Demangle C++17 fold-expressions.

PiperOrigin-RevId: 638068943
Change-Id: I7ffe7df900ec4854d8712885d212854b31e79fea

absl/debugging/internal/demangle.cc

@@ -1885,6 +1885,10 @@ static bool ParseBracedExpression(State *state) {
 //              ::= dt <expression> <unresolved-name> # expr.name
 //              ::= pt <expression> <unresolved-name> # expr->name
 //              ::= sp <expression>         # argument pack expansion
+//              ::= fl <binary operator-name> <expression>
+//              ::= fr <binary operator-name> <expression>
+//              ::= fL <binary operator-name> <expression> <expression>
+//              ::= fR <binary operator-name> <expression> <expression>
 //              ::= sr <type> <unqualified-name> <template-args>
 //              ::= sr <type> <unqualified-name>
 //              ::= u <source-name> <template-arg>* E  # vendor extension
@@ -1990,6 +1994,27 @@ static bool ParseExpression(State *state) {
   }
   state->parse_state = copy;
 
+  // Unary folds (... op pack) and (pack op ...).
+  //
+  // <expression> ::= fl <binary operator-name> <expression>
+  //              ::= fr <binary operator-name> <expression>
+  if ((ParseTwoCharToken(state, "fl") || ParseTwoCharToken(state, "fr")) &&
+      ParseOperatorName(state, nullptr) && ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
+  // Binary folds (init op ... op pack) and (pack op ... op init).
+  //
+  // <expression> ::= fL <binary operator-name> <expression> <expression>
+  //              ::= fR <binary operator-name> <expression> <expression>
+  if ((ParseTwoCharToken(state, "fL") || ParseTwoCharToken(state, "fR")) &&
+      ParseOperatorName(state, nullptr) && ParseExpression(state) &&
+      ParseExpression(state)) {
+    return true;
+  }
+  state->parse_state = copy;
+
   // Object and pointer member access expressions.
   //
   // <expression> ::= (dt | pt) <expression> <unresolved-name>

absl/debugging/internal/demangle_test.cc

@@ -506,6 +506,65 @@ TEST(Demangle, SubobjectAddresses) {
   EXPECT_STREQ("f<>()", tmp);
 }
 
+TEST(Demangle, UnaryFoldExpressions) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> struct S {};
+  //
+  // template <class... T> auto f(T... t) -> S<((sizeof(T) == 4) || ...)> {
+  //   return {};
+  // }
+  //
+  // void g() { f(1, 2L); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // S<((sizeof (int) == 4, sizeof (long) == 4) || ...)> f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfrooeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like with a left fold.
+  //
+  // S<(... || (sizeof (int) == 4, sizeof (long) == 4))> f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXflooeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
+TEST(Demangle, BinaryFoldExpressions) {
+  char tmp[80];
+
+  // Source:
+  //
+  // template <bool b> struct S {};
+  //
+  // template <class... T> auto f(T... t)
+  //     -> S<((sizeof(T) == 4) || ... || false)> {
+  //   return {};
+  // }
+  //
+  // void g() { f(1, 2L); }
+  //
+  // Full LLVM demangling of the instantiation of f:
+  //
+  // S<((sizeof (int) == 4, sizeof (long) == 4) || ... || false)>
+  // f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfRooeqstT_Li4ELb0EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+
+  // The like with a left fold.
+  //
+  // S<(false || ... || (sizeof (int) == 4, sizeof (long) == 4))>
+  // f<int, long>(int, long)
+  EXPECT_TRUE(Demangle("_Z1fIJilEE1SIXfLooLb0EeqstT_Li4EEEDpS1_",
+                       tmp, sizeof(tmp)));
+  EXPECT_STREQ("f<>()", tmp);
+}
+
 TEST(Demangle, SizeofPacks) {
   char tmp[80];