Fix #1597: Incorrect handling of nullability annotations during generic type substitution.

ICSharpCode.Decompiler/TypeSystem/Implementation/NullabilityAnnotatedType.cs

@@ -53,11 +53,16 @@ namespace ICSharpCode.Decompiler.TypeSystem.Implementation
 			IType newBase = baseType.AcceptVisitor(visitor);
 			if (newBase != baseType) {
 				if (newBase.Nullability == Nullability.Nullable) {
-					// T?! -> T?
+					// `T!` with substitution T=`U?` becomes `U?`
 					// This happens during type substitution for generic methods.
 					return newBase;
 				}
-				return newBase.ChangeNullability(nullability);
+				if (newBase.Kind == TypeKind.TypeParameter || newBase.IsReferenceType == true) {
+					return newBase.ChangeNullability(nullability);
+				} else {
+					// `T!` with substitution T=`int` becomes `int`, not `int!`
+					return newBase;
+				}
 			} else {
 				return this;
 			}

ICSharpCode.Decompiler/TypeSystem/ParameterizedType.cs

@@ -140,10 +140,20 @@ namespace ICSharpCode.Decompiler.TypeSystem
 				return b.ToString();
 			}
 		}
-		
+
 		public override string ToString()
 		{
-			return ReflectionName;
+			StringBuilder b = new StringBuilder(genericType.ToString());
+			b.Append('[');
+			for (int i = 0; i < typeArguments.Length; i++) {
+				if (i > 0)
+					b.Append(',');
+				b.Append('[');
+				b.Append(typeArguments[i].ToString());
+				b.Append(']');
+			}
+			b.Append(']');
+			return b.ToString();
 		}
 
 		public IReadOnlyList<IType> TypeArguments => typeArguments;