diff --git a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
--- a/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
+++ b/clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp
@@ -436,9 +436,10 @@
         Errors.back());
     SmallString<100> Message;
     Info.FormatDiagnostic(Message);
-    FullSourceLoc Loc;
-    if (Info.getLocation().isValid() && Info.hasSourceManager())
-      Loc = FullSourceLoc(Info.getLocation(), Info.getSourceManager());
+    FullSourceLoc Loc(Info.getLocation(),
+                      Info.hasSourceManager()
+                          ? Info.getSourceManager()
+                          : Context.DiagEngine->getSourceManager());
     Converter.emitDiagnostic(Loc, DiagLevel, Message, Info.getRanges(),
                              Info.getFixItHints());
   }
diff --git a/clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp b/clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp
--- a/clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp
+++ b/clang-tools-extra/test/clang-tidy/infrastructure/diagnostic.cpp
@@ -25,6 +25,7 @@
 // RUN: not clang-tidy -checks='-*,modernize-use-override' %T/diagnostics/input.cpp -- -DCOMPILATION_ERROR 2>&1 | FileCheck -check-prefix=CHECK6 -implicit-check-not='{{warning:|error:}}' %s
 // RUN: clang-tidy -checks='-*,modernize-use-override,clang-diagnostic-macro-redefined' %s -- -DMACRO_FROM_COMMAND_LINE -std=c++20 | FileCheck -check-prefix=CHECK4 -implicit-check-not='{{warning:|error:}}' %s
 // RUN: clang-tidy -checks='-*,modernize-use-override,clang-diagnostic-macro-redefined,clang-diagnostic-literal-conversion' %s -- -DMACRO_FROM_COMMAND_LINE -std=c++20 -Wno-macro-redefined | FileCheck --check-prefix=CHECK7 -implicit-check-not='{{warning:|error:}}' %s
+// RUN: not clang-tidy -checks='-*,modernize-use-override' %s -- -std=c++20 -DPR64602 | FileCheck -check-prefix=CHECK8 -implicit-check-not='{{warning:|error:}}' %s
 
 // CHECK1: error: no input files [clang-diagnostic-error]
 // CHECK1: error: no such file or directory: '{{.*}}nonexistent.cpp' [clang-diagnostic-error]
@@ -54,3 +55,18 @@
   // CHECK6: :[[@LINE-1]]:3: error: cannot take the address of an rvalue of type 'int' [clang-diagnostic-error]
 }
 #endif
+
+#ifdef PR64602 // Should not crash
+template <class T = void>
+struct S
+{
+    auto foo(auto);
+};
+
+template <>
+auto S<>::foo(auto)
+{
+    return 1;
+}
+// CHECK8: error: template parameter list matching the non-templated nested type 'S<>' should be empty ('template<>') [clang-diagnostic-error]
+#endif