Compiler crash on exception handler construct

[FilippoPolo]

The following F# code:

type test =
    {
        test_field: string[] option
    }
let foo =
    let bar = try (0).ToString() with e -> e.ToString()
    let bar = [|bar|]
    { test_field = Some bar }

When passed through W# 4.6.0.361, results in:

WebSharper error FS9001 : Global error : System.Exception: Undefined variable during writing JavaScript: $3�   at Microsoft.FSharp.Core.PrintfModule.PrintFormatToStringThenFail@1637.Invoke(String message) in E:\A\_work\130\s\src\fsharp\FSharp.Core\printf.fs:line 1637�   at WebSharper.Compiler.JavaScriptWriter.transformId(Environment env, Id id)�   at WebSharper.Compiler.JavaScriptWriter.transformExpr(Environment env, Expression expr)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement(Environment env, Statement statement)�   at WebSharper.Compiler.JavaScriptWriter.add@286-6.Invoke(Statement a)�   at Microsoft.FSharp.Primitives.Basics.List.iter[T](FSharpFunc`2 f, FSharpList`1 x) in E:\A\_work\130\s\src\fsharp\FSharp.Core\local.fs:line 91�   at WebSharper.Compiler.JavaScriptWriter.flatten@283(Environment env, FSharpList`1 s)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement(Environment env, Statement statement)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement@409-15.Invoke(Unit unitVar0)�   at WebSharper.Compiler.JavaScriptWriter.withFuncDecls@323(Environment env, FSharpFunc`2 f)�   at WebSharper.Compiler.JavaScriptWriter.add@286-6.Invoke(Statement a)�   at WebSharper.Compiler.JavaScriptWriter.add@286-6.Invoke(Statement a)�   at Microsoft.FSharp.Primitives.Basics.List.iter[T](FSharpFunc`2 f, FSharpList`1 x) in E:\A\_work\130\s\src\fsharp\FSharp.Core\local.fs:line 91�   at WebSharper.Compiler.JavaScriptWriter.flatten@283(Environment env, FSharpList`1 s)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement(Environment env, Statement statement)�   at WebSharper.Compiler.JavaScriptWriter.transformExpr(Environment env, Expression expr)�   at WebSharper.Compiler.JavaScriptWriter.transformExpr(Environment env, Expression expr)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement(Environment env, Statement statement)�   at WebSharper.Compiler.JavaScriptWriter.add@286-6.Invoke(Statement a)�   at Microsoft.FSharp.Primitives.Basics.List.iter[T](FSharpFunc`2 f, FSharpList`1 x) in E:\A\_work\130\s\src\fsharp\FSharp.Core\local.fs:line 91�   at WebSharper.Compiler.JavaScriptWriter.flatten@283(Environment env, FSharpList`1 s)�   at WebSharper.Compiler.JavaScriptWriter.transformStatement(Environment env, Statement statement)�   at WebSharper.Compiler.JavaScriptWriter.transformExpr(Environment env, Expression expr)�   at WebSharper.Compiler.JavaScriptWriter.transformExpr(Environment env, Expression expr)�   at WebSharper.Compiler.Packager.exprToString(Preferences pref, FSharpFunc`2 getWriter, Expression statement)�   at WebSharper.Compiler.FrontEnd.CreateResources(FSharpOption`1 comp, Info refMeta, Info current, Boolean sourceMap, FSharpOption`1 closures, AssemblyDefinition a)�   at WebSharper.Compiler.FrontEnd.ModifyCecilAssembly(FSharpOption`1 comp, Info refMeta, Info current, Boolean sourceMap, FSharpOption`1 closures, AssemblyDefinition a)�   at WebSharper.FSharp.Program.Compile$cont@219-3(WsConfig config, Loader loader, FSharpList`1 refs, Task`1 wsRefsMeta, Compilation comp, Info currentMeta, Unit unitVar)�   at WebSharper.FSharp.Program.Compile$cont@202-2(WsConfig config, Boolean isBundleOnly, Loader loader, FSharpList`1 refs, Task`1 wsRefsMeta, Compilation comp, Unit unitVar)�   at WebSharper.FSharp.Program.Compile$cont@64(WsConfig config, WarnSettings warnSettings, String thisName, Unit unitVar)�   at WebSharper.FSharp.Program.compileMain$cont@310(String[] argv, Unit unitVar)�   at WebSharper.FSharp.Program.main(String[] argv)

From testing, this seems to only happen with an option of array, and only if the value comes from a try/with construct.

[Jand42]

@FilippoPolo Thanks for the report and the small example!

Indeed, it is some optimizer bug, I managed to shrink the example a more, just a try expression within an array is enough:

let foo = [| (try (0).ToString() with e -> e.ToString()) |]

Looking into it using ASTTests.fsx that can print intermediate expressions as processed by WebSharper compiler.