Don't crash graph checking on invalid syntax

docs/release-notes/.FSharp.Compiler.Service/8.0.300.md

@@ -2,11 +2,11 @@
 
 * Code generated files with > 64K methods and generated symbols crash when loaded. Use infered sequence points for debugging. ([Issue #16399](https://github.com/dotnet/fsharp/issues/16399), [#PR 16514](https://github.com/dotnet/fsharp/pull/16514))
 * `nameof Module` expressions and patterns are processed to link files in `--test:GraphBasedChecking`. ([PR #16550](https://github.com/dotnet/fsharp/pull/16550))
-* Graph Based Checking doesn't throw on invalid parsed input so it can be used for IDE scenarios ([PR #16575](https://github.com/dotnet/fsharp/pull/16575))
+* Graph Based Checking doesn't throw on invalid parsed input so it can be used for IDE scenarios ([PR #16575](https://github.com/dotnet/fsharp/pull/16575), [PR #16588](https://github.com/dotnet/fsharp/pull/16588))
 
 ### Added
 
-* The stackguard depth for ILPdbWriter.unshadowScopes can be modified via the environment variable `FSHARP_ILPdb_UnshadowScopes_StackGuardDepth`([PR #16583](https://github.com/dotnet/fsharp/pull/16583)) 
+* The stackguard depth for ILPdbWriter.unshadowScopes can be modified via the environment variable `FSHARP_ILPdb_UnshadowScopes_StackGuardDepth`([PR #16583](https://github.com/dotnet/fsharp/pull/16583))
 * Parser recovers on complex primary constructor patterns, better tree representation for primary constructor patterns. ([PR #16425](https://github.com/dotnet/fsharp/pull/16425))
 * Name resolution: keep type vars in subsequent checks ([PR #16456](https://github.com/dotnet/fsharp/pull/16456))
 * Higher-order-function-based API for working with the untyped abstract syntax tree. ([PR #16462](https://github.com/dotnet/fsharp/pull/16462))

src/Compiler/Driver/GraphChecking/FileContentMapping.fs

@@ -9,10 +9,9 @@ type Continuations = ((FileContentEntry list -> FileContentEntry list) -> FileCo
 let collectFromOption (mapping: 'T -> 'U list) (t: 'T option) : 'U list = List.collect mapping (Option.toList t)
 
 let longIdentToPath (skipLast: bool) (longId: LongIdent) : LongIdentifier =
-    if skipLast then
-        List.take (longId.Length - 1) longId
-    else
-        longId
+    match skipLast, longId with
+    | true, _ :: _ -> List.take (longId.Length - 1) longId
+    | _ -> longId
     |> List.map (fun ident -> ident.idText)
 
 let synLongIdentToPath (skipLast: bool) (synLongIdent: SynLongIdent) =

tests/FSharp.Compiler.ComponentTests/TypeChecks/Graph/FileContentMappingTests.fs

@@ -122,17 +122,36 @@ module B = C
     | [ TopLevelNamespace "" [ PrefixedIdentifier "C" ] ] -> Assert.Pass()
     | content -> Assert.Fail($"Unexpected content: {content}")
 
-[<Test>]
-let ``Invalid nested module should just be ignored`` () =
-    let content =
-        getContent
-            false
-            """
-module A
 
-module B.C
-"""
+module We_Shouldnt_Crash_With_Invalid_Syntax =
 
-    match content with
-    | [ TopLevelNamespace "" [] ] -> Assert.Pass()
-    | content -> Assert.Fail($"Unexpected content: {content}")
+    [<Test>]
+    let ``Nested module`` () =
+        let content =
+            getContent
+                false
+                """
+    module A
+
+    module B.C
+    """
+
+        match content with
+        | [ TopLevelNamespace "" [] ] -> Assert.Pass()
+        | content -> Assert.Fail($"Unexpected content: {content}")
+
+
+    [<Test>]
+    let ``Module above namespace`` () =
+        let content =
+            getContent
+                false
+                """
+    module
+
+    namespace A.B.C
+    """
+
+        match content with
+        | [ TopLevelNamespace "" [] ] -> Assert.Pass()
+        | content -> Assert.Fail($"Unexpected content: {content}")