From 7a47c78dcbcc3a1858aeeb4d7e26686855b23587 Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Thu, 11 Sep 2025 17:56:06 +0300
Subject: [PATCH 1/8] wip

---
 src/Compiler/Checking/NicePrint.fs            | 22 ++++++++++-------
 src/Compiler/TypedTree/TypedTreeOps.fs        |  1 +
 src/Compiler/TypedTree/TypedTreeOps.fsi       |  5 ++++
 .../TooltipTests.fs                           | 24 ++++++++++++++++++-
 4 files changed, 43 insertions(+), 9 deletions(-)

diff --git a/src/Compiler/Checking/NicePrint.fs b/src/Compiler/Checking/NicePrint.fs
index 00b6f4c0b55..d2aca827887 100644
--- a/src/Compiler/Checking/NicePrint.fs
+++ b/src/Compiler/Checking/NicePrint.fs
@@ -124,6 +124,9 @@ module internal PrintUtilities =
         |> fun reversedArgs -> (true, retTyL) :: reversedArgs
         |> List.fold (fun acc (shouldBreak, layout) -> (if shouldBreak then (---) else (++)) layout acc) emptyL
 
+    let getSuperTypeDenv (denv: DisplayEnv) =
+        denv.UseGenericParameterStyle(PrefixForTopLevel(denv.genericParameterStyle))
+
     let tagNavArbValRef (valRefOpt: ValRef option) tag =
         match valRefOpt with
         | Some vref ->
@@ -169,9 +172,10 @@ module internal PrintUtilities =
 
     let usePrefix (denv: DisplayEnv) (tcref: TyconRef) =
         match denv.genericParameterStyle with
-        | GenericParameterStyle.Implicit -> tcref.IsPrefixDisplay
-        | GenericParameterStyle.Prefix -> true
-        | GenericParameterStyle.Suffix -> false
+        | GenericParameterStyle.Implicit -> tcref.IsPrefixDisplay, denv
+        | GenericParameterStyle.Prefix -> true, denv
+        | GenericParameterStyle.Suffix -> false, denv
+        | GenericParameterStyle.PrefixForTopLevel nested -> true, denv.UseGenericParameterStyle(nested) 
 
     /// <summary>
     /// Creates a layout for TyconRef.
@@ -187,7 +191,7 @@ module internal PrintUtilities =
     /// </param>
     let layoutTyconRefImpl isAttribute (denv: DisplayEnv) (tcref: TyconRef) (demangledPath: string list option) =
 
-        let prefix = usePrefix denv tcref
+        let prefix, denv = usePrefix denv tcref
         let isArray = not prefix && isArrayTyconRef denv.g tcref
         let demangled = 
             if isArray then
@@ -746,7 +750,7 @@ module PrintTypes =
                 let tyLayout =
                     match typarConstraintTy with
                     | TType_app (tyconRef = tc; typeInstantiation = ti)
-                        when ti.Length > 0 && not (usePrefix denv tc) ->
+                        when ti.Length > 0 && not (usePrefix denv tc |> fst) ->
                         layoutTypeWithInfo denv env typarConstraintTy
                         |> bracketL
                     | _ -> layoutTypeWithInfo denv env typarConstraintTy
@@ -975,10 +979,10 @@ module PrintTypes =
 
         // Layout a type application
         | TType_ucase (UnionCaseRef(tc, _), args) ->
-            let prefix = usePrefix denv tc
+            let prefix, denv = usePrefix denv tc
             layoutTypeAppWithInfoAndPrec denv env (layoutTyconRefImpl false denv tc None) prec prefix args
         | TType_app (tc, args, nullness) ->
-            let prefix = usePrefix denv tc
+            let prefix, denv = usePrefix denv tc
             let demangledCompilationPathOpt, args =
                 if not denv.includeStaticParametersInTypeNames then
                     None, args
@@ -2047,6 +2051,7 @@ module TastDefinitionPrinting =
                 GetImmediateInterfacesOfType SkipUnrefInterfaces.Yes g amap m ty
 
         let iimplsLs =
+            let denv = getSuperTypeDenv denv
             iimpls
             |> List.map (fun intfTy -> (if isInterfaceTy g ty then WordL.keywordInherit else WordL.keywordInterface) -* layoutType denv intfTy)
 
