c++: if consteval and consteval propagation [PR115583]

During speculative constant folding of an if consteval, we take the false
branch, but the true branch is an immediate function context, so we don't
want to to cp_fold_immediate it.  So we could check IF_STMT_CONSTEVAL_P
here.  But beyond that, we don't want to do this inside a call, only when
first parsing a function.

	PR c++/115583

gcc/cp/ChangeLog:

	* constexpr.cc (cxx_eval_conditional_expression): Don't
	cp_fold_immediate for if consteval.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/consteval-if13.C: New test.

(cherry picked from commit d5f1948640815a554d106542c2e91e4e117aa3bc)

gcc/cp/constexpr.cc

@@ -3974,10 +3974,13 @@ cxx_eval_conditional_expression (const constexpr_ctx *ctx, tree t,
   if (TREE_CODE (t) == IF_STMT && !val)
     val = void_node;
 
-  /* P2564: a subexpression of a manifestly constant-evaluated expression
-     or conversion is an immediate function context.  */
+  /* P2564: If we aren't in immediate function context (including a manifestly
+     constant-evaluated expression), check any uses of immediate functions in
+     the arm we're discarding.  But don't do this inside a call; we already
+     checked when parsing the function.  */
   if (ctx->manifestly_const_eval != mce_true
       && !in_immediate_context ()
+      && !ctx->call
       && cp_fold_immediate (&TREE_OPERAND (t, zero_p ? 1 : 2),
 			    ctx->manifestly_const_eval))
     {

gcc/testsuite/g++.dg/cpp23/consteval-if13.C

@@ -0,0 +1,17 @@
+// PR c++/115583
+// { dg-do compile { target c++23 } }
+
+consteval int f(int i) {
+  return i;
+}
+const bool b = 0;
+constexpr int g(int i) {
+  if consteval {
+    return f(i);
+  } else {
+    return i;
+  }
+}
+int main() {
+  return g(1);
+}