Fix SRTP get_Item witness resolution for string indexers (dotnet#19757)

Author: Copilot

docs/release-notes/.FSharp.Compiler.Service/11.0.100.md

@@ -61,6 +61,7 @@
 * 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))
 * Allow `| null` nullable annotation on a `[<MeasureAnnotatedAbbreviation>]` over a reference type (e.g. the FSharp.UMX `type string<[<Measure>] 'm> = string` pattern). ([Issue #19657](https://github.com/dotnet/fsharp/issues/19657))
 * Fix `[<Struct>] ?param` optional parameters could not be passed using the explicit `?param = expr` caller-side syntax with a `ValueOption` value. ([Issue #19711](https://github.com/dotnet/fsharp/issues/19711), [PR #19742](https://github.com/dotnet/fsharp/pull/19742))
 * 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))

src/Compiler/TypedTree/TcGlobals.fs

@@ -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

tests/FSharp.Compiler.ComponentTests/Conformance/Types/TypeConstraints/IWSAMsAndSRTPs/IWSAMsAndSRTPsTests.fs

@@ -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\"))")>]
@@ -699,6 +717,7 @@ let main _ =
         |> compileAndRun
         |> shouldSucceed
 
+
     [<InlineData(true)>]        // RealSig
     [<InlineData(false)>]       // Regular
     [<Theory>]
@@ -1955,4 +1974,3 @@ if resultInt <> 0 then failwith $"Expected 0 but got {resultInt}"
         |> asExe
         |> compileAndRun
         |> shouldSucceed
-