How could be an initializer expression is "assignment-expression" by the grammer?

Question

I have this sample of code:

struct S {};

S s = S();

Per my understanding, I see that s = S() in the declaration S s = S() is an init-declarator where s is a declarator and = S() is an initializer.

Also, I see that the initializer (= S()) is a brace-or-equal-initializer which is "= initializer-clause". This means finally that S() is an initializer-clause.

By the grammar, an initializer-clause is either an assignment-expression or a braced-init-list. Indeed, S() might not be a braced-init-list, but it may be an assignment-expression.

If my analysis is correct so far, I have a question here:

How S() is an assignment-expression? Where is the assignment-operator in expression S()?

In other words, the grammar of assignment-expression is:

assignment-expression:
      conditional-expression
      yield-expression
      throw-expression
      logical-or-expression assignment-operator initializer-clause

how S() could be one of the above?

Similarly, the grammar of a throw-expression is

throw-expression:
    throw assignment-expression

how S() in statement throw S(); is an assignment-expression?

Answer 1

How S() is an assignment-expression?

Because it matches the first option conditional-expression as shown below. Note the emphasized part in each section/part and that at the end we reach to simple-type-specifier ( expression-listopt ) which is S().

assignment-expression:

• conditional-expression
• yield-expression
• throw-expression
• logical-or-expression assignment-operator initializer-clause

Next, we go to conditional expression:

conditional-expression:

• logical-or-expression
• logical-or-expression ? expression : assignment-expression

Next, to logical-or-expression:

logical-or-expression:

• logical-and-expression
• logical-or-expression || logical-and-expression

Next, to logical-and-expression:

logical-and-expression:

• inclusive-or-expression
• logical-and-expression && inclusive-or-expression

From inclusive-or-expression:

inclusive-or-expression:

• exclusive-or-expression
• inclusive-or-expression | exclusive-or-expression

Next, exclusive-or-expression

exclusive-or-expression:

• and-expression
• exclusive-or-expression ^ and-expression

Now to and-expression

and-expression:

• equality-expression
• and-expression & equality-expression

From relational-operator

equality-expression:

• relational-expression
• equality-expression == relational-expression
• equality-expression != relational-expression

Next from relational-expression

relational-expression:

• compare-expression
• relational-expression < compare-expression
• relational-expression > compare-expression
• relational-expression <= compare-expression
• relational-expression >= compare-expression

Next to compare-expression

compare-expression:

• shift-expression
• compare-expression <=> shift-expression

Next to shift-expression

shift-expression:

• additive-expression
• shift-expression << additive-expression
• shift-expression >> additive-expression

Next additive-expression

additive-expression:

• multiplicative-expression
• additive-expression + multiplicative-expression
• additive-expression - multiplicative-expression

Next to multiplicative-expression

multiplicative-expression:

• pm-expression
• multiplicative-expression * pm-expression
• multiplicative-expression / pm-expression
• multiplicative-expression % pm-expression

Next to pm-expression

pm-expression:

• cast-expression
• pm-expression .* cast-expression
• pm-expression ->* cast-expression

Next to cast-expression

cast-expression:

• unary-expression
• ( type-id ) cast-expression

Next to unary-expression

unary-expression:

• postfix-expression
• unary-operator cast-expression
• ++ cast-expression
• -- cast-expression
• await-expression
• sizeof unary-expression
• sizeof ( type-id )
• sizeof ... ( identifier )
• alignof ( type-id )
• noexcept-expression
• new-expression
• delete-expression

Finally we reach postfix-expression

postfix-expression:

• primary-expression
• postfix-expression [ expression-listopt ]
• postfix-expression ( expression-listopt )
• simple-type-specifier ( expression-listopt )
• typename-specifier ( expression-listopt )
• simple-type-specifier braced-init-list
• typename-specifier braced-init-list
• postfix-expression . templateopt id-expression
• postfix-expression -> templateopt id-expression
• postfix-expression ++
• postfix-expression --
• dynamic_cast < type-id > ( expression )
• static_cast < type-id > ( expression )
• reinterpret_cast < type-id > ( expression )
• const_cast < type-id > ( expression )
• typeid ( expression )
• typeid ( type-id )