FS0193: internal error in CheckExpressions

[kerams]

Repro steps

let z a = Some ("", "", a = "")

Expected behavior

I guess it should error with The union case 'Some' does not have a field named 'a'., because that's what happens for let z a = Some ("", a = "")? Then again, Some takes one parameter and there are 2 unnamed arguments here (from the compiler's perspective), so the whole thing could be interpreted as a tuple.

Actual behavior

error FS0193: internal error: Index was outside the bounds of the array.

Unhandled exception. System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at FSharp.Compiler.CheckExpressions.args$cont@7599-1(TcFileState cenv, TcEnv env, TcGlobals g, AccessorDomain ad, Item item, FSharpList`1 argTys, FSharpList`1 argNames, Int32 numArgTys, FSharpList`1 args, FSharpList`1 unnamedArgs, FSharpList`1 namedCallerArgs, Unit unitVar) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/CheckExpressions.fs:line 7605
   at FSharp.Compiler.CheckExpressions.TcItemThen(TcFileState cenv, TType overallTy, TcEnv env, UnscopedTyparEnv tpenv, FSharpList`1 tinstEnclosing, Item item, range mItem, FSharpList`1 rest, AfterResolution afterResolution, FSharpList`1 delayed) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/CheckExpressions.fs:line 7594
   at FSharp.Compiler.CheckExpressions.TcExpr(TcFileState cenv, TType ty, TcEnv env, UnscopedTyparEnv tpenv, SynExpr expr) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/CheckExpressions.fs:line 5263
   at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ReraiseIfWatsonable(Exception exn) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/ErrorLogger.fs:line 369
   at FSharp.Compiler.ErrorLogger.ErrorLoggerExtensions.ErrorLogger.ErrorRecovery(ErrorLogger x, Exception exn, range m) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/ErrorLogger.fs:line 416
   at FSharp.Compiler.CheckExpressions.TcExpr(TcFileState cenv, TType ty, TcEnv env, UnscopedTyparEnv tpenv, SynExpr expr) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/CheckExpressions.fs:line 5263
   at FSharp.Compiler.CheckExpressions.TcExprFlex(TcFileState cenv, Boolean flex, Boolean compat, TType ty, TcEnv env, UnscopedTyparEnv tpenv, SynExpr e) in /Users/alfonso/Documents/ncave_fsharp/src/fsharp/CheckExpressions.fs:line 5248

(the stack is from Fable's fcs, but I see the same error in VS)

Known workarounds

let z a = Some ("", "", (a = ""))

Related information

Provide any related information (optional):

• Win 10
• .NET 5.0.300-preview.21228.15
• VS 16.10 Preview 3.0

[cartermp]

so the whole thing could be interpreted as a tuple.

Yes, that's what I would expect to be the case here, especially since adding the parentheses is a workaround that produces a 3-tuple.

[edgarfgp]

@vzarytovskii @dsyme regarding this issue is the expectation:

• Raise a compiler error with a relevant message .
• Interpreted both cases as a tuple

let z a = Some ("", "", a = "")

let z a = Some ("", "", (a = ""))

[dsyme]

It should raise an error with the relevant message