Nullness :: Format string %s should allow nullable string

src/Compiler/Checking/CheckFormatStrings.fs

@@ -449,7 +449,8 @@ let parseFormatStringInternal
                 checkOtherFlags ch
                 collectSpecifierLocation fragLine fragCol 1
                 let i = skipPossibleInterpolationHole (i+1)
-                parseLoop ((posi, g.string_ty)  :: acc) (i, fragLine, fragCol+1) fragments
+                let stringTy = if g.checkNullness && g.langFeatureNullness then g.string_ty_withNull else g.string_ty
+                parseLoop ((posi, stringTy) :: acc) (i, fragLine, fragCol+1) fragments
 
             | 'O' ->
                 checkOtherFlags ch

src/Compiler/TypedTree/TcGlobals.fs

@@ -447,6 +447,7 @@ type TcGlobals(
   let v_FormattableStringFactory_tcref = findSysTyconRef sysCompilerServices "FormattableStringFactory"
   let v_FormattableStringFactory_ty = mkNonGenericTy v_FormattableStringFactory_tcref
   let v_string_ty       = mkNonGenericTy v_string_tcr
+  let v_string_ty_withNull = mkNonGenericTyWithNullness v_string_tcr KnownWithNull
   let v_decimal_ty      = mkSysNonGenericTy sys "Decimal"
   let v_unit_ty         = mkNonGenericTy v_unit_tcr_nice 
   let v_system_Type_ty = mkSysNonGenericTy sys "Type" 
@@ -1336,6 +1337,7 @@ type TcGlobals(
   member _.bool_ty = v_bool_ty
   member _.int_ty = v_int_ty
   member _.string_ty = v_string_ty
+  member _.string_ty_withNull = v_string_ty_withNull
   member _.system_IFormattable_tcref = v_IFormattable_tcref
   member _.system_FormattableString_tcref = v_FormattableString_tcref
   member _.system_FormattableStringFactory_tcref = v_FormattableStringFactory_tcref

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

@@ -221,11 +221,12 @@
     <Compile Include="Language\SequenceExpressionTests.fs" />
     <Compile Include="Language\StaticClassTests.fs" />
     <Compile Include="Language\PrintfFormatTests.fs" />
-  	<Compile Include="Language\InterfaceTests.fs" />
-	  <Compile Include="Language\CopyAndUpdateTests.fs" />
-	  <Compile Include="Language\ConstraintIntersectionTests.fs" />
+    <Compile Include="Language\InterfaceTests.fs" />
+    <Compile Include="Language\CopyAndUpdateTests.fs" />
+    <Compile Include="Language\ConstraintIntersectionTests.fs" />
     <Compile Include="Language\BooleanReturningAndReturnTypeDirectedPartialActivePatternTests.fs" />
     <Compile Include="Language\NullableCsharpImportTests.fs" />
+    <Compile Include="Language\NullableReferenceTypesTests.fs" />
     <Compile Include="ConstraintSolver\PrimitiveConstraints.fs" />
     <Compile Include="ConstraintSolver\MemberConstraints.fs" />
     <Compile Include="ConstraintSolver\ObjInference.fs" />

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

@@ -0,0 +1,74 @@
+module Language.NullableReferenceTypes
+
+open Xunit
+open FSharp.Test.Compiler
+
+let typeCheckWithStrictNullness cu =
+    cu
+    |> withLangVersionPreview
+    |> withCheckNulls
+    |> withWarnOn 3261
+    |> withOptions ["--warnaserror+"]
+    |> typecheck
+
+[<Fact>]
+let ``Printing a nullable string should pass`` () = 
+    FSharp """module MyLibrary
+let maybeNull : string | null = null
+let nonNullString = "abc"
+let printedValueNotNull = sprintf "This is not null: %s" nonNullString
+let printedValueNull = sprintf "This is null: %s" maybeNull
+let interpolated = $"This is fine {maybeNull}"
+let interpolatedAnnotatedNotNull = $"This is fine %s{nonNullString}"
+let interpolatedAnnotatedNullable = $"This is not null %s{maybeNull}"
+let interpolateNullLiteral = $"This is not null %s{null}"
+let sprintfnNullLiteral = sprintf "This is null: %s" null
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+
+[<Fact>]
+let ``Printing a nullable object should pass`` () = 
+    FSharp """module MyLibrary
+let maybeNull : string | null = null
+let maybeUri : System.Uri | null = null
+let okString = "abc"
+let printViaO = sprintf "This is null: %O and this is null %O and this is not null %O" maybeNull maybeUri okString
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+
+[<Fact>]
+let ``Printing a nullable array via percent A should pass`` () = 
+    FSharp """module MyLibrary
+let maybeArray : ((string array) | null) = null
+let arrayOfMaybes : ((string | null) array ) = [|null|]
+let printViaA = sprintf "This is null: %A and this has null inside %A" maybeArray arrayOfMaybes
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
+[<Fact>]
+let ``WhatIf the format itself is null`` () = 
+    FSharp """module MyLibrary
+[<Literal>]
+let thisCannotBeAFormat : string | null = null
+[<Literal>]
+let maybeLiteral : string | null = "abc"
+[<Literal>]
+let maybeLiteralWithHole : string | null = "Look at me %s"
+[<Literal>]
+let notNullLiteral : string = "abc"
+let doStuff() = 
+    printfn notNullLiteral
+    printfn maybeLiteral
+    printfn maybeLiteralWithHole thisCannotBeAFormat
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed