[HLSL] Vector standard conversions (#71098)

HLSL supports vector truncation and element conversions as part of
standard conversion sequences. The vector truncation conversion is a C++
second conversion in the conversion sequence. If a vector truncation is
in a conversion sequence an element conversion may occur after it before
the standard C++ third conversion.

Vector element conversions can be boolean conversions, floating point or
integral conversions or promotions.

[HLSL Draft
Specification](https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf)

---------

Co-authored-by: Aaron Ballman <aaron@aaronballman.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index afe2673..8e76338 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -15676,11 +15676,18 @@ static void CheckImplicitConversion(Sema &S, Expr *E, QualType T,
       if (S.SourceMgr.isInSystemMacro(CC))
         return;
       return DiagnoseImpCast(S, E, T, CC, diag::warn_impcast_vector_scalar);
+    } else if (S.getLangOpts().HLSL &&
+               Target->castAs<VectorType>()->getNumElements() <
+                   Source->castAs<VectorType>()->getNumElements()) {
+      // Diagnose vector truncation but don't return. We may also want to
+      // diagnose an element conversion.
+      DiagnoseImpCast(S, E, T, CC, diag::warn_hlsl_impcast_vector_truncation);
     }
 
     // If the vector cast is cast between two vectors of the same size, it is
-    // a bitcast, not a conversion.
-    if (S.Context.getTypeSize(Source) == S.Context.getTypeSize(Target))
+    // a bitcast, not a conversion, except under HLSL where it is a conversion.
+    if (!S.getLangOpts().HLSL &&
+        S.Context.getTypeSize(Source) == S.Context.getTypeSize(Target))
       return;
 
     Source = cast<VectorType>(Source)->getElementType().getTypePtr();