clang is correct, note the HEAD revision of gcc accepts also accepts this code. This is a well-formed constexpr function, as long as there is value for the argument(s) that allows the function to be evaluated as a core constant expression. In your case 1 is such a value.
This is covered in the draft C++14 standard section 7.1.5 The constexpr specifier [dcl.constexpr] which tells us what is allowed in a constexpr function:
The definition of a constexpr function shall satisfy the following constraints:
it shall not be virtual (10.3);
its return type shall be a literal type;
each of its parameter types shall be a literal type;
its function-body shall be = delete, = default, or a compound-statement that does not contain
an asm-definition,
a goto statement,
a try-block, or
a definition of a variable of non-literal type or of static or thread storage duration or for which
no initialization is performed.
no restriction on throw and it also says (emphasis mine):
For a non-template, non-defaulted constexpr function or a non-template, non-defaulted, non-inheriting
constexpr constructor, if no argument values exist such that an invocation of the function or constructor
could be an evaluated subexpression of a core constant expression (5.19), the program is ill-formed; no
diagnostic required.
and below this paragraph we have the following example, similar to yours:
constexpr int f(bool b)
{ return b ? throw 0 : 0; } // OK
constexpr int f() { return f(true); } // ill-formed, no diagnostic required
throw is not allowed in a core constant expression, which is covered in section 5.19 [expr.const] paragraph 2 which says:
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
and includes the following bullet:
- a throw-expression (15.1).
and so f would not be usable in a core constant expression when n <= 0.
Update
As TemplateRex points out, there are two gcc bugs reports for this:
- Never executed “throw” in constexpr function fails to compile
- C++14] throw-expression is not a valid constant-expression
TemplateRex also notes the fixes are not applied to to 5.3.0 and are only in trunk. No, work arounds are provided.