diff --git a/src/Compiler/Facilities/DiagnosticsLogger.fs b/src/Compiler/Facilities/DiagnosticsLogger.fs
index a686825cffa4..605c7e87ad76 100644
--- a/src/Compiler/Facilities/DiagnosticsLogger.fs
+++ b/src/Compiler/Facilities/DiagnosticsLogger.fs
@@ -375,31 +375,6 @@ type CapturingDiagnosticsLogger(nm, ?eagerFormat) =
         let errors = diagnostics.ToArray()
         errors |> Array.iter diagnosticsLogger.DiagnosticSink
 
-type ConcurrentCapturingDiagnosticsLogger(nm, ?eagerFormat) =
-    inherit DiagnosticsLogger(nm)
-    let mutable errorCount = 0
-    let diagnostics = System.Collections.Concurrent.ConcurrentQueue()
-
-    override _.DiagnosticSink(diagnostic, severity) =
-        let diagnostic =
-            match eagerFormat with
-            | None -> diagnostic
-            | Some f -> f diagnostic
-
-        if severity = FSharpDiagnosticSeverity.Error then
-            Interlocked.Increment &errorCount |> ignore
-
-        diagnostics.Enqueue(diagnostic, severity)
-
-    override _.ErrorCount = errorCount
-
-    member _.Diagnostics = diagnostics |> Seq.toList
-
-    member _.CommitDelayedDiagnostics(diagnosticsLogger: DiagnosticsLogger) =
-        // Eagerly grab all the errors and warnings from the mutable collection
-        let errors = diagnostics.ToArray()
-        errors |> Array.iter diagnosticsLogger.DiagnosticSink
-
 /// Type holds thread-static globals for use by the compiler.
 type DiagnosticsAsyncState =
     static let buildPhase = new AsyncLocal<BuildPhase voption>()
diff --git a/src/Compiler/Facilities/DiagnosticsLogger.fsi b/src/Compiler/Facilities/DiagnosticsLogger.fsi
index 4102cce79e7d..0d4c6aec67ac 100644
--- a/src/Compiler/Facilities/DiagnosticsLogger.fsi
+++ b/src/Compiler/Facilities/DiagnosticsLogger.fsi
@@ -226,21 +226,6 @@ type CapturingDiagnosticsLogger =
 
     override ErrorCount: int
 
-/// Represents a DiagnosticsLogger that captures all diagnostics, optionally formatting them
-/// eagerly.
-type ConcurrentCapturingDiagnosticsLogger =
-    inherit DiagnosticsLogger
-
-    new: nm: string * ?eagerFormat: (PhasedDiagnostic -> PhasedDiagnostic) -> ConcurrentCapturingDiagnosticsLogger
-
-    member CommitDelayedDiagnostics: diagnosticsLogger: DiagnosticsLogger -> unit
-
-    override DiagnosticSink: diagnostic: PhasedDiagnostic * severity: FSharpDiagnosticSeverity -> unit
-
-    member Diagnostics: (PhasedDiagnostic * FSharpDiagnosticSeverity) list
-
-    override ErrorCount: int
-
 /// Thread statics for the installed diagnostic logger
 [<Class>]
 type DiagnosticsAsyncState =
@@ -318,7 +303,7 @@ val error: exn: exn -> 'T
 /// Reports an informational diagnostic
 val informationalWarning: exn: exn -> unit
 
-val simulateError: diagnostic: PhasedDiagnostic -> 'T
+val simulateError: diagnostic: PhasedDiagnostic -> 'Tt
 
 val diagnosticSink: diagnostic: PhasedDiagnostic * severity: FSharpDiagnosticSeverity -> unit