Templated opAssign do not forward on Alias this.

[dlangBugzillaToGithub]

SrMordred reported this on 2016-08-24T13:14:11Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=16426

CC List

• John Hall
• RazvanN
• Steven Schveighoffer (@schveiguy)

Description

import std.stdio;

struct Base{
	void opAssign(T)(T x){
		writeln("assigning : ",x);
	}		
}
struct Derived{
	Base parent;
	alias parent this;
}

void main()
{
	Base a;
	a = 10; //ok
	
	Derived b;
	b = 20;	//Error
}

Output:

Error: function f937.Derived.opAssign (Derived p) is not callable using argument types (int)

[dlangBugzillaToGithub]

john.michael.hall commented on 2018-03-22T19:46:50Z

Structs have opAssign defined by default. Alias this only forwards undefined lookups. Below is a workaround.

import std.stdio : writeln;

struct Base{
	void opAssign(T)(T x){
		writeln("assigning : ",x);
	}		
}
struct Derived{
	Base parent;
	alias parent this;

	void opAssign(T)(T x){
		parent = x;
	}	
}

void main()
{
	Base a;
	a = 10; //ok
	
	Derived b;
	b = 20;	//Error
}

[dlangBugzillaToGithub]

schveiguy (@schveiguy) commented on 2020-09-04T12:14:51Z

Ran into this today.

If the compiler is going to generate an opAssign, it should generate one that forwards to the alias-this member. It should be something equivalent to adding the following overload to the existing generated opAssign:

auto ref opAssign(T)(auto ref T val) if (!is(T == typeof(this)) && __traits(compiles, aliasThisMember = val))
{
   aliasThisMember = val;
}

BTW, thanks for the workaround. For a wrapped single alias-this member, it works perfectly.