[ADT] Use static_assert in PackedVector

[kazutakahirata]

This patch replaces an intentionally undefined template
specialization:

template class PackedVector<T, 0>;

with:

static_assert(BitNum > 0, "BitNum must be > 0");

This way, the compiler diagnostic on a use of PackedVector<T, 0>
improves from:

error: implicit instantiation of undefined template
'llvm::PackedVector<unsigned int, 0>'

to:

error: static assertion failed due to requirement '0U > 0': BitNum
must be > 0

[llvmbot]

@llvm/pr-subscribers-llvm-adt

Author: Kazu Hirata (kazutakahirata)

Changes

This patch replaces an intentionally undefined template
specialization:

template <typename T> class PackedVector<T, 0>;

with:

static_assert(BitNum > 0, "BitNum must be > 0");

This way, the compiler diagnostic on a use of PackedVector<T, 0>
improves from:

error: implicit instantiation of undefined template
'llvm::PackedVector<unsigned int, 0>'

to:

error: static assertion failed due to requirement '0U > 0': BitNum
must be > 0

---

Full diff: https://github.com/llvm/llvm-project/pull/164142.diff

1 Files Affected:

• (modified) llvm/include/llvm/ADT/PackedVector.h (+2-3)

diff --git a/llvm/include/llvm/ADT/PackedVector.h b/llvm/include/llvm/ADT/PackedVector.h
index 09c20e39d1552..57e41979b4ce2 100644
--- a/llvm/include/llvm/ADT/PackedVector.h
+++ b/llvm/include/llvm/ADT/PackedVector.h
@@ -29,6 +29,8 @@ namespace llvm {
 /// an assertion.
 template <typename T, unsigned BitNum, typename BitVectorTy = BitVector>
 class PackedVector {
+  static_assert(BitNum > 0, "BitNum must be > 0");
+
   BitVectorTy Bits;
   // Keep track of the number of elements on our own.
   // We always maintain Bits.size() == NumElements * BitNum.
@@ -133,9 +135,6 @@ class PackedVector {
   BitVectorTy &raw_bits() { return Bits; }
 };
 
-// Leave BitNum=0 undefined.
-template <typename T> class PackedVector<T, 0>;
-
 } // end namespace llvm
 
 #endif // LLVM_ADT_PACKEDVECTOR_H