@@ -2181,7 +2186,8 @@ module TastDefinitionPrinting =
                 | _ -> ()
             ]
 
-        let inheritsL = 
+        let inheritsL =
+            let denv = getSuperTypeDenv denv
             inherits
             |> List.map (fun super -> WordL.keywordInherit ^^ (layoutType denv super))
 
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fs b/src/Compiler/TypedTree/TypedTreeOps.fs
index 368dd9a99ce..3d4aee1e2b2 100644
--- a/src/Compiler/TypedTree/TypedTreeOps.fs
+++ b/src/Compiler/TypedTree/TypedTreeOps.fs
@@ -3171,6 +3171,7 @@ type GenericParameterStyle =
     | Implicit
     | Prefix
     | Suffix
+    | PrefixForTopLevel of nested: GenericParameterStyle
 
 [<NoEquality; NoComparison>]
 type DisplayEnv = 
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
index 06200be47f7..c0e2a7848bb 100755
--- a/src/Compiler/TypedTree/TypedTreeOps.fsi
+++ b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -1061,6 +1061,11 @@ type GenericParameterStyle =
     | Prefix
     /// Force the suffix style: int List
     | Suffix
+    /// Force the prefix style for top-level types in interface/inherit declarations:
+    ///     inherit seq<int list>
+    /// instead of
+    ///     inherit int list seq
+    | PrefixForTopLevel of nested: GenericParameterStyle
 
 [<NoEquality; NoComparison>]
 type DisplayEnv =
diff --git a/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs b/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
index 92d30a3e4cd..fe4806ddd3a 100644
--- a/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
+++ b/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
@@ -535,4 +535,26 @@ open System
 let doIt(myAction : Action<int>) = myAc{caret}tion.Invoke(42)
 """
     |> assertAndGetSingleToolTipText
-    |> Assert.shouldBeEquivalentTo ("""val myAction: Action<int>""" |> normalize)
\ No newline at end of file
+    |> Assert.shouldBeEquivalentTo ("""val myAction: Action<int>""" |> normalize)
+
+[<Fact>]
+let ``Super type should be formatted in the prefix style`` () =
+    Checker.getTooltip """
+namespace Foo
+
+type A{caret} =
+    inherit seq<int list>
+"""
+    |> assertAndGetSingleToolTipText
+    |> Assert.shouldBeEquivalentTo "type A =\n  inherit seq<int list>"
+
+[<Fact>]
+let ``Interface impl should be formatted in the prefix style`` () =
+    Checker.getTooltip """
+namespace Foo
+
+type A{caret} =
+    interface seq<int list> with
+"""
+    |> assertAndGetSingleToolTipText
+    |> Assert.shouldBeEquivalentTo "type A =\n  interface seq<int list>"

From 8e4200e42464e43066a4d52143a806463f1dc943 Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Thu, 11 Sep 2025 22:22:33 +0300
Subject: [PATCH 2/8] release notes

---
 docs/release-notes/.FSharp.Compiler.Service/10.0.100.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
index 06b39cc019d..a707b87e14f 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
@@ -24,6 +24,7 @@
 * Fix Show XML doc for enum fields in external metadata ([Issue #17939](https://github.com/dotnet/fsharp/issues/17939#issuecomment-3137410105), [PR #18800](https://github.com/dotnet/fsharp/pull/18800))
 * Fix nullable types formatting in `FSharpType.Format` and tooltips to include parentheses. ([PR #18842](https://github.com/dotnet/fsharp/pull/18842))
 * TypeMismatchDiagnosticExtendedData: fix expected and actual types calculation. ([Issue ](https://github.com/dotnet/fsharp/pull/18851))
+* Nice print: format generic types in inherit/interface declarations using a prefix style. ([PR #18897](https://github.com/dotnet/fsharp/pull/18897))
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

From 968bf9fcb3211c1025c51e79ec0c049d35d7e4f4 Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Thu, 11 Sep 2025 22:55:06 +0300
Subject: [PATCH 3/8] ilverify

