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Make conditional operators on objects rvalues #8989

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4 changes: 4 additions & 0 deletions docs/ReleaseNotes.md
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Expand Up @@ -67,6 +67,10 @@ line upon naming the release. Refer to previous for appropriate section names.

- Fixed an optimizer crash when scalarizing an out-of-bounds vector access
[#8940](https://github.com/microsoft/DirectXShaderCompiler/issues/8940).
- Assigning to a conditional operator on objects, such as `(c ? a : b) = buf`
with resources, is now an "expression is not assignable" error like for
other types, instead of asserting or being silently dropped
[#8579](https://github.com/microsoft/DirectXShaderCompiler/issues/8579).

### Upcoming Preview Release

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8 changes: 7 additions & 1 deletion tools/clang/lib/Sema/SemaHLSL.cpp
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Expand Up @@ -11159,8 +11159,14 @@ HLSLExternalSource::CheckVectorConditional(ExprResult &Cond, ExprResult &LHS,
rightObjectKind == AR_TOBJ_MATRIX;

if (!leftIsSimple || !rightIsSimple) {
if (leftObjectKind == AR_TOBJ_OBJECT && leftObjectKind == AR_TOBJ_OBJECT) {
if (leftObjectKind == AR_TOBJ_OBJECT && rightObjectKind == AR_TOBJ_OBJECT) {
if (leftType == rightType) {
// As for the other types, the result is not an lvalue, so it can't be
// assigned to.
if (LHS.get()->isLValue())

@bob80905 Joshua Batista (bob80905) Sep 30, 2026

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I believe we want to gate this behavior on the language version, and preserve this older behavior, according to this document:
https://hlsl-tc57.github.io/tc57/proposal/0017/
See if you can add a condition here to ensure that the language version is at least 202x.

LHS.set(CreateLValueToRValueCast(LHS.get()));
if (RHS.get()->isLValue())
RHS.set(CreateLValueToRValueCast(RHS.get()));
return leftType;
}
}
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22 changes: 22 additions & 0 deletions tools/clang/test/SemaHLSL/conditional-object-not-assignable.hlsl
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@@ -0,0 +1,22 @@
// RUN: %dxc -Tcs_6_0 -verify %s

// Like for scalars, vectors and matrices, a conditional operator on objects
// yields an rvalue, so it can't be assigned to.

RWByteAddressBuffer gBuf0 : register(u0);
RWByteAddressBuffer gBuf1 : register(u1);

[numthreads(1, 1, 1)]
void main(uint3 tid : SV_DispatchThreadID) {
RWByteAddressBuffer a = gBuf0;
RWByteAddressBuffer b = gBuf1;
(true ? a : b) = gBuf1; /* expected-error {{expression is not assignable}} */

uint x = 1, y = 2;
(true ? x : y) = 5; /* expected-error {{expression is not assignable}} */

// Using the result as an rvalue is still fine.
RWByteAddressBuffer c = true ? a : b;
c.Store(0, 1);
(false ? a : b).Store(4, 2);
}
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