Refine the implementation of hiding __is_deducible type trait.

Apply approach 3.

Author: hokein

clang/include/clang/Basic/TokenKinds.def

@@ -537,6 +537,9 @@ TYPE_TRAIT_1(__is_referenceable, IsReferenceable, KEYCXX)
 TYPE_TRAIT_1(__can_pass_in_regs, CanPassInRegs, KEYCXX)
 TYPE_TRAIT_2(__reference_binds_to_temporary, ReferenceBindsToTemporary, KEYCXX)
 TYPE_TRAIT_2(__reference_constructs_from_temporary, ReferenceConstructsFromTemporary, KEYCXX)
+// IsDeducible is only used internally by clang for CTAD implementation and
+// is not exposed to users.
+TYPE_TRAIT_2(/*EmptySpellingName*/, IsDeducible, KEYCXX)
 
 // Embarcadero Expression Traits
 EXPRESSION_TRAIT(__is_lvalue_expr, IsLValueExpr, KEYCXX)

clang/include/clang/Basic/TypeTraits.h

@@ -26,12 +26,8 @@ enum TypeTrait {
 #include "clang/Basic/TokenKinds.def"
   ,
 #define TYPE_TRAIT_2(Spelling, Name, Key) BTT_##Name,
-  // IsDeducible is only used internally by clang for CTAD implementation and
-  // is not exposed to users.
-  TYPE_TRAIT_2(/**/, IsDeducible, KEYCXX)
 #include "clang/Basic/TokenKinds.def"
-  // +1 for the IsDeducible enumerator.
-  BTT_Last = UTT_Last + 1 // BTT_Last == last BTT_XX in the enum.
+  BTT_Last = UTT_Last
 
 #define TYPE_TRAIT_2(Spelling, Name, Key) +1
 #include "clang/Basic/TokenKinds.def"

clang/lib/Basic/TypeTraits.cpp

@@ -13,15 +13,13 @@
 #include "clang/Basic/TypeTraits.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <cassert>
+#include <cstring>
 using namespace clang;
 
 static constexpr const char *TypeTraitNames[] = {
 #define TYPE_TRAIT_1(Spelling, Name, Key) #Name,
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_2(Spelling, Name, Key) #Name,
-    // IsDeducible is only used internally by clang for CTAD implementation and
-    // is not exposed to users.
-    TYPE_TRAIT_2(/**/, IsDeducible, KEYCXX)
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_N(Spelling, Name, Key) #Name,
 #include "clang/Basic/TokenKinds.def"
@@ -31,9 +29,6 @@ static constexpr const char *TypeTraitSpellings[] = {
 #define TYPE_TRAIT_1(Spelling, Name, Key) #Spelling,
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_2(Spelling, Name, Key) #Spelling,
-    // __is_deducible is only used internally by clang for CTAD implementation
-    // and is not exposed to users.
-    TYPE_TRAIT_2(__is_deducible, /**/, KEYCXX)
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_N(Spelling, Name, Key) #Spelling,
 #include "clang/Basic/TokenKinds.def"
@@ -65,9 +60,6 @@ static constexpr const unsigned TypeTraitArities[] = {
 #define TYPE_TRAIT_1(Spelling, Name, Key) 1,
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_2(Spelling, Name, Key) 2,
-    // IsDeducible is only used internally by clang for CTAD implementation and
-    // is not exposed to users.
-    TYPE_TRAIT_2(/**/, IsDeducible, KEYCXX)
 #include "clang/Basic/TokenKinds.def"
 #define TYPE_TRAIT_N(Spelling, Name, Key) 0,
 #include "clang/Basic/TokenKinds.def"
@@ -90,6 +82,15 @@ const char *clang::getTraitName(UnaryExprOrTypeTrait T) {
 
 const char *clang::getTraitSpelling(TypeTrait T) {
   assert(T <= TT_Last && "invalid enum value!");
+  if (T == BTT_IsDeducible) {
+    // The __is_deducible is an internal-only type trait. To hide it from
+    // external users, we define it with an empty spelling name, preventing the
+    // clang parser from recognizing its token kind.
+    // However, other components such as the AST dump still require the real
+    // type trait name. Therefore, we return the real name when needed.
+    assert(std::strlen(TypeTraitSpellings[T]) == 0);
+    return "__is_deducible";
+  }
   return TypeTraitSpellings[T];
 }