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Clarify spec for "+=" and friends #1429

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Clarify spec for "+=" and friends #1429

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30 changes: 29 additions & 1 deletion spec/expression.dd
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Original file line number Diff line number Diff line change
Expand Up @@ -96,10 +96,38 @@ $(H4 Assignment Operator Expressions)
$(P except that:)

$(UL
$(LI operand $(D a) is only evaluated once)
$(LI operand $(D a) is only evaluated once, see below.)
$(LI overloading $(I op) uses a different function than overloading $(I op)= does)
$(LI the left operand of $(D >>>=) does not undergo integral promotions before shifting)
)

$(P The expression evaluates according to

--------------
void doOpAssignment(ref int x, int y)
{
x = x op y;
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There is a cast missing in there. I did not put it in the forum thread as I used int explicitly, but it is needed in the general case. For instance :

byte a = 3;
a += 3; // This is ok because of the cast, but won't work because of int promotion without).

}
doOpAssignment(a, b);
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@andralex andralex Jul 12, 2016 •
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The description needs to include the cast. I'd say a correct formulation goes as follows:

The expression `a @= b`, where `@` stands for any of the binary operators, 
is evaluated as if it were lowered as follows:

(function ref(ref x, y) { return x = cast(typeof(x)) (x @ y); })(a, b)

See also (sadly) https://issues.dlang.org/show_bug.cgi?id=16271 which blocks this. Other than that this definition is really solid because defines everything by "redirecting" to function calls.

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cc @WalterBright

--------------

thus the following unittest passes

--------------
int sum;
int add7tosum_return1() { sum += 7; return 1; }
unittest
{
sum = 0;
sum += add7tosum_return1();
assert(sum == 8); // "eight" !

sum = 0;
sum = cast(typeof(sum))(sum + add7tosum_return1());
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Why is the cast necessary?

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It isn't. But I made it look exactly the same as the given "semantic equivalent" above.

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It is necessary if all isn't int.

assert(sum == 1); // "one" !
}
--------------
)

$(H3 Conditional Expressions)

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