[clang] [NFC][Clang] Pre-commit test for assertion failure in template specialization error recovery (PR #208151)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Aditi Medhane (AditiRM)

<details>
<summary>Changes</summary>

Pre-commit test documenting the current crash behaviour when handling explicit template specialization after instantiation.
The compiler currently crashes with an assertion failure in `getASTRecordLayout()` when attempting to get layout information for an invalid declaration during error recovery. This test documents this behavior before the fix.

---
Full diff: https://github.com/llvm/llvm-project/pull/208151.diff


1 Files Affected:

- (added) clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp (+40) 


``````````diff
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..9120fa45330b8
--- /dev/null
+++ b/clang/test/SemaCXX/explicit-specialization-after-instantiation-crash.cpp
@@ -0,0 +1,40 @@
+// Test that explicit template specialization after instantiation
+// is handled gracefully without assertion failure.
+//
+// RUN: not --crash %clang_cc1 -fsyntax-only -verify %s 2>&1 | FileCheck %s
+//
+// REQUIRES: asserts
+
+// This test case triggers an assertion failure in getASTRecordLayout()
+// when the compiler encounters explicit specialization after instantiation
+// and attempts to get layout information during error recovery.
+
+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 -11 {{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'}}
+}
+
+// Verify the assertion failure occurs
+// CHECK: Assertion
+// CHECK: isInvalidDecl
+// CHECK: Cannot get layout of invalid decl
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/208151