Report ERR_NoConversionForNubDefaultParam even if default parameter value is not default(T).

Fixes #6186.

src/Compilers/CSharp/Portable/Symbols/Source/ParameterHelpers.cs

@@ -268,7 +268,7 @@ internal static class ParameterHelpers
 
                 hasErrors = true;
             }
-            else if (conversion.IsNullable && defaultExpression.Kind == BoundKind.DefaultOperator && !defaultExpression.Type.IsNullableType() &&
+            else if (conversion.IsNullable && !defaultExpression.Type.IsNullableType() &&
                 !(parameterType.GetNullableUnderlyingType().IsEnumType() || parameterType.GetNullableUnderlyingType().IsIntrinsicType()))
             {
                 // We can do:

src/Compilers/CSharp/Test/Symbol/Symbols/SymbolErrorTests.cs

@@ -15366,7 +15366,7 @@ List<IFoo> IBar<string>.GetList()
                 Diagnostic(ErrorCode.ERR_GenericsUsedInNoPIAType));
         }
 
-        [Fact]
+        [Fact, WorkItem(6186, "https://github.com/dotnet/roslyn/issues/6186")]
         public void CS1770ERR_NoConversionForNubDefaultParam()
         {
             var text = @"using System;
@@ -15378,12 +15378,22 @@ public enum E { None }
     public void Foo1(int? x = default(int)) { }
     public void Foo2(E? x = default(E)) { }
     public void Foo3(DateTime? x = default(DateTime?)) { }
+    public void Foo4(DateTime? x = new DateTime?()) { }
 
     // Error:
-    public void Foo(DateTime? x = default(DateTime)) { }
+    public void Foo11(DateTime? x = default(DateTime)) { }
+    public void Foo12(DateTime? x = new DateTime()) { }
 }";
-            var comp = DiagnosticsUtils.VerifyErrorsAndGetCompilationWithMscorlib(text,
-                new ErrorDescription { Code = 1770, Line = 12, Column = 31 });
+            var comp = CreateCompilationWithMscorlib(text);
+
+            comp.VerifyDiagnostics(
+    // (13,33): error CS1770: A value of type 'DateTime' cannot be used as default parameter for nullable parameter 'x' because 'DateTime' is not a simple type
+    //     public void Foo11(DateTime? x = default(DateTime)) { }
+    Diagnostic(ErrorCode.ERR_NoConversionForNubDefaultParam, "x").WithArguments("System.DateTime", "x").WithLocation(13, 33),
+    // (14,33): error CS1770: A value of type 'DateTime' cannot be used as default parameter for nullable parameter 'x' because 'DateTime' is not a simple type
+    //     public void Foo12(DateTime? x = new DateTime()) { }
+    Diagnostic(ErrorCode.ERR_NoConversionForNubDefaultParam, "x").WithArguments("System.DateTime", "x").WithLocation(14, 33)
+                );
         }
 
         [Fact]