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

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.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D130437

clang/docs/ReleaseNotes.rst

@@ -194,6 +194,8 @@ Bug Fixes
   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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

clang/lib/AST/ExprConstant.cpp

@@ -5266,10 +5266,14 @@ static EvalStmtResult EvaluateStmt(StmtResult &Result, EvalInfo &Info,
       }
     }
     bool Cond;
-    if (IS->isConsteval())
+    if (IS->isConsteval()) {
       Cond = IS->isNonNegatedConsteval();
-    else if (!EvaluateCond(Info, IS->getConditionVariable(), IS->getCond(),
-                           Cond))
+      // 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()) {

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

@@ -26,3 +26,30 @@ constexpr void f() {
 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;
+}
+
+}