---
 .../ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl   | 2 +-
 .../ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
index 6a676b52c7a..42cf5fbfe1d 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
@@ -60,7 +60,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CompilerConfig+TcConfig::.ctor([FSharp.Compiler.Service]FSharp.Compiler.CompilerConfig+TcConfigBuilder, bool)][offset 0x00000634][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.PatternMatchCompilation::isProblematicClause([FSharp.Compiler.Service]FSharp.Compiler.PatternMatchCompilation+MatchClause)][offset 0x00000065][found Byte] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharp.Compiler.PatternMatchCompilation::.cctor()][offset 0x00000015][found Boolean] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2078-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000BE][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2083-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000BE][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+PrintUtilities::layoutXmlDoc([FSharp.Compiler.Service]FSharp.Compiler.TypedTreeOps+DisplayEnv, bool, [FSharp.Compiler.Service]FSharp.Compiler.Xml.XmlDoc, [FSharp.Compiler.Service]FSharp.Compiler.Text.Layout)][offset 0x00000033][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateNamespaceName(string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<Microsoft.FSharp.Core.CompilerServices.ITypeProvider>, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string)][offset 0x00000063][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateExpectedName([FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string[], string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<FSharp.Compiler.TypeProviders+ProvidedType>)][offset 0x000000AD][found Char] Unexpected type on the stack.
diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
index 3a9313580a0..fc0562112ea 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
@@ -65,7 +65,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.IlxGen::HashRangeSorted([S.P.CoreLib]System.Collections.Generic.IDictionary`2<!!0,System.Tuple`2<int32,!!1>>)][offset 0x00000012][found ref '[FSharp.Compiler.Service]FSharp.Compiler.IlxGen+HashRangeSorted@1875<T1>'][expected ref '[FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`2<int32,T0>,T0>'] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.PatternMatchCompilation::isProblematicClause([FSharp.Compiler.Service]FSharp.Compiler.PatternMatchCompilation+MatchClause)][offset 0x00000040][found Byte] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharp.Compiler.PatternMatchCompilation::.cctor()][offset 0x0000000B][found Boolean] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2078-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000B3][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2083-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000B3][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+PrintUtilities::layoutXmlDoc([FSharp.Compiler.Service]FSharp.Compiler.TypedTreeOps+DisplayEnv, bool, [FSharp.Compiler.Service]FSharp.Compiler.Xml.XmlDoc, [FSharp.Compiler.Service]FSharp.Compiler.Text.Layout)][offset 0x00000034][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateNamespaceName(string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<Microsoft.FSharp.Core.CompilerServices.ITypeProvider>, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string)][offset 0x00000074][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateExpectedName([FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string[], string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<FSharp.Compiler.TypeProviders+ProvidedType>)][offset 0x000000A8][found Char] Unexpected type on the stack.

From b669abb975241ad246902353db45f37b17b3e075 Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Mon, 15 Sep 2025 20:00:30 +0300
Subject: [PATCH 4/8] wip

---
 src/Compiler/Checking/NicePrint.fs            | 17 ++++-------
 src/Compiler/Symbols/Symbols.fs               |  3 ++
 src/Compiler/Symbols/Symbols.fsi              |  5 ++++
 src/Compiler/TypedTree/TypedTreeOps.fs        | 10 ++++++-
 src/Compiler/TypedTree/TypedTreeOps.fsi       |  4 ++-
 .../FSharp.Compiler.Service.Tests/Symbols.fs  | 28 +++++++++++++++++++
 .../TooltipTests.fs                           | 10 +++++++
 ....Compiler.Service_Debug_netstandard2.0.bsl |  4 +--
 ...ompiler.Service_Release_netstandard2.0.bsl |  4 +--
 9 files changed, 67 insertions(+), 18 deletions(-)

diff --git a/src/Compiler/Checking/NicePrint.fs b/src/Compiler/Checking/NicePrint.fs
index d2aca827887..94a239da048 100644
--- a/src/Compiler/Checking/NicePrint.fs
+++ b/src/Compiler/Checking/NicePrint.fs
@@ -124,9 +124,6 @@ module internal PrintUtilities =
         |> fun reversedArgs -> (true, retTyL) :: reversedArgs
         |> List.fold (fun acc (shouldBreak, layout) -> (if shouldBreak then (---) else (++)) layout acc) emptyL
 
