[clang] Fix incorrect constant folding of `if consteval`

[cor3ntin]

Summary

Fixes https://github.com/llvm/llvm-project/issues/55638.

if consteval was evaluated incorrectly when in a
non-constant context that could be constant-folded.

[cor3ntin]

Fix test

[aaron.ballman]

LGTM!

clang/docs/ReleaseNotes.rst

- Clang now correctly emits symbols for implicitly instantiated constexpr
   template function. Fixes `Issue 55560 <https://github.com/llvm/llvm-project/issues/55560>`_.
 - Clang now checks ODR violations when merging concepts from different modules.
   Note that this may possibly break existing code, and is done so intentionally.
   Fixes `Issue 56310 <https://github.com/llvm/llvm-project/issues/56310>`_.
 - Clang will now look through type sugar when checking a member function is a
   move assignment operator. Fixes `Issue 56456 <https://github.com/llvm/llvm-project/issues/56456>`_.
 - Fixed a crash when a variable with a bool enum type that has no definition
   used in comparison operators. Fixes `Issue 56560 <https://github.com/llvm/llvm-project/issues/56560>`_.
+- Fix that ``if consteval`` could evaluate to ``true`` at runtime because it was incorrectly
+  constant folded. Fixes `Issue 55638 <https://github.com/llvm/llvm-project/issues/55638>`_.
 
 Improvements to Clang's diagnostics
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 - ``-Wliteral-range`` will warn on floating-point equality comparisons with
   constants that are not representable in a casted value. For example,
   ``(float) f == 0.1`` is always false.
 - ``-Winline-namespace-reopened-noninline`` now takes into account that the
   ``inline`` keyword must appear on the original but not necessarily all

clang/lib/AST/ExprConstant.cpp

if (const Stmt *Init = IS->getInit()) {
       EvalStmtResult ESR = EvaluateStmt(Result, Info, Init);
       if (ESR != ESR_Succeeded) {
         if (ESR != ESR_Failed && !Scope.destroy())
           return ESR_Failed;
         return ESR;
       }
     }
     bool Cond;
-    if (IS->isConsteval())
+    if (IS->isConsteval()) {
       Cond = IS->isNonNegatedConsteval();
-    else if (!EvaluateCond(Info, IS->getConditionVariable(), IS->getCond(),
+      // If we are not in a constant context, if consteval should not evaluate
+      // to true.
+      if (!Info.InConstantContext)
+        Cond = !Cond;
+    } else if (!EvaluateCond(Info, IS->getConditionVariable(), IS->getCond(),
                              Cond))
       return ESR_Failed;
 
     if (const Stmt *SubStmt = Cond ? IS->getThen() : IS->getElse()) {
       EvalStmtResult ESR = EvaluateStmt(Result, Info, SubStmt);
       if (ESR != ESR_Succeeded) {
         if (ESR != ESR_Failed && !Scope.destroy())
           return ESR_Failed;
         return ESR;

clang/test/CodeGenCXX/cxx2b-consteval-if.cpp

constexpr void f() {
   } else {
     should_not_be_used(); // CHECK-NOT: call {{.*}}should_not_be_used
   }
 }
 
 void g() {
   f();
 }
+
+namespace GH55638 {
+
+constexpr bool is_constant_evaluated() noexcept {
+  if consteval { return true; } else { return false; }
+}
+
+constexpr int compiletime(int) {
+   return 2;
+}
+
+constexpr int runtime(int) {
+   return 1;
+}
+
+constexpr int test(int x) {
+  if(is_constant_evaluated())
+    return compiletime(x);  // CHECK-NOT: call {{.*}}compiletime
+   return runtime(x);  // CHECK: call {{.*}}runtime
+}
+
+int f(int x) {
+  x = test(x);
+  return x;
+}
+
+}