Why can constructor syntax not be used with the "unsigned int" type?

Question

Why is the following illegal in C++?

auto x = unsigned int(0); 

Whereas the following are all OK:

auto y = int(0); auto z = unsigned(0); auto w = float(0); 

or in general:

auto t = Type(... c-tor-args ...); 

(with the exception of Type being unsigned int).

Answer 1

The syntax is Explicit type conversion (functional notation) here. According to the grammatical rule, it only works with simple type specifier or typedef specifier (i.e. a single-word type name).

(emphasis mine)

2) The functional cast expression consists of a simple type specifier or a typedef specifier (in other words, a single-word type name: unsigned int(expression) or int*(expression) are not valid), followed by a single expression in parentheses. This cast expression is exactly equivalent to the corresponding C-style cast expression.

You can change it to c-style cast expression or static_cast, or use it with typedef specifier as @Jean-FrançoisFabre suggested.

auto x1 = (unsigned int)(0); auto x2 = static_cast<unsigned int>(0); 

Quotes from the standard, $5.2.3/1 Explicit type conversion (functional notation) [expr.type.conv]

A simple-type-specifier ([dcl.type.simple]) or typename-specifier ([temp.res]) followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.

And $7.1.7.2/1 Simple type specifiers [dcl.type.simple]

The simple type specifiers are

simple-type-specifier:     nested-name-specifieropt type-name     nested-name-specifier template simple-template-id     nested-name-specifieropt template-name     char     char16_t     char32_t     wchar_t     bool     short     int     long     signed     unsigned     float     double     void     auto     decltype-specifier type-name:     class-name     enum-name     typedef-name     simple-template-id decltype-specifier:   decltype ( expression )   decltype ( auto )