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SRTP "member get_Item" causes "FS0192: internal error" when used on strings #18093
Description
Please provide a succinct description of the issue.
I was attempting to index into array-like collections using SRTP and encountered the error using strings. It appears the underlying cause is that the indexer method is not necessarily get_Item and may be overridden using [<IndexerName>]. This is the case in String.cs with the attribute [IndexerName("Chars")] that results in a get_Chars method instead of get_Item.
Provide the steps required to reproduce the problem:
It can be reproduced using FSI
let inline indexInto (slice: ^T when ^T: (member get_Item: int -> ^U)) i : ^U =
slice.get_Item i
let array = [| 1; 2; 3; 4; 5 |] // Ok
let list = ResizeArray<int>(array) // Ok
let str = "abcde"
printfn $"{indexInto array 2}" // Ok
printfn $"{indexInto list 2}" // Ok
printfn $"{indexInto str 2}" // error FS0192: internal error: unknown builtin witness 'get_ItemDynamic'
let inline indexChars (slice: ^T when ^T: (member get_Chars: int -> ^U)) i : ^U =
slice.get_Chars i
printfn $"{indexChars array 2}" // The type 'int array' does not support the operator 'get_Chars'
printfn $"{indexChars str 2}" // OkExpected behavior
My initial expectation was that indexInto would allow indexing into any type that had an indexer (using either get_Item or [i]) but after learning about [<IndexerName>] I'm no longer sure if this should be the case. Some suggestions for a resolution:
- Change the compiler's understanding of
member get_Itemin SRTP
Make get_Item a compiler-known method that checks for usage of [<IndexerName>] on the target type and inserts the appropriate call.
Despite being "magical" this is probably the expected behavior of most F# developers unaware of the existence of [<IndexerName>].
- Change the error and provide alternate mechanism to combine SRTP and indexers
The error should become "The type 'string' does not support the operator 'get_Item'" rather than "FS0192 internal error" to match other type errors.
Consider some other syntax to allow indexers to be accessed for all types using SRTP:
member []member Item[][<Indexer>] member get_Itemmember op_Item
let inline indexInto (slice: ^T when ^T: (member []: int -> ^U)) i : ^U =
slice.[i] // or slice[i]Known workarounds
- Instead of SRTP, use interfaces to access elements (it's noteworthy that
Stringdoesn't implementIReadOnlyListwhich would seem appropriate for this case) - Use
Spanwhich unifies some indexable types but has other usage limitations - Wrap the string and re-implement the indexer
[<Struct>]
type Wrapper =
| Wrapper of string
member this.Item with get i =
let (Wrapper s) = this
s.[i]
printfn $"{indexInto (Wrapper str) 2}"Metadata
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T-Gro commentedon Dec 2, 2024
This is an unfortunate consequence on how indexer for strings is done.
I don't think I have seen this used elsewhere, in any other common type.
From the proposals, the
member []syntax looks good and would also enable dropping theget_Itemworkaround/limitation. For treating it as a bugfix though, I am afraid the audience for this issue is too small to justify a language change (happy to be corrected! ).Either a change of the compiler message, or a known compile-time replacement from
get_Iteminto any method with the[<IndexerName>]attribute are more likely.Thank you for describing the issue in depth and listing good workarounds - this will be a nice reference in case anyone else encounters the limitation for the intersection of SRTP's and indexing into a string.
T-Gro commentedon May 18, 2026
🤖 Copilot implementation hints
Fix
One-line typo fix in
src/Compiler/TypedTree/TcGlobals.fsatTryMakeOperatorAsBuiltInWitnessInfo(~line 1932).The string case for
get_Itemmatches[stringTy; _; _](3 elements) butCompiledObjectAndArgumentTypesforstring.get_Item(int)is[stringTy; intTy](2 elements). It never matches, falls through tofailwithf.Change:
to:
Instructions
let inline indexInto (s: ^a) i = (^a: (member get_Item: int -> ^b) (s, i)) in indexInto "abc" 2failing with FS0192, then fix.dotnet test tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj --filter "SRTP" -c Debugexpert-reviewskill to review your work.dotnet fantomas .on changed files.