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add note about VRP not applying to assignment operator exps #23101

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add note about VRP not applying to assignment operator exps #23101

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10 changes: 10 additions & 0 deletions spec/expression.dd
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Expand Up @@ -448,6 +448,16 @@ $(H3 $(LNAME2 assignment_operator_expressions, Assignment Operator Expressions))
the binary operators. Still the left operand must be an lvalue.
)

$(P $(DDSUBLINK spec/type.html, vrp, Value Range Propagation) does not apply to assignment operator expressions:)
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@dkorpel dkorpel May 8, 2026

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VRP is part of implicit conversions. For built-in types, a op= b is defined as doing a cast / explicit conversion, so VRP isn't even a possibility. And for custom types, where an implicit conversion happens to the argument type, VRP does apply:

struct S
{
	auto opOpAssign(string op)(ubyte value)
	{
		return this;
	}
}

void f(S b, uint i)
{
	b *= i;        // Error
	b *= i & 0xFF; // No error
}

So the statement is either irrelevant or incorrect, it's best left out.

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@WalterBright WalterBright May 9, 2026

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The spec for a op= b can certainly be adapted to do VRP. As for opOpAssign, it is up to the user implementation of it to use VRP or not.

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@dkorpel dkorpel May 11, 2026

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it is up to the user implementation of it to use VRP or not.

Then the PR should demonstrate that instead of stating something false.


---
void f(ubyte b, int i)
{
b = b * i; // Error: cannot implicitly convert expression `cast(int)b * i` of type `int` to `ubyte`
b *= i; // no error
}
---

$(H2 $(LNAME2 conditional_expressions, Conditional Expressions))

$(GRAMMAR
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