llvm: fix lowering pointer to final zero-width field of a comptime value

 * Handle a `null` return from `llvmFieldIndex`.
 * Add a behavior test to test this code path.
 * Reword this test name, which incorrectly described how pointers to
   zero-bit fields behave, and instead describe the actual test.

1 files changed, 15 insertions, 7 deletions

diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 7af987f4d6..39c797e12e 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -4033,16 +4033,24 @@ pub const DeclGen = struct {
                             const final_llvm_ty = (try dg.lowerType(ptr_child_ty)).pointerType(0);
                             break :blk field_addr.constIntToPtr(final_llvm_ty);
                         }
-                        bitcast_needed = !field_ty.eql(ptr_child_ty, dg.module);
 
                         var ty_buf: Type.Payload.Pointer = undefined;
-                        const llvm_field_index = llvmFieldIndex(parent_ty, field_index, target, &ty_buf).?;
-                        const indices: [2]*llvm.Value = .{
-                            llvm_u32.constInt(0, .False),
-                            llvm_u32.constInt(llvm_field_index, .False),
-                        };
+
                         const parent_llvm_ty = try dg.lowerType(parent_ty);
-                        break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
+                        if (llvmFieldIndex(parent_ty, field_index, target, &ty_buf)) |llvm_field_index| {
+                            bitcast_needed = !field_ty.eql(ptr_child_ty, dg.module);
+                            const indices: [2]*llvm.Value = .{
+                                llvm_u32.constInt(0, .False),
+                                llvm_u32.constInt(llvm_field_index, .False),
+                            };
+                            break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
+                        } else {
+                            bitcast_needed = !parent_ty.eql(ptr_child_ty, dg.module);
+                            const indices: [1]*llvm.Value = .{
+                                llvm_u32.constInt(1, .False),
+                            };
+                            break :blk parent_llvm_ty.constInBoundsGEP(parent_llvm_ptr, &indices, indices.len);
+                        }
                     },
                     .Pointer => {
                         assert(parent_ty.isSlice());