[clang] [Clang] Fix assertion failure in getASTRecordLayout for invalid declarations (PR #208153)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Aditi Medhane (AditiRM)

<details>
<summary>Changes</summary>

When encountering invalid declarations during error recovery (e.g., explicit template specialization after instantiation), `getASTRecordLayout()` would assert and crash instead of handling the error gracefully.

This patch adds a check for invalid/incomplete declarations and returns a minimal safe layout (1-byte size/alignment) to allow error recovery to continue, enabling the compiler to report all errors and exit cleanly.

Before this fix:
- Assertion failed: `!D->isInvalidDecl()`
- Exit code: 134 (SIGABRT)

After this fix:
- Clean error messages
- Exit code: 1

Test updated: `clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp`
Addresses this issue : https://github.com/llvm/llvm-project/issues/207953

---
Full diff: https://github.com/llvm/llvm-project/pull/208153.diff


2 Files Affected:

- (modified) clang/lib/AST/RecordLayoutBuilder.cpp (+26) 
- (added) clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp (+42) 


``````````diff
diff --git a/clang/lib/AST/RecordLayoutBuilder.cpp b/clang/lib/AST/RecordLayoutBuilder.cpp
index 854f88f20b2b6..6b5027772885d 100644
--- a/clang/lib/AST/RecordLayoutBuilder.cpp
+++ b/clang/lib/AST/RecordLayoutBuilder.cpp
@@ -3384,6 +3384,32 @@ ASTContext::getASTRecordLayout(const RecordDecl *D) const {
   // not a complete definition (which is what isCompleteDefinition() tests)
   // until we *finish* parsing the definition.
   D = D->getDefinition();
+
+  // Handle invalid declarations gracefully during error recovery
+  // This can happen when there are template specialization errors
+  if (!D || D->isInvalidDecl() || !D->isCompleteDefinition()) {
+    // Check if we already have a cached layout
+    const ASTRecordLayout *Entry = ASTRecordLayouts[D];
+    if (Entry)
+      return *Entry;
+
+    // Create a minimal safe layout for error recovery
+    // Use 1-byte size and alignment to avoid division by zero or other issues
+    ASTRecordLayout *NewEntry = new (*this)
+        ASTRecordLayout(*this,
+                        CharUnits::One(),    // Size = 1 byte
+                        CharUnits::One(),    // Alignment = 1 byte
+                        CharUnits::One(),    // PreferredAlignment = 1 byte
+                        CharUnits::One(),    // UnadjustedAlignment = 1 byte
+                        CharUnits::One(),    // RequiredAlignment = 1 byte
+                        CharUnits::One(),    // DataSize = 1 byte
+                        ArrayRef<uint64_t>() // Empty field offsets
+        );
+
+    ASTRecordLayouts[D] = NewEntry;
+    return *NewEntry;
+  }
+
   assert(D && "Cannot get layout of forward declarations!");
   assert(!D->isInvalidDecl() && "Cannot get layout of invalid decl!");
   assert(D->isCompleteDefinition() && "Cannot layout type before complete!");
diff --git a/clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp b/clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp
new file mode 100644
index 0000000000000..37b00db4d3877
--- /dev/null
+++ b/clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp
@@ -0,0 +1,42 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+//
+// Test that explicit template specialization after instantiation
+// is handled gracefully without assertion failure.
+//
+// Before the fix, this code triggered an assertion failure:
+//   Assertion failed: !D->isInvalidDecl() && "Cannot get layout of invalid decl!"
+//   Location: clang/lib/AST/RecordLayoutBuilder.cpp:3388
+//   Exit code: 134 (SIGABRT)
+//
+// After the fix:
+//   Clean error messages are reported
+//   Exit code: 1
+//
+// The fix adds error handling in getASTRecordLayout() to return a minimal
+// safe layout (1-byte size/alignment) for invalid declarations, allowing
+// error recovery to continue and report all errors.
+
+template <typename T>
+struct X {
+    struct Y {
+        Y() : v(0) {}
+        int v;
+        int getValue();
+    } y;
+};
+
+template <typename T>
+int X<T>::Y::getValue() {
+    return ++v;
+}
+
+// expected-error at +1 {{explicit specialization of 'Y' after instantiation}}
+template <> struct X<int>::Y { int getValue() { return 55; } };
+// expected-note at -10 {{implicit instantiation first required here}}
+
+extern template class X<int>::Y;
+
+int main() {
+    X<int> x;
+    return x.y.getValue(); // expected-error {{no member named 'getValue' in 'X<int>::Y'}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/208153