[clang] Do not crash on undefined template partial specialization

Before checking that template partial specialization is "reachable",
ensure it exists.

Fixes https://github.com/llvm/llvm-project/issues/61356

Reviewed By: shafik, erichkeane

Differential Revision: https://reviews.llvm.org/D148330

clang/docs/ReleaseNotes.rst

@@ -325,6 +325,8 @@ Bug Fixes in This Version
   member pointer as an invalid expression.
 - Fix crash when member function contains invalid default argument.
   (`#62122 <https://github.com/llvm/llvm-project/issues/62122>`_)
+- Fix crash when handling undefined template partial specialization
+  (`#61356 <https://github.com/llvm/llvm-project/issues/61356>`_)
 
 Bug Fixes to Compiler Builtins
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/Sema/SemaCXXScopeSpec.cpp

@@ -131,7 +131,8 @@ DeclContext *Sema::computeDeclContext(const CXXScopeSpec &SS,
             // 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);

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}}
+
+}