RFR: 8258853: Support separate function declaration and definition with ENABLE_IF-based SFINAE

[Kim Barrett]

On Wed, 23 Dec 2020 01:05:29 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change which adds the ENABLE_IF_PARAM macro.  This is
> used in the definition of a function template when that definition is
> separate from the declaration and the declaration uses ENABLE_IF.
> 
> For the separate definition to match the declaration, the non-type template
> parameter for SFINAE in the two places must be "equivalent", where that's
> defined in terms of tokens.  Rather than requiring a separate definition
> match the ENABLE_IF expansion directly, the new macro is provided; it is
> maintained in parallel with ENABLE_IF to ensure the needed consistency.
> 
> Alternative names for the new macro are possible.
> 
> (1) This gives the following usage:
> 
> template<typename T, ENABLE_IF(std::is_integral<T>::value)>
> void foo(T x) { ... }
> 
> and this for separate declaration and definition.  
> 
> template<typename T, ENABLE_IF(std::is_integral<T>::value)>
> void foo(T x);
> 
> // later separate definition
> template<typename T, ENABLE_IF_PARAM(std::is_integral<T>::value)>
> void foo(T x) { ... }
> 
> (2) An alternative that isn't being proposed here, but which could be done
> instead, would be to drop the ENABLE_IF macro and use a type alias, with
> associated change of syntax. Suppose the name is EnableIfX (should probably
> be EnableIf, but need to deal with name conflict). It would be defined as
> 
> template<bool cond> using EnableIfX = std::enable_if_t<cond, int>;
> 
> and used as
> 
> template<typename T, EnableIfX<std::is_integral<T>::value> = 0>
> void foo(T x) { ... }
> 
> and this for separate declaration and definition. 
> 
> template<typename T, EnableIfX<std::is_integral<T>::value> = 0>
> void foo(T x);
> 
> // later separate definition
> template<typename T, EnableIfX<std::is_integral<T>::value>>
> void foo(T x) { ... }
> 
> (3) Or we could not provide any syntactic help, and just write it all out everywhere:
> 
> template<typename T, std::enable_if_t<std::is_integral<T>::value, int> = 0>
> void foo(T x) { ... }
> 
> and this for separate declaration and definition.  
> 
> template<typename T, std::enable_if_t<std::is_integral<T>::value, int> = 0>
> void foo(T x);
> 
> // later separate definition
> template<typename T, std::enable_if_t<std::is_integral<T>::value, int>> 
> void foo(T x) { ... }
> 
> Testing:
> mach5 tier1
> This includes some new gtests for the macros.
> There aren't any current non-test uses of ENABLE_IF_PARAM yet.

Some offline discussion with jrose has led to some intersting discoveries that require changes, so I'm withdrawing this for now.

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PR: https://git.openjdk.java.net/jdk/pull/1873