Don't treat invalid parameters as being unused

The misc-unused-parameters check would trigger false positive warnings
about the parameter being unused when the parameter declaration was
invalid. No longer issue the warning in that case on the assumption
that most parameters are used in practice, so the extra diagnostic is
most likely a false positive.

Fixes #56152

clang-tools-extra/clang-tidy/misc/UnusedParametersCheck.cpp

@@ -135,6 +135,9 @@ void UnusedParametersCheck::warnOnUnusedParameter(
     const MatchFinder::MatchResult &Result, const FunctionDecl *Function,
     unsigned ParamIndex) {
   const auto *Param = Function->getParamDecl(ParamIndex);
+  // Don't bother to diagnose invalid parameters as being unused.
+  if (Param->isInvalidDecl())
+    return;
   auto MyDiag = diag(Param->getLocation(), "parameter %0 is unused") << Param;
 
   if (!Indexer) {

clang-tools-extra/docs/ReleaseNotes.rst

@@ -110,6 +110,9 @@ Improvements to clang-tidy
   from suppressing diagnostics associated with macro arguments. This fixes
   `Issue 55134 <https://github.com/llvm/llvm-project/issues/55134>`_.
 
+- Invalid parameters are no longer treated as being implicitly unused for the
+  `-misc-unused-parameters` check. This fixes `Issue 56152 <https://github.com/llvm/llvm-project/issues/56152>`_.
+
 New checks
 ^^^^^^^^^^
 

clang-tools-extra/test/clang-tidy/checkers/misc-unused-invalid-parameter.cpp

@@ -0,0 +1,10 @@
+// RUN: %check_clang_tidy -fix-errors %s misc-unused-parameters %t
+
+namespace GH56152 {
+// There's no way to know whether the parameter is used or not if the parameter
+// is an invalid declaration. Ensure the diagnostic is suppressed in this case.
+void func(unknown_type value) { // CHECK-MESSAGES: :[[@LINE]]:11: error: unknown type name 'unknown_type'
+  value += 1;
+}
+}
+