Fix #3180: Spell out type arguments of a null-conditional extension call

When the extension method call cannot be written without type arguments
(another extension method with the same name and a different arity is
found first), the call falls back to the static form. For 'a?.M()' that
fallback produced 'Enumerable.Count(s?)', which is not valid C#. Writing
the type arguments excludes the extension methods of a different arity,
so for a null-conditional target the type arguments are tried before
giving up.

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -932,6 +932,12 @@ namespace ICSharpCode.Decompiler.Tests
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3180([ValueSource(nameof(roslynOnlyOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AssemblyCustomAttributes([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3180.cs

@@ -0,0 +1,31 @@
+using System.Collections;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty.Issue3180
+{
+	internal static class Extensions
+	{
+		public static int Count(this IEnumerable source)
+		{
+			return 5;
+		}
+	}
+
+	internal class NullConditional
+	{
+		public static int StaticCall(IEnumerable<string> s)
+		{
+			return Enumerable.Count(s);
+		}
+
+		public static int CountOrZero(IEnumerable<string> s)
+		{
+#if EXPECTED_OUTPUT
+			return s?.Count<string>() ?? 0;
+#else
+			return (s != null) ? Enumerable.Count(s) : 0;
+#endif
+		}
+	}
+}