Reset sticky diagnostic state after a rejected InstantiateTemplate

The rejection (e.g. a type argument for a non-type parameter) emits its
error diagnostic outside any incremental parse, and DiagnosticsEngine's
ErrorOccurred flag is sticky: the next IncrementalParser::Parse reported
the stale flag as its own failure and then soft-reset it, poisoning
exactly one parse. Contain the rejection with a DiagnosticErrorTrap-based
RAII that restores a clean diagnostic state when the instantiation
introduced errors - the same cleanup IncrementalParser performs on a real
parse failure and InstantiateFunctionDefinition performs inline.

Co-developed-with-the-help-of: Claude Code (Fable 5, human in the loop)

Author: Emery Conrad

lib/CppInterOp/CppInterOp.cpp

@@ -5206,6 +5206,35 @@ std::string ObjToString(const char* TyRef, void* obj) {
   return INTEROP_RETURN(getInterp().toString(TyRef, obj));
 }
 
+namespace {
+/// Traps diagnostics emitted by Sema entry points invoked outside any
+/// incremental parse and, when errors were introduced, restores a clean
+/// diagnostic state on scope exit — the same cleanup IncrementalParser
+/// performs on a real parse failure. Without it, DiagnosticsEngine's sticky
+/// ErrorOccurred flag makes the next IncrementalParser::Parse report the
+/// stale flag as its own failure (and then soft-reset it), spuriously
+/// failing exactly one parse.
+class StickyDiagnosticResetRAII {
+  clang::DiagnosticsEngine* m_Diags;
+  clang::DiagnosticErrorTrap m_Trap;
+
+public:
+  StickyDiagnosticResetRAII(clang::DiagnosticsEngine& Diags)
+      : m_Diags(&Diags), m_Trap(Diags) {}
+  StickyDiagnosticResetRAII(const StickyDiagnosticResetRAII&) = delete;
+  StickyDiagnosticResetRAII&
+  operator=(const StickyDiagnosticResetRAII&) = delete;
+  StickyDiagnosticResetRAII(StickyDiagnosticResetRAII&&) = delete;
+  StickyDiagnosticResetRAII& operator=(StickyDiagnosticResetRAII&&) = delete;
+  ~StickyDiagnosticResetRAII() {
+    if (m_Trap.hasErrorOccurred()) {
+      m_Diags->Reset(/*soft=*/true);
+      m_Diags->getClient()->clear();
+    }
+  }
+};
+} // namespace
+
 static Decl* InstantiateTemplate(TemplateDecl* TemplateD,
                                  TemplateArgumentListInfo& TLI, Sema& S,
                                  bool instantiate_body) {
@@ -5303,6 +5332,10 @@ DeclRef InstantiateTemplate(compat::Interpreter& I, DeclRef tmpl,
   auto* TmplD = unwrap<TemplateDecl>(tmpl);
   // We will create a new decl, push a transaction.
   compat::SynthesizingCodeRAII RAII(&getInterp());
+  // A rejected instantiation emits its error diagnostic outside any
+  // incremental parse; contain it so the sticky error state cannot fail the
+  // parse that follows.
+  StickyDiagnosticResetRAII DiagGuard(S.getDiagnostics());
   return InstantiateTemplate(TmplD, TemplateArgs, S, instantiate_body);
 }
 

unittests/CppInterOp/ScopeReflectionTest.cpp

@@ -1245,6 +1245,30 @@ TYPED_TEST(CPPINTEROP_TEST_MODE, ScopeReflection_InstantiateTemplate) {
   EXPECT_TRUE(TA4_1.getAsIntegral() == 3);
 }
 
+TYPED_TEST(CPPINTEROP_TEST_MODE,
+           ScopeReflection_RejectedInstantiationDoesNotPoisonNextParse) {
+  std::vector<Decl*> Decls;
+  std::string code = R"(
+    struct PoisonTag {};
+    template <unsigned N> struct PoisonBuf { int t; };
+  )";
+
+  GetAllTopLevelDecls(code, Decls);
+
+  // A type argument for the non-type parameter N: Sema rejects the
+  // instantiation and emits an error diagnostic outside any incremental
+  // parse.
+  Cpp::TypeRef TagTy = Cpp::GetTypeFromScope(Decls[0]);
+  std::vector<Cpp::TemplateArgInfo> args = {TagTy.data};
+  EXPECT_FALSE(Cpp::InstantiateTemplate(Decls[1], args));
+
+  // The sticky DiagnosticsEngine error state must not fail the parses that
+  // follow (the poison was one-shot: only the first parse after the
+  // rejection reported failure).
+  EXPECT_EQ(Cpp::Declare("int rejected_instantiation_probe_1 = 1;"), 0);
+  EXPECT_EQ(Cpp::Declare("int rejected_instantiation_probe_2 = 2;"), 0);
+}
+
 TYPED_TEST(CPPINTEROP_TEST_MODE, ScopeReflection_GetClassTemplateArgs) {
   std::vector<Decl*> Decls;
   std::string code = R"(