-    let getSuperTypeDenv (denv: DisplayEnv) =
-        denv.UseGenericParameterStyle(PrefixForTopLevel(denv.genericParameterStyle))
-
     let tagNavArbValRef (valRefOpt: ValRef option) tag =
         match valRefOpt with
         | Some vref ->
@@ -175,7 +172,7 @@ module internal PrintUtilities =
         | GenericParameterStyle.Implicit -> tcref.IsPrefixDisplay, denv
         | GenericParameterStyle.Prefix -> true, denv
         | GenericParameterStyle.Suffix -> false, denv
-        | GenericParameterStyle.PrefixForTopLevel nested -> true, denv.UseGenericParameterStyle(nested) 
+        | GenericParameterStyle.TopLevelPrefix nested -> true, denv.UseGenericParameterStyle(nested) 
 
     /// <summary>
     /// Creates a layout for TyconRef.
@@ -748,12 +745,8 @@ module PrintTypes =
         | Some typarConstraintTy ->
             if Zset.contains typar env.singletons then
                 let tyLayout =
-                    match typarConstraintTy with
-                    | TType_app (tyconRef = tc; typeInstantiation = ti)
-                        when ti.Length > 0 && not (usePrefix denv tc |> fst) ->
-                        layoutTypeWithInfo denv env typarConstraintTy
-                        |> bracketL
-                    | _ -> layoutTypeWithInfo denv env typarConstraintTy
+                    let denv = denv.UseTopLevelPrefixGenericParameterStyle()
+                    layoutTypeWithInfo denv env typarConstraintTy
 
                 leftL (tagPunctuation "#") ^^ tyLayout
             else
@@ -2051,7 +2044,7 @@ module TastDefinitionPrinting =
                 GetImmediateInterfacesOfType SkipUnrefInterfaces.Yes g amap m ty
 
         let iimplsLs =
-            let denv = getSuperTypeDenv denv
+            let denv = denv.UseTopLevelPrefixGenericParameterStyle()
             iimpls
             |> List.map (fun intfTy -> (if isInterfaceTy g ty then WordL.keywordInherit else WordL.keywordInterface) -* layoutType denv intfTy)
 
@@ -2187,7 +2180,7 @@ module TastDefinitionPrinting =
             ]
 
         let inheritsL =
-            let denv = getSuperTypeDenv denv
+            let denv = denv.UseTopLevelPrefixGenericParameterStyle()
             inherits
             |> List.map (fun super -> WordL.keywordInherit ^^ (layoutType denv super))
 
diff --git a/src/Compiler/Symbols/Symbols.fs b/src/Compiler/Symbols/Symbols.fs
index fb413eccea7..28045551725 100644
--- a/src/Compiler/Symbols/Symbols.fs
+++ b/src/Compiler/Symbols/Symbols.fs
@@ -216,6 +216,9 @@ type FSharpDisplayContext(denv: TcGlobals -> DisplayEnv) =
     member _.WithSuffixGenericParameters () =
         FSharpDisplayContext(fun g -> { denv g with genericParameterStyle = GenericParameterStyle.Suffix }  )
 
+    member x.WithTopLevelPrefixGenericParameters () =
+        FSharpDisplayContext(fun g -> (denv g).UseTopLevelPrefixGenericParameterStyle())
+
 // delay the realization of 'item' in case it is unresolved
 type FSharpSymbol(cenv: SymbolEnv, item: unit -> Item, access: FSharpSymbol -> CcuThunk -> AccessorDomain -> bool) =
 
diff --git a/src/Compiler/Symbols/Symbols.fsi b/src/Compiler/Symbols/Symbols.fsi
index 701e9a8fe3c..0fec9922171 100644
--- a/src/Compiler/Symbols/Symbols.fsi
+++ b/src/Compiler/Symbols/Symbols.fsi
@@ -74,6 +74,11 @@ type FSharpDisplayContext =
     /// for example `int list`
     member WithSuffixGenericParameters: unit -> FSharpDisplayContext
 
