From d98784b97238f9c8b3bfdf7687ee710e83ca9f1e Mon Sep 17 00:00:00 2001
From: ATHARVA <89757222+jaswanthsanjay88@users.noreply.github.com>
Date: Thu, 10 Sep 2026 19:29:24 +0000
Subject: [PATCH] Fix crash when initializing class with omitted base class
 (#7740)

### Description
When looking up default initializers for class elements in
`ConvertStructToClass`, the compiler previously assumed that every
member looked up from the class scope was a `FieldDecl` and called
`GetAs<SemIR::FieldDecl>` directly.

For a derived class with a base class, looking up `base` returns a
`BaseDecl`, which caused a `CHECK` assertion failure when casting to
`FieldDecl`. Use `TryGetAs<SemIR::FieldDecl>` instead so non-`FieldDecl`
entries like `BaseDecl` are recognized as having no default initializer,
cleanly diagnosing that the `base` field is missing.

Fixes #7722

Assisted-by: Google Deepmind Antigravity
---
 toolchain/check/convert.cpp                   |  7 ++++--
 .../inheritance/fail_init_missing_base.carbon | 25 +++++++++++++++++++
 2 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon

diff --git a/toolchain/check/convert.cpp b/toolchain/check/convert.cpp
index 4f7f96973792..8a74cbf57b6c 100644
--- a/toolchain/check/convert.cpp
+++ b/toolchain/check/convert.cpp
@@ -920,8 +920,11 @@ static auto ConvertStructToClass(Context& context, SemIR::StructType src_type,
         dest_class_scope.GetEntry(*entry_id).result.target_inst_id();
     LoadImportRef(context, field_inst_id);
     field_inst_id = context.constant_values().GetConstantInstId(field_inst_id);
-    auto field_decl = context.insts().GetAs<SemIR::FieldDecl>(field_inst_id);
-    auto field = context.fields().Get(field_decl.field_id);
+    auto field_decl = context.insts().TryGetAs<SemIR::FieldDecl>(field_inst_id);
+    if (!field_decl) {
+      return SemIR::InstId::None;
+    }
+    auto field = context.fields().Get(field_decl->field_id);
     if (!field.initializer_id.has_value()) {
       return SemIR::InstId::None;
     }
diff --git a/toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon b/toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon
new file mode 100644
index 000000000000..43146bdbf285
--- /dev/null
+++ b/toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon
@@ -0,0 +1,25 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/class/inheritance/fail_init_missing_base.carbon
+
+base class X {}
+
+class Z {
+  extend base: X;
+}
+
+fn Run() {
+  // CHECK:STDERR: fail_init_missing_base.carbon:[[@LINE+4]]:14: error: missing value for field `base` in struct initialization [StructInitMissingFieldInLiteral]
+  // CHECK:STDERR:   var _: Z = {};
+  // CHECK:STDERR:              ^~
+  // CHECK:STDERR:
+  var _: Z = {};
+}