates/pgx/queryCode.tmpl

@@ -33,7 +33,9 @@ func (q *Queries) {{.MethodName}}(ctx context.Context, db DBTX, {{.Arg.Pair}}) (
 func (q *Queries) {{.MethodName}}(ctx context.Context, {{.Arg.Pair}}) ({{.Ret.DefineType}}, error) {
 	row := q.db.QueryRow(ctx, {{.ConstantName}}, {{.Arg.Params}})
 {{- end}}
+	{{- if ne .Arg.Pair .Ret.Pair }}
 	var {{.Ret.Name}} {{.Ret.Type}}
+	{{- end}}
 	err := row.Scan({{.Ret.Scan}})
 	return {{.Ret.ReturnName}}, err
 }

internal/endtoend/testdata/single_param_conflict/postgresql/pgx/v4/go/db.go

@@ -0,0 +1,13 @@
+{
+	"version": "1",
+	"packages": [
+		{
+			"engine": "postgresql",
+			"sql_package": "pgx/v4",
+			"path": "go",
+			"name": "querytest",
+			"schema": "query.sql",
+			"queries": "query.sql"
+		}
+	]
+}

internal/endtoend/testdata/single_param_conflict/postgresql/go/models.go renamed to internal/endtoend/testdata/single_param_conflict/postgresql/pgx/v4/go/models.go

@@ -0,0 +1,37 @@
+-- Example queries for sqlc
+CREATE TABLE authors (
+  id   BIGSERIAL PRIMARY KEY,
+  name TEXT      NOT NULL,
+  bio  text
+);
+
+-- name: GetAuthorIDByID :one
+SELECT  id
+FROM    authors
+WHERE   id = $1
+LIMIT   1;
+
+-- name: GetAuthorByID :one
+SELECT  id, name, bio
+FROM    authors
+WHERE   id = $1
+LIMIT   1;
+
+-- https://github.com/kyleconroy/sqlc/issues/1290
+CREATE TABLE users (
+  sub UUID PRIMARY KEY
+);
+
+-- name: GetUser :one
+SELECT  sub
+FROM    users
+WHERE   sub = $1
+LIMIT   1;
+
+-- https://github.com/kyleconroy/sqlc/issues/1235
+
+-- name: SetDefaultName :one
+UPDATE  authors
+SET     name = "Default Name"
+WHERE   id = $1
+RETURNING id;