+    /// Causes type signatures to be formatted with prefix-style generic parameters for a top level type
+    /// while nested types inherit generic parameters style from the current `FSharpDisplayContext` instance,
+    /// for example, `int list seq` becomes `seq<int list>`
+    member WithTopLevelPrefixGenericParameters: unit -> FSharpDisplayContext
+
 /// Represents a symbol in checked F# source code or a compiled .NET component.
 ///
 /// The subtype of the symbol may reveal further information and can be one of FSharpEntity, FSharpUnionCase
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fs b/src/Compiler/TypedTree/TypedTreeOps.fs
index 3d4aee1e2b2..3c5d2dcd20b 100644
--- a/src/Compiler/TypedTree/TypedTreeOps.fs
+++ b/src/Compiler/TypedTree/TypedTreeOps.fs
@@ -3171,7 +3171,7 @@ type GenericParameterStyle =
     | Implicit
     | Prefix
     | Suffix
-    | PrefixForTopLevel of nested: GenericParameterStyle
+    | TopLevelPrefix of nested: GenericParameterStyle
 
 [<NoEquality; NoComparison>]
 type DisplayEnv = 
@@ -3256,6 +3256,14 @@ type DisplayEnv =
 
     member denv.UseGenericParameterStyle style =
         { denv with genericParameterStyle = style }
+    
+    member denv.UseTopLevelPrefixGenericParameterStyle() =
+        let nestedStyle =
+            match denv.genericParameterStyle with
+            | TopLevelPrefix(nested) -> nested
+            | style -> style
+
+        { denv with genericParameterStyle = TopLevelPrefix(nestedStyle) }
 
     static member InitialForSigFileGeneration g =
         let denv =
diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
index c0e2a7848bb..128ae33dddd 100755
--- a/src/Compiler/TypedTree/TypedTreeOps.fsi
+++ b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -1065,7 +1065,7 @@ type GenericParameterStyle =
     ///     inherit seq<int list>
     /// instead of
     ///     inherit int list seq
-    | PrefixForTopLevel of nested: GenericParameterStyle
+    | TopLevelPrefix of nested: GenericParameterStyle
 
 [<NoEquality; NoComparison>]
 type DisplayEnv =
@@ -1116,6 +1116,8 @@ type DisplayEnv =
 
     member UseGenericParameterStyle: GenericParameterStyle -> DisplayEnv
 
+    member UseTopLevelPrefixGenericParameterStyle: unit -> DisplayEnv
+
     static member InitialForSigFileGeneration: TcGlobals -> DisplayEnv
 
 val tagEntityRefName: xref: EntityRef -> name: string -> TaggedText
diff --git a/tests/FSharp.Compiler.Service.Tests/Symbols.fs b/tests/FSharp.Compiler.Service.Tests/Symbols.fs
index b5126bf1ca6..f27033ee4d2 100644
--- a/tests/FSharp.Compiler.Service.Tests/Symbols.fs
+++ b/tests/FSharp.Compiler.Service.Tests/Symbols.fs
@@ -449,6 +449,34 @@ let tester2: int Group = []
 
