[clang] diagnose invalid std::tuple_size sizes

Fixes #159563

Author: mizvekov

clang/docs/ReleaseNotes.rst

@@ -262,6 +262,8 @@ Improvements to Clang's diagnostics
   Moved the warning for a missing (though implied) attribute on a redeclaration into this group.
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
+- Implemented diagnostics when retrieving the tuple size for types where its specialization of `std::tuple_size`
+  produces an invalid size (either negative or greater than the implementation limit). (#GH159563)
 - Fixed fix-it hint for fold expressions. Clang now correctly places the suggested right
   parenthesis when diagnosing malformed fold expressions. (#GH151787)
 - Added fix-it hint for when scoped enumerations require explicit conversions for binary operations. (#GH24265)
@@ -347,8 +349,8 @@ Bug Fixes in This Version
   and vector of 4 ``float`` values. (#GH155405)
 - Fixed inconsistent shadow warnings for lambda capture of structured bindings.
   Previously, ``[val = val]`` (regular parameter) produced no warnings with ``-Wshadow``
-  while ``[a = a]`` (where ``a`` is from ``auto [a, b] = std::make_pair(1, 2)``) 
-  incorrectly produced warnings. Both cases now consistently show no warnings with 
+  while ``[a = a]`` (where ``a`` is from ``auto [a, b] = std::make_pair(1, 2)``)
+  incorrectly produced warnings. Both cases now consistently show no warnings with
   ``-Wshadow`` and show uncaptured-local warnings with ``-Wshadow-all``. (#GH68605)
 - Fixed a failed assertion with a negative limit parameter value inside of
   ``__has_embed``. (#GH157842)

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -638,6 +638,9 @@ def err_decomp_decl_std_tuple_element_not_specialized : Error<
 def err_decomp_decl_std_tuple_size_not_constant : Error<
   "cannot decompose this type; 'std::tuple_size<%0>::value' "
   "is not a valid integral constant expression">;
+def err_decomp_decl_std_tuple_size_invalid
+    : Error<"cannot decompose this type; 'std::tuple_size<%0>::value' "
+            "is not a valid size: %1">;
 def note_in_binding_decl_init : Note<
   "in implicit initialization of binding declaration %0">;
 def err_arg_is_not_destructurable : Error<

clang/lib/Sema/SemaDeclCXX.cpp

@@ -1222,6 +1222,16 @@ static IsTupleLike isTupleLike(Sema &S, SourceLocation Loc, QualType T,
   if (E.isInvalid())
     return IsTupleLike::Error;
 
+  if (Size < 0 || Size >= UINT_MAX) {
+    llvm::SmallVector<char, 16> Str;
+    Size.toString(Str);
+    S.Diag(Loc, diag::err_decomp_decl_std_tuple_size_invalid)
+        << printTemplateArgs(S.Context.getPrintingPolicy(), Args,
+                             /*Params=*/nullptr)
+        << StringRef(Str.data(), Str.size());
+    return IsTupleLike::Error;
+  }
+
   return IsTupleLike::TupleLike;
 }
 

clang/test/SemaCXX/builtin-structured-binding-size.cpp

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify
-// RUN: %clang_cc1 %s -std=c++2c -fsyntax-only -verify -fexperimental-new-constant-interpreter
+// RUN: %clang_cc1 %s -triple=x86_64 -std=c++2c -fsyntax-only -verify
+// RUN: %clang_cc1 %s -triple=x86_64 -std=c++2c -fsyntax-only -verify -fexperimental-new-constant-interpreter
 
 
 struct S0 {};
@@ -229,3 +229,19 @@ static_assert(__is_same_as(tag_of_t<S1>, int));
 static_assert(__is_same_as(tag_of_t<int>, int)); // error
 // expected-error@-1 {{constraints not satisfied for alias template 'tag_of_t' [with T = int]}}
 // expected-note@#tag-of-constr {{because substituted constraint expression is ill-formed: type 'int' cannot be decomposed}}
+
+struct MinusOne;
+template <> struct ::std::tuple_size<MinusOne> {
+  static constexpr int value = -1;
+};
+int minus_one = __builtin_structured_binding_size(MinusOne);
+// expected-error@-1 {{cannot decompose this type; 'std::tuple_size<MinusOne>::value' is not a valid size: -1}}
+// expected-error@-2 {{type 'MinusOne' cannot be decomposed}}
+
+struct UintMax;
+template <> struct ::std::tuple_size<UintMax> {
+  static constexpr unsigned value = -1;
+};
+int uint_max = __builtin_structured_binding_size(UintMax);
+// expected-error@-1 {{cannot decompose this type; 'std::tuple_size<UintMax>::value' is not a valid size: 4294967295}}
+// expected-error@-2 {{type 'UintMax' cannot be decomposed}}