[clang] Limit the maximum level of fold-expr expansion.

Introduce a new diagnostic, and respect the bracket-depth (256) by default.

Differential Revision: https://reviews.llvm.org/D86936

diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
index e1601da..ec0c0fd 100644 (file)
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -5092,6 +5092,9 @@ def err_fold_expression_empty : Error<
   "with no fallback value">;
 def err_fold_expression_bad_operand : Error<
   "expression not permitted as operand of fold expression">;
+def err_fold_expression_limit_exceeded: Error<
+  "instantiating fold expression with %0 arguments exceeded expression nesting "
+  "limit of %1">, DefaultFatal, NoSFINAE;
 
 def err_unexpected_typedef : Error<
   "unexpected type name %0: expected expression">;

diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index 4c8293f..6457b19 100644 (file)
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -28,6 +28,7 @@
 #include "clang/AST/StmtCXX.h"
 #include "clang/AST/StmtObjC.h"
 #include "clang/AST/StmtOpenMP.h"
+#include "clang/Basic/DiagnosticParse.h"
 #include "clang/Basic/OpenMPKinds.h"
 #include "clang/Sema/Designator.h"
 #include "clang/Sema/Lookup.h"
@@ -13193,6 +13194,18 @@ TreeTransform<Derived>::TransformCXXFoldExpr(CXXFoldExpr *E) {
         E->getEllipsisLoc(), RHS.get(), E->getEndLoc(), NumExpansions);
   }
 
+  // Formally a fold expression expands to nested parenthesized expressions.
+  // Enforce this limit to avoid creating trees so deep we can't safely traverse
+  // them.
+  if (NumExpansions && SemaRef.getLangOpts().BracketDepth < NumExpansions) {
+    SemaRef.Diag(E->getEllipsisLoc(),
+                 clang::diag::err_fold_expression_limit_exceeded)
+        << *NumExpansions << SemaRef.getLangOpts().BracketDepth
+        << E->getSourceRange();
+    SemaRef.Diag(E->getEllipsisLoc(), diag::note_bracket_depth);
+    return ExprError();
+  }
+
   // The transform has determined that we should perform an elementwise
   // expansion of the pattern. Do so.
   ExprResult Result = getDerived().TransformExpr(E->getInit());

diff --git a/clang/test/SemaCXX/fold_expr_expansion_limit.cpp b/clang/test/SemaCXX/fold_expr_expansion_limit.cpp
new file mode 100644 (file)
index 0000000..600278d
--- /dev/null
+++ b/clang/test/SemaCXX/fold_expr_expansion_limit.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -fsyntax-only -fbracket-depth 2 -verify -std=c++17 %s
+
+template <class T, T... V> struct seq {
+  constexpr bool zero() { return (true && ... && (V == 0)); }; // expected-error {{instantiating fold expression with 3 arguments exceeded expression nesting limit of 2}} \
+                                                                  expected-note {{use -fbracket-depth}}
+};
+constexpr unsigned N = 3;
+auto x = __make_integer_seq<seq, int, N>{};
+static_assert(!x.zero(), ""); // expected-note {{in instantiation of member function}}