             | other -> failwithf "myArr was supposed to be a value, but is %A"  other
 
+    [<Fact>]
+    let ``FSharpType.Format with top-level prefix generic parameters style`` () =
+        let _, checkResults = getParseAndCheckResults """
+let f (x: int list seq) = ()
+"""
+        let symbolUse = findSymbolUseByName "x" checkResults
+        let symbol = symbolUse.Symbol :?> FSharpMemberOrFunctionOrValue
+        let typeArg = symbol.FullType
+        let displayContext = symbolUse.DisplayContext
+
+        let topLevelPrefixStyle =
+            displayContext.WithTopLevelPrefixGenericParameters()
+
+        let topLevelPrefixWithNestedSuffixStyle1 =
+            displayContext.WithPrefixGenericParameters().WithTopLevelPrefixGenericParameters()
+
+        // Idempotent
+        let topLevelPrefixWithNestedSuffixStyle2 =
+            topLevelPrefixWithNestedSuffixStyle1.WithTopLevelPrefixGenericParameters()
+
+        [ typeArg.Format(topLevelPrefixStyle)
+          typeArg.Format(topLevelPrefixWithNestedSuffixStyle1)
+          typeArg.Format(topLevelPrefixWithNestedSuffixStyle2) ]
+        |> shouldBe [
+            "seq<int list>"
+            "seq<list<int>>"
+            "seq<list<int>>" ]
+
     [<Fact>]
     let ``Unfinished long ident type `` () =
         let _, checkResults = getParseAndCheckResults """
diff --git a/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs b/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
index fe4806ddd3a..517e4a8b59d 100644
--- a/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
+++ b/tests/FSharp.Compiler.Service.Tests/TooltipTests.fs
@@ -558,3 +558,13 @@ type A{caret} =
 """
     |> assertAndGetSingleToolTipText
     |> Assert.shouldBeEquivalentTo "type A =\n  interface seq<int list>"
+
+[<Fact>]
+let ``Flexible generic type should be formatted in the prefix style`` () =
+    Checker.getTooltip """
+module Foo
+
+let f (x{caret}: #seq<int list>) = ()
+"""
+    |> assertAndGetSingleToolTipText
+    |> Assert.shouldBeEquivalentTo "val x: #seq<int list>"
diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
index 42cf5fbfe1d..3eefca84aee 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Debug_netstandard2.0.bsl
@@ -40,7 +40,7 @@
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-516::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x00000082][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-516::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x0000008B][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-516::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x00000094][found Char] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$Symbols+fullName@2498-1::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000015][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$Symbols+fullName@2501-1::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000015][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CreateILModule+MainModuleBuilder::ConvertProductVersionToILVersionInfo(string)][offset 0x00000011][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.StaticLinking+TypeForwarding::followTypeForwardForILTypeRef([FSharp.Compiler.Service]FSharp.Compiler.AbstractIL.IL+ILTypeRef)][offset 0x00000010][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CompilerOptions::getCompilerOption([FSharp.Compiler.Service]FSharp.Compiler.CompilerOptions+CompilerOption, [FSharp.Core]Microsoft.FSharp.Core.FSharpOption`1<int32>)][offset 0x000000E6][found Char] Unexpected type on the stack.
@@ -60,7 +60,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CompilerConfig+TcConfig::.ctor([FSharp.Compiler.Service]FSharp.Compiler.CompilerConfig+TcConfigBuilder, bool)][offset 0x00000634][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.PatternMatchCompilation::isProblematicClause([FSharp.Compiler.Service]FSharp.Compiler.PatternMatchCompilation+MatchClause)][offset 0x00000065][found Byte] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharp.Compiler.PatternMatchCompilation::.cctor()][offset 0x00000015][found Boolean] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2083-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000BE][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2076-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000BE][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+PrintUtilities::layoutXmlDoc([FSharp.Compiler.Service]FSharp.Compiler.TypedTreeOps+DisplayEnv, bool, [FSharp.Compiler.Service]FSharp.Compiler.Xml.XmlDoc, [FSharp.Compiler.Service]FSharp.Compiler.Text.Layout)][offset 0x00000033][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateNamespaceName(string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<Microsoft.FSharp.Core.CompilerServices.ITypeProvider>, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string)][offset 0x00000063][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateExpectedName([FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string[], string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<FSharp.Compiler.TypeProviders+ProvidedType>)][offset 0x000000AD][found Char] Unexpected type on the stack.
diff --git a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
index fc0562112ea..bbf9187b1a7 100644
--- a/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
+++ b/tests/ILVerify/ilverify_FSharp.Compiler.Service_Release_netstandard2.0.bsl
@@ -39,7 +39,7 @@
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-531::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x00000064][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-531::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x0000006D][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$ServiceLexing+clo@924-531::Invoke([FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`3<FSharp.Compiler.Parser+token,int32,int32>,Microsoft.FSharp.Core.Unit>)][offset 0x00000076][found Char] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$Symbols+fullName@2498-3::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000030][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$Symbols+fullName@2501-3::Invoke([FSharp.Core]Microsoft.FSharp.Core.Unit)][offset 0x00000030][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Driver+ProcessCommandLineFlags@286-1::Invoke(string)][offset 0x0000000B][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.Driver+ProcessCommandLineFlags@286-1::Invoke(string)][offset 0x00000014][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.CreateILModule+MainModuleBuilder::ConvertProductVersionToILVersionInfo(string)][offset 0x00000010][found Char] Unexpected type on the stack.
@@ -65,7 +65,7 @@
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.IlxGen::HashRangeSorted([S.P.CoreLib]System.Collections.Generic.IDictionary`2<!!0,System.Tuple`2<int32,!!1>>)][offset 0x00000012][found ref '[FSharp.Compiler.Service]FSharp.Compiler.IlxGen+HashRangeSorted@1875<T1>'][expected ref '[FSharp.Core]Microsoft.FSharp.Core.FSharpFunc`2<System.Tuple`2<int32,T0>,T0>'] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.PatternMatchCompilation::isProblematicClause([FSharp.Compiler.Service]FSharp.Compiler.PatternMatchCompilation+MatchClause)][offset 0x00000040][found Byte] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : <StartupCode$FSharp-Compiler-Service>.$FSharp.Compiler.PatternMatchCompilation::.cctor()][offset 0x0000000B][found Boolean] Unexpected type on the stack.
-[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2083-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000B3][found Char] Unexpected type on the stack.
+[IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+TastDefinitionPrinting+meths@2076-3::Invoke([FSharp.Compiler.Service]FSharp.Compiler.Infos+MethInfo)][offset 0x000000B3][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.NicePrint+PrintUtilities::layoutXmlDoc([FSharp.Compiler.Service]FSharp.Compiler.TypedTreeOps+DisplayEnv, bool, [FSharp.Compiler.Service]FSharp.Compiler.Xml.XmlDoc, [FSharp.Compiler.Service]FSharp.Compiler.Text.Layout)][offset 0x00000034][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateNamespaceName(string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<Microsoft.FSharp.Core.CompilerServices.ITypeProvider>, [FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string)][offset 0x00000074][found Char] Unexpected type on the stack.
 [IL]: Error [StackUnexpected]: : FSharp.Compiler.TypeProviders::ValidateExpectedName([FSharp.Compiler.Service]FSharp.Compiler.Text.Range, string[], string, [FSharp.Compiler.Service]FSharp.Compiler.Tainted`1<FSharp.Compiler.TypeProviders+ProvidedType>)][offset 0x000000A8][found Char] Unexpected type on the stack.

