Error on invalid declarations in type definitions.

docs/release-notes/.FSharp.Compiler.Service/10.0.100.md

@@ -17,6 +17,7 @@
 * Completion: fix previous namespace considered opened [PR #18609](https://github.com/dotnet/fsharp/pull/18609)
 * Fix active pattern typechecking regression. ([Issue #18638](https://github.com/dotnet/fsharp/issues/18638), [PR #18642](https://github.com/dotnet/fsharp/pull/18642))
 * Fix nullness warnings when casting non-nullable values to `IEquatable<T>` to match C# behavior. ([Issue #18759](https://github.com/dotnet/fsharp/issues/18759))
+* Warn when a module definition is encountered inside a type definition.([Issue #10066](https://github.com/dotnet/fsharp/issues/10066), [PR #18813](https://github.com/dotnet/fsharp/pull/18813))
 * Fix IsByRefLikeAttribute types being incorrectly suppressed in completion lists. Types like `Span<T>` and `ReadOnlySpan<T>` now appear correctly in IntelliSense.
 
 ### Changed

docs/release-notes/.Language/preview.md

@@ -15,5 +15,6 @@
 ### Fixed
 
 * Warn on uppercase identifiers in patterns. ([PR #15816](https://github.com/dotnet/fsharp/pull/15816))
+* Warn when a module definition is encountered inside a type definition.([Issue #10066](https://github.com/dotnet/fsharp/issues/10066), [PR #18813](https://github.com/dotnet/fsharp/pull/18813))
 
 ### Changed

src/Compiler/FSComp.txt

@@ -1802,6 +1802,7 @@ featureSupportWarnWhenUnitPassedToObjArg,"Warn when unit is passed to a member a
 featureUseBangBindingValueDiscard,"Allows use! _ = ... in computation expressions"
 featureBetterAnonymousRecordParsing,"Support for better anonymous record parsing"
 featureScopedNowarn,"Support for scoped enabling / disabling of warnings by #warn and #nowarn directives, also inside modules"
+featureWarnOnUnexpectedModuleDefinitionsInsideTypes,"Warn when a module definition is encountered inside a type definition"
 featureAllowLetOrUseBangTypeAnnotationWithoutParens,"Allow let! and use! type annotations without requiring parentheses"
 3874,lexWarnDirectiveMustBeFirst,"#nowarn/#warnon directives must appear as the first non-whitespace characters on a line"
 3875,lexWarnDirectiveMustHaveArgs,"Warn directives must have warning number(s) as argument(s)"

src/Compiler/Facilities/LanguageFeatures.fs

@@ -102,6 +102,7 @@ type LanguageFeature =
     | UseBangBindingValueDiscard
     | BetterAnonymousRecordParsing
     | ScopedNowarn
+    | WarnOnUnexpectedModuleDefinitionsInsideTypes
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
 
@@ -243,6 +244,7 @@ type LanguageVersion(versionText) =
 
                 // F# preview (still preview in 10.0)
                 LanguageFeature.FromEndSlicing, previewVersion // Unfinished features --- needs work
+                LanguageFeature.WarnOnUnexpectedModuleDefinitionsInsideTypes, previewVersion
             ]
 
     static let defaultLanguageVersion = LanguageVersion("default")
@@ -410,6 +412,7 @@ type LanguageVersion(versionText) =
         | LanguageFeature.UseBangBindingValueDiscard -> FSComp.SR.featureUseBangBindingValueDiscard ()
         | LanguageFeature.BetterAnonymousRecordParsing -> FSComp.SR.featureBetterAnonymousRecordParsing ()
         | LanguageFeature.ScopedNowarn -> FSComp.SR.featureScopedNowarn ()
+        | LanguageFeature.WarnOnUnexpectedModuleDefinitionsInsideTypes -> FSComp.SR.featureWarnOnUnexpectedModuleDefinitionsInsideTypes ()
         | LanguageFeature.AllowTypedLetUseAndBang -> FSComp.SR.featureAllowLetOrUseBangTypeAnnotationWithoutParens ()
         | LanguageFeature.ReturnFromFinal -> FSComp.SR.featureReturnFromFinal ()
 

