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Fix SRTP get_Item witness resolution for string indexers #19757

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Fix SRTP get_Item witness resolution for string indexers #19757

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f96e69f
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Copilot May 18, 2026
150a7cd
Fix SRTP get_Item witness handling for string indexers
Copilot May 18, 2026
864a717
Add release notes for SRTP get_Item string indexer fix
T-Gro May 25, 2026
48c2a5e
Merge remote-tracking branch 'origin/main' into copilot/fix-srtp-memb…
T-Gro May 25, 2026
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1 change: 1 addition & 0 deletions docs/release-notes/.FSharp.Compiler.Service/11.0.100.md
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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@
* Fix parallel compilation of scripts ([PR #19649](https://github.com/dotnet/fsharp/pull/19649))
* Fix parser recovery, name resolution, and code completion for unfinished enum patterns ([PR #19708](https://github.com/dotnet/fsharp/pull/19708))
* Parser: fix unexpected diagnostics in debug builds, improve error messages ([PR #19730](https://github.com/dotnet/fsharp/pull/19730))
* Fix internal error when resolving SRTP `get_Item` witness for `string` indexers (`unknown builtin witness 'get_ItemDynamic'`). ([Issue #18093](https://github.com/dotnet/fsharp/issues/18093), [PR #19757](https://github.com/dotnet/fsharp/pull/19757))
* Fix signature conformance: overloaded member with unit parameter `M(())` now matches sig `member M: unit -> unit`. ([Issue #19596](https://github.com/dotnet/fsharp/issues/19596), [PR #19615](https://github.com/dotnet/fsharp/pull/19615))

### Added
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2 changes: 1 addition & 1 deletion src/Compiler/TypedTree/TcGlobals.fs
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Expand Up @@ -1929,7 +1929,7 @@ type TcGlobals(
Some (g.array_get_info, [retTy], argExprs)
| "set_Item", [arrTy; _; elemTy], _, [_; _; _] when isArrayTy g arrTy ->
Some (g.array_set_info, [elemTy], argExprs)
| "get_Item", [stringTy; _; _], _, [_; _] when isStringTy g stringTy ->
| "get_Item", [stringTy; _], _, [_; _] when isStringTy g stringTy ->
Some (g.getstring_info, [], argExprs)
| "op_UnaryPlus", [aty], _, [_] ->
// Call Operators.id
Expand Down
20 changes: 19 additions & 1 deletion ...er.ComponentTests/Conformance/Types/TypeConstraints/IWSAMsAndSRTPs/IWSAMsAndSRTPsTests.fs
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Expand Up @@ -287,6 +287,24 @@ let main _ =
IL_000a: ret
}"""]

[<Fact>]
let ``SRTP get_Item works on strings`` () =
FSharp """
let inline indexInto (slice: ^T when ^T: (member get_Item: int -> ^U)) i : ^U =
slice.get_Item i

[<EntryPoint>]
let main _ =
if indexInto "abcde" 2 <> 'c' then
failwith "Unexpected result"

0
"""
|> asExe
|> withOptions ["--nowarn:77"]
|> compileAndRun
|> shouldSucceed

[<Theory>]
[<InlineData("let inline f_set_Item<'T when 'T : (member Item: int -> string with set) >(x: 'T) = (^T : (member Item: int -> string with set) (x, 3, \"a\"))")>]
[<InlineData("let inline f_set_Item<'T when 'T : (member set_Item: int * string -> unit) >(x: 'T) = (^T : (member set_Item: int * string -> unit) (x, 3, \"a\"))")>]
Expand Down Expand Up @@ -699,6 +717,7 @@ let main _ =
|> compileAndRun
|> shouldSucceed


[<InlineData(true)>] // RealSig
[<InlineData(false)>] // Regular
[<Theory>]
Expand Down Expand Up @@ -1955,4 +1974,3 @@ if resultInt <> 0 then failwith $"Expected 0 but got {resultInt}"
|> asExe
|> compileAndRun
|> shouldSucceed

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