Reuse synTypar names when typar is resolved.

src/Compiler/Checking/CheckDeclarations.fs

@@ -3948,7 +3948,17 @@ module TcDeclarations =
             let resInfo = TypeNameResolutionStaticArgsInfo.FromTyArgs synTypars.Length
             let _, tcref =
                 match ResolveTypeLongIdent cenv.tcSink cenv.nameResolver ItemOccurence.Binding OpenQualified envForDecls.NameEnv ad longPath resInfo PermitDirectReferenceToGeneratedType.No with
-                | Result res -> res
+                | Result res ->
+                    // Update resolved type parameters with the names from the source.
+                    let _, tcref = res
+                    if tcref.TyparsNoRange.Length = synTypars.Length then
+                        (tcref.TyparsNoRange, synTypars)
+                        ||> List.zip
+                        |> List.iter (fun (typar, SynTyparDecl.SynTyparDecl(_, SynTypar(ident = untypedIdent))) ->
+                            typar.SetIdent(untypedIdent)
+                        )
+
+                    res
                 | res when inSig && List.isSingleton longPath ->
                     errorR(Deprecated(FSComp.SR.tcReservedSyntaxForAugmentation(), m))
                     ForceRaise res

tests/FSharp.Compiler.ComponentTests/Signatures/TypeTests.fs

@@ -75,3 +75,60 @@ namespace Foo.Types
     val EndLine: int
 
     val EndColumn: int"""
+
+[<Fact>]
+let ``Type extension uses type parameters names from source`` () =
+    FSharp """
+module Extensions
+
+type List<'E> with
+
+    member this.X = this.Head
+"""
+    |> printSignatures
+    |> should equal
+        """
+module Extensions
+type List<'E> with
+
+  member X: 'E"""
+
+[<Fact>]
+let ``Type extension with constraints uses type parameters names from source`` () =
+    FSharp """
+module Extensions
+
+type Map<'K, 'V when 'K: comparison> with
+
+    member m.X (t: 'T) (k: 'K) = Some k, ({| n = [|k|] |}, 0)
+"""
+    |> printSignatures
+    |> should equal
+        """
+module Extensions
+type Map<'K,'V when 'K: comparison> with
+
+  member X: t: 'T -> k: 'K -> 'K option * ({| n: 'K array |} * int) when 'K: comparison"""
+
+[<Fact>]
+let ``Type extension with lowercase type parameters names from source`` () =
+    FSharp """
+module Extensions
+
+open System.Collections.Concurrent
+
+type ConcurrentDictionary<'key, 'value> with
+
+  member x.TryFind key =
+    match x.TryGetValue key with
+    | true, value -> Some value
+    | _ -> None
+"""
+    |> printSignatures
+    |> should equal
+        """
+module Extensions
+type System.Collections.Concurrent.ConcurrentDictionary<'key,'value> with
+
+  member TryFind: key: 'key -> 'value option"""
+