From 0d489b9ea1f0ac29b67fbc518b6a67a19f138cae Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Mon, 15 Sep 2025 20:09:36 +0300
Subject: [PATCH 5/8] fix

---
 src/Compiler/TypedTree/TypedTreeOps.fsi | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/src/Compiler/TypedTree/TypedTreeOps.fsi b/src/Compiler/TypedTree/TypedTreeOps.fsi
index 128ae33dddd..ed847cc5839 100755
--- a/src/Compiler/TypedTree/TypedTreeOps.fsi
+++ b/src/Compiler/TypedTree/TypedTreeOps.fsi
@@ -1061,10 +1061,8 @@ type GenericParameterStyle =
     | Prefix
     /// Force the suffix style: int List
     | Suffix
-    /// Force the prefix style for top-level types in interface/inherit declarations:
-    ///     inherit seq<int list>
-    /// instead of
-    ///     inherit int list seq
+    /// Force the prefix style for a top-level type,
+    /// for example, `seq<int list>` instead of `int list seq`
     | TopLevelPrefix of nested: GenericParameterStyle
 
 [<NoEquality; NoComparison>]

From 46b330d93b1611fa84eb2c7d54f6243113cbfabb Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Mon, 15 Sep 2025 20:57:30 +0300
Subject: [PATCH 6/8] surface area

---
 .../FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl        | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl
index 31ecae647c8..cf2c03ce7df 100644
--- a/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl
+++ b/tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl
@@ -5164,6 +5164,7 @@ FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisp
 FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisplayContext WithPrefixGenericParameters()
 FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisplayContext WithShortTypeNames(Boolean)
 FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisplayContext WithSuffixGenericParameters()
+FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisplayContext WithTopLevelPrefixGenericParameters()
 FSharp.Compiler.Symbols.FSharpDisplayContext: FSharp.Compiler.Symbols.FSharpDisplayContext get_Empty()
 FSharp.Compiler.Symbols.FSharpEntity: Boolean Equals(System.Object)
 FSharp.Compiler.Symbols.FSharpEntity: Boolean HasAssemblyCodeRepresentation

