Don't call GetGenericsArguments unless IsGeneric is true

Fixes #8452

src/Scripting/CSharpTest/ObjectFormatterTests.cs

@@ -4,6 +4,7 @@
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Threading;
 using Microsoft.CodeAnalysis.Scripting;
@@ -777,6 +778,15 @@ public void DebuggerProxy_ArrayBuilder()
             );
         }
 
+        [WorkItem(8542, "https://github.com/dotnet/roslyn/issues/8452")]
+        [Fact]
+        public void FormatConstructorSignature()
+        {
+            var constructor = typeof(object).GetTypeInfo().DeclaredConstructors.Single();
+            var signature = ((CommonObjectFormatter)s_formatter).FormatMethodSignature(constructor);
+            Assert.Equal("object..ctor()", signature); // Checking for exceptions, more than particular output.
+        }
+
         // The stack trace contains line numbers.  We use a #line directive
         // so that the baseline doesn't need to be updated every time this
         // file changes.

src/Scripting/Core/Hosting/ObjectFormatter/CommonObjectFormatter.cs

@@ -101,7 +101,7 @@ public override string FormatUnhandledException(Exception e)
         /// Returns a method signature display string. Used to display stack frames.
         /// </summary>
         /// <returns>Null if the method is a compiler generated method that shouldn't be displayed to the user.</returns>
-        protected virtual string FormatMethodSignature(MethodBase method)
+        protected internal virtual string FormatMethodSignature(MethodBase method)
         {
             var pooled = PooledStringBuilder.GetInstance();
             var builder = pooled.Builder;
@@ -112,7 +112,10 @@ protected virtual string FormatMethodSignature(MethodBase method)
             builder.Append(TypeNameFormatter.FormatTypeName(declaringType, options));
             builder.Append('.');
             builder.Append(method.Name);
+            if (method.IsGenericMethod)
+            {
                 builder.Append(TypeNameFormatter.FormatTypeArguments(method.GetGenericArguments(), options));
+            }
 
             builder.Append('(');
 

src/Scripting/Core/Hosting/ObjectFormatter/CommonTypeNameFormatter.cs

@@ -93,7 +93,7 @@ public virtual string FormatTypeArguments(Type[] typeArguments, CommonTypeNameFo
 
             if (typeArguments.Length == 0)
             {
-                return "";
+                throw new ArgumentException(null, nameof(typeArguments));
             }
 
             var pooled = PooledStringBuilder.GetInstance();