src/Compiler/Facilities/LanguageFeatures.fsi

@@ -93,6 +93,7 @@ type LanguageFeature =
     | UseBangBindingValueDiscard
     | BetterAnonymousRecordParsing
     | ScopedNowarn
+    | WarnOnUnexpectedModuleDefinitionsInsideTypes
     | AllowTypedLetUseAndBang
     | ReturnFromFinal
 

src/Compiler/SyntaxTree/ParseHelpers.fs

@@ -1065,6 +1065,39 @@ let leadingKeywordIsAbstract =
     | SynLeadingKeyword.StaticAbstractMember _ -> true
     | _ -> false
 
+[<return: Struct>]
+let (|NestedModuleAt|_|) =
+    function
+    | SynModuleDecl.NestedModule(trivia = { ModuleKeyword = Some mKeyword }) -> ValueSome mKeyword
+    | _ -> ValueNone
+
+[<return: Struct>]
+let (|TypesFollowedByModules|_|) =
+    function
+    | [ SynModuleDecl.Types(typeDefns = defns) ], rest -> ValueSome(defns, rest)
+    | _ -> ValueNone
+
+let getLastTypeColumn (defns: SynTypeDefn list) =
+    defns
+    |> List.choose (function
+        | SynTypeDefn(trivia = { LeadingKeyword = mKeyword }) -> Some mKeyword.Range)
+    |> List.tryLast
+    |> Option.defaultValue range0
+
+let checkInvalidDeclsInTypeDefn (moduleDecls1: SynModuleDecl list) (moduleDecls2: SynModuleDecl list) (lexBuf: Lexbuf) =
+    match moduleDecls1, moduleDecls2 with
+    | TypesFollowedByModules(defns, rest) ->
+        let lastTypeColumn = getLastTypeColumn defns
+
+        for defn in rest do
+            match defn with
+            | NestedModuleAt mKeyword ->
+                if lexBuf.SupportsFeature(LanguageFeature.WarnOnUnexpectedModuleDefinitionsInsideTypes) then
+                    if mKeyword.StartColumn > lastTypeColumn.StartColumn then
+                        warning (Error(FSComp.SR.parsInvalidDeclarationSyntax (), mKeyword))
+            | _ -> ()
+    | _ -> ()
+
 /// Unified helper for creating let/let!/use/use! expressions
 /// Creates either SynExpr.LetOrUse or SynExpr.LetOrUseBang based on isBang parameter
 /// Handles all four cases: 'let', 'let!', 'use', and 'use!'

src/Compiler/SyntaxTree/ParseHelpers.fsi

@@ -7,6 +7,7 @@ open FSharp.Compiler.Syntax
 open FSharp.Compiler.SyntaxTrivia
 open FSharp.Compiler.Features
 open FSharp.Compiler.Text
+open FSharp.Compiler.UnicodeLexing
 open FSharp.Compiler.Xml
 open Internal.Utilities.Text.Lexing
 open Internal.Utilities.Text.Parsing
@@ -270,3 +271,7 @@ val mkSynField:
         SynField
 
 val leadingKeywordIsAbstract: SynLeadingKeyword -> bool
+
+/// Check for invalid declarations inside type definitions
+val checkInvalidDeclsInTypeDefn:
+    moduleDecls1: SynModuleDecl list -> moduleDecls2: SynModuleDecl list -> lexBuf: Lexbuf -> unit

src/Compiler/pars.fsy

@@ -1271,8 +1271,13 @@ moduleDefnsOrExpr:
 
