diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -303,6 +303,8 @@
 - Fix a failed assertion due to an invalid source location when trying to form
   a coverage report for an unresolved constructor expression.
   (`#62105 <https://github.com/llvm/llvm-project/issues/62105>`_)
+- Fix crash when handling undefined template partial specialization
+  (`#61356 <https://github.com/llvm/llvm-project/issues/61356>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
diff --git a/clang/lib/Sema/SemaCXXScopeSpec.cpp b/clang/lib/Sema/SemaCXXScopeSpec.cpp
--- a/clang/lib/Sema/SemaCXXScopeSpec.cpp
+++ b/clang/lib/Sema/SemaCXXScopeSpec.cpp
@@ -131,7 +131,8 @@
             // entering the context, and that can't happen in a SFINAE context.
             assert(!isSFINAEContext() && "partial specialization scope "
                                          "specifier in SFINAE context?");
-            if (!hasReachableDefinition(PartialSpec))
+            if (PartialSpec->hasDefinition() &&
+                !hasReachableDefinition(PartialSpec))
               diagnoseMissingImport(SS.getLastQualifierNameLoc(), PartialSpec,
                                     MissingImportKind::PartialSpecialization,
                                     true);
diff --git a/clang/test/SemaCXX/undefined-partial-specialization.cpp b/clang/test/SemaCXX/undefined-partial-specialization.cpp
new file mode 100644
--- /dev/null
+++ b/clang/test/SemaCXX/undefined-partial-specialization.cpp
@@ -0,0 +1,15 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+// RUN: %clang_cc1 -std=c++20 -verify %s
+
+namespace GH61356 {
+
+template <typename T, bool b>
+class boo {void foo();};
+
+template <typename T>
+class boo<T, true>;
+
+template<typename T>
+void boo<T, true>::foo(){} // expected-error{{out-of-line definition of 'foo' from class 'boo<type-parameter-0-0, true>' without definition}}
+
+}