Guard against unformattable caller name

Src/FluentAssertions/Execution/MessageBuilder.cs

@@ -91,7 +91,14 @@ private string FormatArgumentPlaceholders(string failureMessage, object[] failur
         {
             string[] values = failureArgs.Select(a => Formatter.ToString(a, formattingOptions)).ToArray();
 
-            return string.Format(CultureInfo.InvariantCulture, failureMessage, values);
+            try
+            {
+                return string.Format(CultureInfo.InvariantCulture, failureMessage, values);
+            }
+            catch (FormatException formatException)
+            {
+                return $"**WARNING** failure message '{failureMessage}' could not be formatted with string.Format{Environment.NewLine}{formatException.StackTrace}";
+            }
         }
 
         private string SanitizeReason(string reason)

docs/_pages/releases.md

@@ -21,6 +21,7 @@ sidebar:
 * `OccurredEvent` ordering on monitored object is now done via thread-safe counter - [#1773](https://github.com/fluentassertions/fluentassertions/pull/1773)
 * Avoid a `NullReferenceException` when testing an application compiled with .NET Native - [#1776](https://github.com/fluentassertions/fluentassertions/pull/1776)
 * `[Not]Contain(key, value)` for dictionary-like enumerables incorrectly checked if the key was present - [#1786](https://github.com/fluentassertions/fluentassertions/pull/1786)
+* Avoid throwing a `FormatException` when caller name determination returns an unformattable string - [#1788](https://github.com/fluentassertions/fluentassertions/pull/1788)
 
 ## 6.3.0