 /* A sequence of definitions in a namespace or module */
 moduleDefns:
+  | moduleDefnOrDirective moduleDefnOrDirective
+      { checkInvalidDeclsInTypeDefn $1 $2 parseState.LexBuffer
+        $1 @ $2 }
+
   | moduleDefnOrDirective moduleDefns
-      { $1 @ $2 }
+      { checkInvalidDeclsInTypeDefn $1 $2 parseState.LexBuffer
+        $1 @ $2 }
 
   | moduleDefnOrDirective topSeparators moduleDefnsOrExpr
       { $1 @ $3 }

tests/FSharp.Compiler.ComponentTests/ErrorMessages/WarnOnUnexpectedModuleDefinitions.fs

@@ -0,0 +1,106 @@
+// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
+namespace ErrorMessages
+
+open Xunit
+open FSharp.Test.Compiler
+
+module ``Module definition is encountered inside a type definition`` =
+
+    [<Fact>]
+    let ``Warn when a module definition is encountered inside a type definition``() =
+        Fsx """
+type IFace =
+    abstract F : int -> int
+    module M =
+        let f () = ()
+
+type C () =
+    member _.F () = 3
+    module M2 =
+        let f () = ()
+
+type U =
+    | A
+    | B
+    module M3 =
+        let f () = ()
+
+type R =
+    { A : int }
+    module M4 =
+        let f () = ()
+
+type A = A
+
+module M4 = begin end
+module M5 = begin end
+module M6 = begin end
+
+type B =
+    | B
+    module M7 = begin end
+    module M8 = begin end
+    module M9 = begin end
+
+module ThisIsFine =
+    let f () = ()
+
+type D = D
+    """
+        |> withLangVersionPreview
+        |> typecheck
+        |> shouldFail
+        |> withDiagnostics [
+            (Warning 554, Line 31, Col 5, Line 31, Col 11, "Invalid declaration syntax")
+            (Warning 554, Line 32, Col 5, Line 32, Col 11, "Invalid declaration syntax")
+            (Warning 554, Line 33, Col 5, Line 33, Col 11, "Invalid declaration syntax")
+            (Warning 554, Line 20, Col 5, Line 20, Col 11, "Invalid declaration syntax")
+            (Warning 554, Line 15, Col 5, Line 15, Col 11, "Invalid declaration syntax")
+            (Warning 554, Line 9, Col 5, Line 9, Col 11, "Invalid declaration syntax");
+            (Warning 554, Line 4, Col 5, Line 4, Col 11, "Invalid declaration syntax")
+        ]
+
+    [<Fact>]
+    let ``Don't warn when a module definition is encountered inside a type definition``() =
+        Fsx """
+type IFace =
+    abstract F : int -> int
+    module M =
+        let f () = ()
+
+type C () =
+    member _.F () = 3
+    module M2 =
+        let f () = ()
+
+type U =
+    | A
+    | B
+    module M3 =
+        let f () = ()
+
+type R =
+    { A : int }
+    module M4 =
+        let f () = ()
+
+type A = A
+
+module M5 = begin end
+module M6 = begin end
+module M7 = begin end
+
+type B =
+    | B
+    module M8 = begin end
+    module M9 = begin end
+    module M10 = begin end
+
+module ThisIsFine =
+    let f () = ()
+
+type D = D
+    """
+        |> withLangVersion10
+        |> typecheck
+        |> shouldSucceed

tests/FSharp.Compiler.ComponentTests/FSharp.Compiler.ComponentTests.fsproj

@@ -221,6 +221,7 @@
     <Compile Include="ErrorMessages\FS0988AtEndOfFile.fs" />
     <Compile Include="ErrorMessages\Repro1548.fs" />
     <Compile Include="ErrorMessages\WarnIfDiscardedInList.fs" />
+    <Compile Include="ErrorMessages\WarnOnUnexpectedModuleDefinitions.fs" />
     <Compile Include="ErrorMessages\UnionCasePatternMatchingErrors.fs" />
     <Compile Include="ErrorMessages\InterfaceImplInAugmentationsTests.fs" />
     <Compile Include="ErrorMessages\ExtendedDiagnosticDataTests.fs" />