An implicit numeric cast is only unnecessary if it doesn't have the chance to lose information.

src/EditorFeatures/CSharpTest/Diagnostics/RemoveUnnecessaryCast/RemoveUnnecessaryCastTests.cs

@@ -2111,6 +2111,22 @@ public A(object x)
 }");
         }
 
+        [WorkItem(39042, "https://github.com/dotnet/roslyn/issues/39042")]
+        [Fact, Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryCast)]
+        public async Task DontRemoveNecessaryCastForImplicitNumericCastsThatLoseInformation()
+        {
+            await TestMissingInRegularAndScriptAsync(
+@"using System;
+
+class A
+{
+    public A(long x)
+    {
+        long y = (long)[|(double)x|];
+    }
+}");
+        }
+
         #region Interface Casts
 
         [WorkItem(545889, "http://vstfdevdiv:8080/DevDiv2/DevDiv/_workitems/edit/545889")]

src/Workspaces/CSharp/Portable/Extensions/CastExpressionSyntaxExtensions.cs

@@ -500,7 +500,8 @@ public static bool IsUnnecessaryCast(this CastExpressionSyntax cast, SemanticMod
                     expressionToCastType.IsImplicit &&
                     (expressionToCastType.IsNumeric || expressionToCastType.IsConstantExpression))
                 {
-                    return true;
+                    // Some implicit numeric conversions can cause loss of precision and must not be removed.
+                    return !IsRequiredImplicitNumericConversion(expressionType, castType);
                 }
 
                 if (!castToOuterType.IsBoxing &&