c++: Fix compile-time-hog in cp_fold_immediate_r [PR111660]

My recent patch introducing cp_fold_immediate_r caused exponential
compile time with nested COND_EXPRs.  The problem is that the COND_EXPR
case recursively walks the arms of a COND_EXPR, but after processing
both arms it doesn't end the walk; it proceeds to walk the
sub-expressions of the outermost COND_EXPR, triggering again walking
the arms of the nested COND_EXPR, and so on.  This patch brings the
compile time down to about 0m0.030s.

The ff_fold_immediate flag is unused after this patch but since I'm
using it in the P2564 patch, I'm not removing it now.  Maybe at_eof
can be used instead and then we can remove ff_fold_immediate.

	PR c++/111660

gcc/cp/ChangeLog:

	* cp-gimplify.cc (cp_fold_immediate_r) <case COND_EXPR>: Don't
	handle it here.
	(cp_fold_r): Handle COND_EXPR here.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/hog1.C: New test.
	* g++.dg/cpp2a/consteval36.C: New test.

2 files changed, 99 insertions, 0 deletions

diff --git a/gcc/testsuite/g++.dg/cpp0x/hog1.C b/gcc/testsuite/g++.dg/cpp0x/hog1.C
new file mode 100644
index 0000000..105a2e9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/hog1.C
@@ -0,0 +1,77 @@
+// PR c++/111660
+// { dg-do compile { target c++11 } }
+
+enum Value {
+  LPAREN,
+  RPAREN,
+  LBRACE,
+  RBRACE,
+  LBRACK,
+  RBRACK,
+  CONDITIONAL,
+  COLON,
+  SEMICOLON,
+  COMMA,
+  PERIOD,
+  BIT_OR,
+  BIT_AND,
+  BIT_XOR,
+  BIT_NOT,
+  NOT,
+  LT,
+  GT,
+  MOD,
+  ASSIGN,
+  ADD,
+  SUB,
+  MUL,
+  DIV,
+  PRIVATE_NAME,
+  STRING,
+  TEMPLATE_SPAN,
+  IDENTIFIER,
+  WHITESPACE,
+  ILLEGAL,
+};
+
+constexpr Value GetOneCharToken(char c) {
+  return
+      c == '(' ? LPAREN :
+      c == ')' ? RPAREN :
+      c == '{' ? LBRACE :
+      c == '}' ? RBRACE :
+      c == '[' ? LBRACK :
+      c == ']' ? RBRACK :
+      c == '?' ? CONDITIONAL :
+      c == ':' ? COLON :
+      c == ';' ? SEMICOLON :
+      c == ',' ? COMMA :
+      c == '.' ? PERIOD :
+      c == '|' ? BIT_OR :
+      c == '&' ? BIT_AND :
+      c == '^' ? BIT_XOR :
+      c == '~' ? BIT_NOT :
+      c == '!' ? NOT :
+      c == '<' ? LT :
+      c == '>' ? GT :
+      c == '%' ? MOD :
+      c == '=' ? ASSIGN :
+      c == '+' ? ADD :
+      c == '-' ? SUB :
+      c == '*' ? MUL :
+      c == '/' ? DIV :
+      c == '#' ? PRIVATE_NAME :
+      c == '"' ? STRING :
+      c == '\'' ? STRING :
+      c == '`' ? TEMPLATE_SPAN :
+      c == '\\' ? IDENTIFIER :
+      c == ' ' ? WHITESPACE :
+      c == '\t' ? WHITESPACE :
+      c == '\v' ? WHITESPACE :
+      c == '\f' ? WHITESPACE :
+      c == '\r' ? WHITESPACE :
+      c == '\n' ? WHITESPACE :
+      ILLEGAL;
+}
+
+int main() {}
diff --git a/gcc/testsuite/g++.dg/cpp2a/consteval36.C b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
new file mode 100644
index 0000000..9c470e4
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/consteval36.C
@@ -0,0 +1,22 @@
+// PR c++/111660
+// { dg-do compile { target c++20 } }
+
+consteval int id (int i) { return i; }
+
+void
+g (int i)
+{
+  1 ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((i ? 1 : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? i : 1), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((i ? -i : i), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : id (i)), id (42), 1); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : id (42)), id (i)); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : id (42)), id (i), 1); // { dg-error "'i' is not a constant expression" }
+  id (i) ? 1 : ((1 ? 1 : 1), id (i)); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((1 ? 1 : id (i)), id (i)); // { dg-error "'i' is not a constant expression" }
+  1 ? id (i) : ((1 ? 1 : id (i)), id (i)); // { dg-error "'i' is not a constant expression" }
+  1 ? 1 : ((id (i) ? 1 : 1), id (i)); // { dg-error "'i' is not a constant expression" }
+}