MSSQL version 11+ (2012+) backend calls try_cast instead of cast

try_cast results in a more elegant failure (returns NA) if a cast is not possible ('cast' will throw an error).
Address issue tidyverse#380

R/backend-mssql.R

@@ -47,7 +47,7 @@
 
 #' @export
 `sql_translate_env.Microsoft SQL Server` <- function(con) {
-  sql_variant(
+  mssql_scalar <-
     sql_translator(.parent = base_odbc_scalar,
 
       `!`           = function(x) {
@@ -150,8 +150,30 @@
           sql_expr(DATEPART(QUARTER, !!x))
         }
       },
+    )
 
-    ),
+  if (sub("0.*", "", DBI::dbGetInfo(con)$db.version) >= 11) {
+    # version 11 equates to MSSQL 2012
+    # if MSSQL 2012+, use sql_try_cast, allows more graceful return of invalid values
+    mssql_scalar <- sql_translator(.parent = mssql_scalar,
+                                   as.logical    = sql_try_cast("BIT"),
+
+                                   as.Date       = sql_try_cast("DATE"),
+                                   as.POSIXct    = sql_try_cast("TIMESTAMP"),
+                                   as.numeric    = sql_try_cast("FLOAT"),
+                                   as.double     = sql_try_cast("FLOAT"),
+                                   as.integer    = sql_try_cast("NUMERIC"),
+                                   # in MSSQL, NUMERIC converts to integer
+                                   as.integer64  = sql_try_cast("BIGINT"),
+                                   as.character  = sql_try_cast("VARCHAR(MAX)"),
+
+                                   as_date = sql_try_cast("DATE"),
+                                   as_datetime = sql_try_cast("DATETIME2")
+    )
+  }
+
+  sql_variant(
+    mssql_scalar,
     sql_translator(.parent = base_odbc_agg,
       sd            = sql_aggregate("STDEV", "sd"),
       var           = sql_aggregate("VAR", "var"),

R/translate-sql-helpers.R

@@ -231,6 +231,16 @@ sql_cast <- function(type) {
   }
 }
 
+#' @rdname sql_variant
+#' @export
+sql_try_cast <- function(type) {
+  type <- sql(type)
+  function(x) {
+    sql_expr(try_cast(!!x %as% !!type))
+    # try_cast available in MSSQL 2012+
+  }
+}
+
 #' @rdname sql_variant
 #' @export
 sql_log <- function() {