Fix #17719

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -7530,9 +7530,9 @@ and TcInterpolatedStringExpr cenv (overallTy: OverallTy) env m tpenv (parts: Syn
             let concatenableExprs = if canLower then concatenable [] fillExprs parts else []
 
             match concatenableExprs with
-            | [p1; p2; p3; p4] -> mkStaticCall_String_Concat4 g m p1 p2 p3 p4, tpenv
-            | [p1; p2; p3] -> mkStaticCall_String_Concat3 g m p1 p2 p3, tpenv
-            | [p1; p2] -> mkStaticCall_String_Concat2 g m p1 p2, tpenv
+            | [p1; p2; p3; p4] -> TcPropagatingExprLeafThenConvert cenv overallTy g.string_ty env m (fun () -> mkStaticCall_String_Concat4 g m p1 p2 p3 p4, tpenv)
+            | [p1; p2; p3] -> TcPropagatingExprLeafThenConvert cenv overallTy g.string_ty env m (fun () -> mkStaticCall_String_Concat3 g m p1 p2 p3, tpenv)
+            | [p1; p2] -> TcPropagatingExprLeafThenConvert cenv overallTy g.string_ty env m (fun () -> mkStaticCall_String_Concat2 g m p1 p2, tpenv)
             | [p1] -> p1, tpenv
             | _ ->
 

tests/FSharp.Compiler.ComponentTests/EmittedIL/StringFormatAndInterpolation.fs

@@ -88,4 +88,46 @@ IL_0014:  call       string [runtime]System.String::Concat(string,
                                                                   string,
                                                                   string,
                                                                   string)
-IL_0019:  ret"""]
+IL_0019:  ret"""]
+
+    [<Fact>]
+    let ``Interpolated string with concat converts to span implicitly`` () =
+        let compilation = 
+                FSharp $"""
+        module InterpolatedStringByefLikes
+        type Foo() =
+
+                let sb = System.Text.StringBuilder()
+
+                member _.Bar(s: System.ReadOnlySpan<char>) = sb.Append(s) |> ignore
+
+        let [<EntryPoint>] main _ =
+            let foo = Foo()
+            let foos = "foo"
+            foo.Bar($"{{foos}} is bar")
+            0
+                """
+
+        compilation |> compile |> shouldSucceed |> ignore
+
+        compilation |> asExe |> compileAndRun |> shouldSucceed |> ignore
+
+        compilation |> compile |> shouldSucceed |> verifyIL ["""
+.locals init (class InterpolatedStringByefLikes/Foo V_0,
+        valuetype [runtime]System.ReadOnlySpan`1<char> V_1,
+        class [runtime]System.Text.StringBuilder V_2)
+IL_0000:  newobj     instance void InterpolatedStringByefLikes/Foo::.ctor()
+IL_0005:  stloc.0
+IL_0006:  ldstr      "foo"
+IL_000b:  ldstr      " is bar"
+IL_0010:  call       string [runtime]System.String::Concat(string,
+                                                                string)
+IL_0015:  call       valuetype [runtime]System.ReadOnlySpan`1<char> [runtime]System.String::op_Implicit(string)
+IL_001a:  stloc.1
+IL_001b:  ldloc.0
+IL_001c:  ldfld      class [runtime]System.Text.StringBuilder InterpolatedStringByefLikes/Foo::sb
+IL_0021:  ldloc.1
+IL_0022:  callvirt   instance class [runtime]System.Text.StringBuilder [runtime]System.Text.StringBuilder::Append(valuetype [runtime]System.ReadOnlySpan`1<char>)
+IL_0027:  stloc.2
+IL_0028:  ldc.i4.0
+IL_0029:  ret"""]