Cleanup: Adjust GSpan and GArray asserts

Previously, 4e9e44ad made changes to allow GSpan and GMutableSpan to not
have a type when they are empty. This mirrors the same change in the
conversion from GArray to both span types and when converting to an
actual typed Span<> or MutableSpan<> via typed().

Fixes #125013

Pull Request: https://projects.blender.org/blender/blender/pulls/125018

source/blender/blenlib/BLI_generic_array.hh

@@ -169,14 +169,14 @@ class GArray {
 
   operator GSpan() const
   {
-    BLI_assert(type_ != nullptr);
-    return GSpan(*type_, data_, size_);
+    BLI_assert(size_ == 0 || type_ != nullptr);
+    return GSpan(type_, data_, size_);
   }
 
   operator GMutableSpan()
   {
-    BLI_assert(type_ != nullptr);
-    return GMutableSpan(*type_, data_, size_);
+    BLI_assert(size_ == 0 || type_ != nullptr);
+    return GMutableSpan(type_, data_, size_);
   }
 
   GSpan as_span() const

source/blender/blenlib/BLI_generic_span.hh

@@ -85,7 +85,8 @@ class GSpan {
 
   template<typename T> Span<T> typed() const
   {
-    BLI_assert(type_->is<T>());
+    BLI_assert(size_ == 0 || type_ != nullptr);
+    BLI_assert(type_ == nullptr || type_->is<T>());
     return Span<T>(static_cast<const T *>(data_), size_);
   }
 
@@ -212,7 +213,8 @@ class GMutableSpan {
 
   template<typename T> MutableSpan<T> typed() const
   {
-    BLI_assert(type_->is<T>());
+    BLI_assert(size_ == 0 || type_ != nullptr);
+    BLI_assert(type_ == nullptr || type_->is<T>());
     return MutableSpan<T>(static_cast<T *>(data_), size_);
   }
 

source/blender/blenlib/tests/BLI_generic_array_test.cc

@@ -133,4 +133,13 @@ TEST(generic_array, AssignDefault)
   EXPECT_EQ(array.data(), nullptr);
 }
 
+TEST(generic_array, DefaultConstructor)
+{
+  GArray<> array;
+
+  EXPECT_TRUE(array.is_empty());
+  EXPECT_TRUE(array.as_mutable_span().is_empty());
+  EXPECT_TRUE(array.as_span().is_empty());
+}
+
 }  // namespace blender::tests