Nullness :: reduce false warnings, do not infer a value to be (string|null) when used within 'printfn "%s"'

src/Compiler/Checking/CheckFormatStrings.fs

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

src/Compiler/TypedTree/TcGlobals.fs

@@ -449,7 +449,7 @@ 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_string_ty_ambivalent = mkNonGenericTyWithNullness v_string_tcr KnownAmbivalentToNull
   let v_decimal_ty      = mkSysNonGenericTy sys "Decimal"
   let v_unit_ty         = mkNonGenericTy v_unit_tcr_nice 
   let v_system_Type_ty = mkSysNonGenericTy sys "Type" 
@@ -1341,7 +1341,7 @@ 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 _.string_ty_ambivalent = v_string_ty_ambivalent
   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/Language/NullableReferenceTypesTests.fs

@@ -61,7 +61,7 @@ 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 interpolated = $"This is fine %s{maybeNull}"
 let interpolatedAnnotatedNotNull = $"This is fine %s{nonNullString}"
 let interpolatedAnnotatedNullable = $"This is not null %s{maybeNull}"
 let interpolateNullLiteral = $"This is not null %s{null}"
@@ -96,6 +96,19 @@ let printViaA = sprintf "This is null: %A and this has null inside %A" maybeArra
     |> typeCheckWithStrictNullness
     |> shouldSucceed
 
+[<Fact>]
+let ``Type inference with sprintfn`` () = 
+    FSharp """module MyLibrary
+let needsString(x:string) = ()
+
+let myTopFunction inferedVal = 
+    printfn "This is it %s" inferedVal  // There was a regression infering this to be (string | null)
+    needsString inferedVal
+"""
+    |> asLibrary
+    |> typeCheckWithStrictNullness
+    |> shouldSucceed
+
 [<Fact>]
 let ``WhatIf the format itself is null`` () = 
     FSharp """module MyLibrary