Bugfix :: Nullness :: Unsafe cast no longer requires generic type to …

…be `not null` typar (#18343)

docs/release-notes/.FSharp.Compiler.Service/9.0.300.md

@@ -12,6 +12,7 @@
 * Cancellable: fix leaking cancellation token ([PR #18295](https://github.com/dotnet/fsharp/pull/18295))
 * Fix NRE when accessing nullable fields of types within their equals/hash/compare methods ([PR #18296](https://github.com/dotnet/fsharp/pull/18296))
 * Fix nullness warning for overrides of generic code with nullable type instance ([Issue #17988](https://github.com/dotnet/fsharp/issues/17988), [PR #18337](https://github.com/dotnet/fsharp/pull/18337))
+* Unsafe downcast from `obj` to generic `T` no longer requires `not null` constraint on `T`([Issue #18275](https://github.com/dotnet/fsharp/issues/18275), [PR #18343](https://github.com/dotnet/fsharp/pull/18343))
 
 ### Added
 * Added missing type constraints in FCS. ([PR #18241](https://github.com/dotnet/fsharp/pull/18241))

src/Compiler/Checking/Expressions/CheckExpressions.fs

@@ -2976,11 +2976,9 @@ let TcRuntimeTypeTest isCast isOperator (cenv: cenv) denv m tgtTy srcTy =
     if isSealedTy g srcTy then
         error(RuntimeCoercionSourceSealed(denv, srcTy, m))
 
-    if isSealedTy g tgtTy || isTyparTy g tgtTy || not (isInterfaceTy g srcTy) then
-        if isCast then
-            AddCxTypeMustSubsumeType (ContextInfo.RuntimeTypeTest isOperator) denv cenv.css m NoTrace srcTy tgtTy
-        else
-            AddCxTypeMustSubsumeType ContextInfo.NoContext denv cenv.css m NoTrace srcTy tgtTy
+    if (isSealedTy g tgtTy || isTyparTy g tgtTy || not (isInterfaceTy g srcTy)) && not (isObjTyAnyNullness g srcTy) then
+        let context = if isCast then ContextInfo.RuntimeTypeTest isOperator else ContextInfo.NoContext
+        AddCxTypeMustSubsumeType context denv cenv.css m NoTrace srcTy tgtTy
 
     if isErasedType g tgtTy then
         if isCast then

tests/FSharp.Compiler.ComponentTests/Language/Nullness/NullableReferenceTypesTests.fs

@@ -150,10 +150,8 @@ let doNotWarnOnDowncastRepeatedNestedNullable(o:objnull) = o :? list<((AB | null
     |> shouldFail
     |> withDiagnostics
                 [ Error 3264, Line 4, Col 39, Line 4, Col 47, "Nullness warning: Downcasting from 'objnull' into 'AB' can introduce unexpected null values. Cast to 'AB|null' instead or handle the null before downcasting."
-                  Error 3261, Line 5, Col 42, Line 5, Col 59, "Nullness warning: The types 'obj' and 'AB | null' do not have compatible nullability."
                   Error 3060, Line 5, Col 42, Line 5, Col 59, "This type test or downcast will erase the provided type 'AB | null' to the type 'AB'"
                   Error 3060, Line 6, Col 41, Line 6, Col 55, "This type test or downcast will erase the provided type 'AB | null' to the type 'AB'"
-                  Error 3261, Line 7, Col 51, Line 7, Col 97, "Nullness warning: The types 'obj' and 'AB | null array | null list | null' do not have compatible nullability."
                   Error 3060, Line 7, Col 51, Line 7, Col 97, "This type test or downcast will erase the provided type 'List<AB | null array | null> | null' to the type 'List<AB array>'"]
 
 
@@ -1381,6 +1379,20 @@ dict["ok"] <- 42
     |> typeCheckWithStrictNullness
     |> shouldSucceed
 
+[<InlineData("t :?> 'T")>]
+[<InlineData("(downcast t) : 'T")>]
+[<InlineData("t |> unbox<'T>")>]
+[<InlineData("(t : objnull) :?> 'T")>]
+[<Theory>]
+let ``Unsafe cast should not insist on not null constraint``(castOp:string) =
+
+    FSharp $"""module MyLibrary
+let test<'T> () =
+    let t = obj()
+    {castOp}"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
 
 
 [<InlineData("null")>]