Check if the module appears to be incorrectly nested based on indentation

Author: edgarfgp

src/Compiler/pars.fsy

@@ -1271,6 +1271,14 @@ moduleDefnsOrExpr:
 
 /* A sequence of definitions in a namespace or module */
 moduleDefns:
+  | moduleDefnOrDirective moduleDefnOrDirective
+      { match $1, $2 with
+        | [SynModuleDecl.Types _], [SynModuleDecl.NestedModule(_, _, _, _, _, { ModuleKeyword = Some m })] ->
+            // Check if the module appears to be incorrectly nested based on indentation
+            errorR(Error(FSComp.SR.parsInvalidDeclarationSyntax(), m))
+            $1 @ $2
+        | _ -> $1 @ $2 }
+
   | moduleDefnOrDirective moduleDefns
       { $1 @ $2 }
 

tests/service/data/SyntaxTree/Type/Module Inside Type Defn 01.fs

@@ -0,0 +1,6 @@
+module Module
+
+type IFace =
+    abstract F : int -> int
+    module M =
+        let f () = ()

tests/service/data/SyntaxTree/Type/Module Inside Type Defn 02.fs

@@ -0,0 +1,6 @@
+module Module
+
+type C () =
+    member _.F () = 3
+    module M2 =
+        let f () = ()

tests/service/data/SyntaxTree/Type/Module Inside Type Defn 03.fs

@@ -0,0 +1,7 @@
+module Module
+
+type U =
+    | A
+    | B
+    module M3 =
+        let f () = ()

tests/service/data/SyntaxTree/Type/Module Inside Type Defn 04.fs

@@ -0,0 +1,6 @@
+module Module
+
+type R =
+    { A : int }
+    module M4 =
+        let f () = ()

tests/service/data/SyntaxTree/Type/Module Inside Type Defn 05.fs

@@ -0,0 +1,8 @@
+module Module
+
+type IFace =
+    interface
+        abstract F : int -> int
+        module M =
+            let f () = f ()
+    end