From 074938645bff029b2eb742d82c25b67cf33d2aaf Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Mon, 15 Sep 2025 22:10:10 +0300
Subject: [PATCH 7/8] update release notes

---
 docs/release-notes/.FSharp.Compiler.Service/10.0.100.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
index a707b87e14f..638ccefdb23 100644
--- a/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
+++ b/docs/release-notes/.FSharp.Compiler.Service/10.0.100.md
@@ -4,6 +4,7 @@
 * Add support for tail calls in computation expressions ([PR #18804](https://github.com/dotnet/fsharp/pull/18804))
 * Add `--typecheck-only` flag support for F# Interactive (FSI) scripts to type-check without execution. ([Issue #18686](https://github.com/dotnet/fsharp/issues/18686))
 * Diagnostics: add extended data for 'No constructors' error ([PR #18863](https://github.com/dotnet/fsharp/pull/18863))
+* FSharpType.Format: support top-level prefix generic types style. ([PR #18897](https://github.com/dotnet/fsharp/pull/18897))
 
 ### Fixed
 
@@ -24,7 +25,7 @@
 * Fix Show XML doc for enum fields in external metadata ([Issue #17939](https://github.com/dotnet/fsharp/issues/17939#issuecomment-3137410105), [PR #18800](https://github.com/dotnet/fsharp/pull/18800))
 * Fix nullable types formatting in `FSharpType.Format` and tooltips to include parentheses. ([PR #18842](https://github.com/dotnet/fsharp/pull/18842))
 * TypeMismatchDiagnosticExtendedData: fix expected and actual types calculation. ([Issue ](https://github.com/dotnet/fsharp/pull/18851))
-* Nice print: format generic types in inherit/interface declarations using a prefix style. ([PR #18897](https://github.com/dotnet/fsharp/pull/18897))
+* Format top-level generic types using a prefix style in inherit/interface declarations and flexible type annotations. ([PR #18897](https://github.com/dotnet/fsharp/pull/18897))
 
 ### Changed
 * Use `errorR` instead of `error` in `CheckDeclarations.fs` when possible. ([PR #18645](https://github.com/dotnet/fsharp/pull/18645))

From 0ed71838859c37fcc439b2889531cf90275a24bd Mon Sep 17 00:00:00 2001
From: "Alexey.Berezhnykh" <alexey.berezhnykh@jetbrains.com>
Date: Mon, 15 Sep 2025 23:08:53 +0300
Subject: [PATCH 8/8] fixes

---
 tests/FSharp.Compiler.Service.Tests/Symbols.fs | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/tests/FSharp.Compiler.Service.Tests/Symbols.fs b/tests/FSharp.Compiler.Service.Tests/Symbols.fs
index f27033ee4d2..dc39cc6f7f7 100644
--- a/tests/FSharp.Compiler.Service.Tests/Symbols.fs
+++ b/tests/FSharp.Compiler.Service.Tests/Symbols.fs
@@ -462,16 +462,16 @@ let f (x: int list seq) = ()
         let topLevelPrefixStyle =
             displayContext.WithTopLevelPrefixGenericParameters()
 
-        let topLevelPrefixWithNestedSuffixStyle1 =
+        let topLevelPrefixWithNestedPrefixStyle1 =
             displayContext.WithPrefixGenericParameters().WithTopLevelPrefixGenericParameters()
 
-        // Idempotent
-        let topLevelPrefixWithNestedSuffixStyle2 =
-            topLevelPrefixWithNestedSuffixStyle1.WithTopLevelPrefixGenericParameters()
+        // Should be idempotent
+        let topLevelPrefixWithNestedPrefixStyle2 =
+            topLevelPrefixWithNestedPrefixStyle1.WithTopLevelPrefixGenericParameters()
 
         [ typeArg.Format(topLevelPrefixStyle)
-          typeArg.Format(topLevelPrefixWithNestedSuffixStyle1)
-          typeArg.Format(topLevelPrefixWithNestedSuffixStyle2) ]
+          typeArg.Format(topLevelPrefixWithNestedPrefixStyle1)
+          typeArg.Format(topLevelPrefixWithNestedPrefixStyle2) ]
         |> shouldBe [
             "seq<int list>"
             "seq<list<int>>"