Is a glvalue integral constant expression a constant expression?

Question

N4527 5.20 [expr.const]p3

An integral constant expression is an expression of integral or unscoped enumeration type, implici

Answer 1

Is a an integral constant expression?

In the following contexts:

int b[a]{};
static_assert(a,"");
switch(1){
  case a:
    ;
}

yes, a is an integral constant expression. Starting with your first quote:

An integral constant expression is an expression of integral or unscoped enumeration type, implicitly converted to a prvalue, where the converted expression is a core constant expression.

'a' is an integral type, in your cases it will be implicitly converted to a prvalue, so now is a a core constant expression? Yes, if we go back to paragraph 2 which defines what is not a core constant expression:

A conditional-expression e is a core constant expression unless the evaluation of e, following the rules of the abstract machine (1.9), would evaluate one of the following expressions

it has the following clause:

an lvalue-to-rvalue conversion (4.1) unless it is applied to

with the following exception:

a non-volatile glvalue of integral or enumeration type that refers to a complete non-volatile const object with a preceding initialization, initialized with a constant expression, or

which applies to a since it is non-volatile, const and is initialized with a constant expression.

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Is a a constant expression?

In the same contexts as above, yes, since we can see from the quote above it is a core constant expression.

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Is a glvalue integral constant expression a constant expression?

No, in order for it to be a integral constant expression it must be converted to a prvalue and threfore can not be a glvalue.