Make nullness equivalent warning message clearer.

src/Compiler/Driver/CompilerDiagnostics.fs

@@ -438,7 +438,7 @@ module OldStyleMessages =
     let ConstraintSolverTupleDiffLengthsE () = Message("ConstraintSolverTupleDiffLengths", "%d%d")
     let ConstraintSolverInfiniteTypesE () = Message("ConstraintSolverInfiniteTypes", "%s%s")
     let ConstraintSolverMissingConstraintE () = Message("ConstraintSolverMissingConstraint", "%s")
-    let ConstraintSolverNullnessWarningEquivWithTypesE () = Message("ConstraintSolverNullnessWarningEquivWithTypes", "%s%s")
+    let ConstraintSolverNullnessWarningEquivWithTypesE () = Message("ConstraintSolverNullnessWarningEquivWithTypes", "%s")
     let ConstraintSolverNullnessWarningWithTypesE () = Message("ConstraintSolverNullnessWarningWithTypes", "%s%s")
     let ConstraintSolverNullnessWarningWithTypeE () = Message("ConstraintSolverNullnessWarningWithType", "%s")
     let ConstraintSolverNullnessWarningE () = Message("ConstraintSolverNullnessWarning", "%s")
@@ -678,8 +678,7 @@ type Exception with
 
             let t1, t2, _cxs = NicePrint.minimalStringsOfTwoTypes denv ty1 ty2
 
-            os.Append(ConstraintSolverNullnessWarningEquivWithTypesE().Format t1 t2)
-            |> ignore
+            os.Append(ConstraintSolverNullnessWarningEquivWithTypesE().Format t1) |> ignore
 
             if m.StartLine <> m2.StartLine then
                 os.Append(SeeAlsoE().Format(stringOfRange m)) |> ignore

src/Compiler/FSStrings.resx

@@ -130,7 +130,7 @@
     <value>A type parameter is missing a constraint '{0}'</value>
   </data>
   <data name="ConstraintSolverNullnessWarningEquivWithTypes" xml:space="preserve">
-    <value>Nullness warning: The types '{0}' and '{1}' do not have equivalent nullability.</value>
+    <value>Nullness warning: A non-nullable '{0}' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression.</value>
   </data>
   <data name="ConstraintSolverNullnessWarningWithTypes" xml:space="preserve">
     <value>Nullness warning: The types '{0}' and '{1}' do not have compatible nullability.</value>

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

@@ -107,9 +107,9 @@ let y:System.Collections.Generic.LinkedListNode<string> = ll.First
     |> shouldFail
     |> withDiagnostics 
          [ Error 3261, Line 4, Col 59, Line 4, Col 66, "Nullness warning: The types 'System.Collections.Generic.LinkedListNode<string>' and 'System.Collections.Generic.LinkedListNode<string> | null' do not have compatible nullability."
-           Error 3261, Line 4, Col 59, Line 4, Col 66, "Nullness warning: The types 'System.Collections.Generic.LinkedListNode<string>' and 'System.Collections.Generic.LinkedListNode<string> | null' do not have equivalent nullability."
+           Error 3261, Line 4, Col 59, Line 4, Col 66, "Nullness warning: A non-nullable 'System.Collections.Generic.LinkedListNode<string>' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."
            Error 3261, Line 5, Col 59, Line 5, Col 67, "Nullness warning: The types 'System.Collections.Generic.LinkedListNode<string>' and 'System.Collections.Generic.LinkedListNode<string> | null' do not have compatible nullability."
-           Error 3261, Line 5, Col 59, Line 5, Col 67, "Nullness warning: The types 'System.Collections.Generic.LinkedListNode<string>' and 'System.Collections.Generic.LinkedListNode<string> | null' do not have equivalent nullability."]
+           Error 3261, Line 5, Col 59, Line 5, Col 67, "Nullness warning: A non-nullable 'System.Collections.Generic.LinkedListNode<string>' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression." ]
 
 [<FactForNETCOREAPP>]
 let ``Nullable directory info show warn on prop access`` () = 
@@ -219,12 +219,12 @@ let ``Consumption of nullable C# - no generics, just strings in methods and fiel
     |> shouldFail
     |> withDiagnostics [
             Error 3261, Line 5, Col 40, Line 5, Col 85, "Nullness warning: The types 'string' and 'string | null' do not have compatible nullability."
-            Error 3261, Line 5, Col 40, Line 5, Col 85, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
-            Error 3261, Line 14, Col 34, Line 14, Col 62, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
+            Error 3261, Line 5, Col 40, Line 5, Col 85, "Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."
+            Error 3261, Line 14, Col 34, Line 14, Col 62, "Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."
             Error 3261, Line 16, Col 35, Line 16, Col 39, "Nullness warning: The type 'string' does not support 'null'."
-            Error 3261, Line 25, Col 85, Line 25, Col 97, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
-            Error 3261, Line 28, Col 99, Line 28, Col 111, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."
-            Error 3261, Line 30, Col 97, Line 30, Col 109, "Nullness warning: The types 'string' and 'string | null' do not have equivalent nullability."]
+            Error 3261, Line 25, Col 85, Line 25, Col 97, "Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."
+            Error 3261, Line 28, Col 99, Line 28, Col 111, "Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."
+            Error 3261, Line 30, Col 97, Line 30, Col 109, "Nullness warning: A non-nullable 'string' was expected but this expression is nullable. Consider either changing the target to also be nullable, or use pattern matching to safely handle the null case of this expression."]
 
 
 [<FactForNETCOREAPP>]