Implement P1937 consteval in unevaluated contexts

In an unevaluated contexts, consteval functions should not be
immediately evaluated.

Author: cor3ntin

clang/lib/Sema/SemaExpr.cpp

@@ -16641,7 +16641,8 @@ void Sema::CheckUnusedVolatileAssignment(Expr *E) {
 }
 
 ExprResult Sema::CheckForImmediateInvocation(ExprResult E, FunctionDecl *Decl) {
-  if (!E.isUsable() || !Decl || !Decl->isConsteval() || isConstantEvaluated() ||
+  if (isUnevaluatedContext() || !E.isUsable() || !Decl ||
+      !Decl->isConsteval() || isConstantEvaluated() ||
       RebuildingImmediateInvocation)
     return E;
 
@@ -18758,8 +18759,8 @@ void Sema::MarkDeclRefReferenced(DeclRefExpr *E, const Expr *Base) {
       OdrUse = false;
 
   if (auto *FD = dyn_cast<FunctionDecl>(E->getDecl()))
-    if (!isConstantEvaluated() && FD->isConsteval() &&
-        !RebuildingImmediateInvocation)
+    if (!isUnevaluatedContext() && !isConstantEvaluated() &&
+        FD->isConsteval() && !RebuildingImmediateInvocation)
       ExprEvalContexts.back().ReferenceToConsteval.insert(E);
   MarkExprReferenced(*this, E->getLocation(), E->getDecl(), E, OdrUse,
                      RefsMinusAssignments);

clang/test/CXX/basic/basic.def.odr/p2-typeid.cpp

@@ -1,4 +1,5 @@
 // RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -std=c++20 -fsyntax-only -verify %s
 
 // C++ [basic.def.odr]p2:
 //   An expression is potentially evaluated unless it [...] is the
@@ -16,7 +17,7 @@ struct Poly {
 
 struct NonPoly { };
 
-template<typename T, typename Result = T> 
+template<typename T, typename Result = T>
 struct X {
   Result f(T t) { return t + t; } // expected-error{{invalid operands}}
 
@@ -34,3 +35,33 @@ void test(X<Poly> xp, X<Poly, Poly&> xpr, X<NonPoly> xnp, X<NonPoly, NonPoly&> x
   // Triggers an error (as it should);
   xpr.g(Poly()); // expected-note{{instantiation of member function}}
 }
+
+#if __cplusplus >= 202002L
+
+namespace unevaluated {
+
+struct S {
+  void f();
+};
+struct T {
+  virtual void f();
+};
+
+consteval S *null_s() { return nullptr; }
+consteval S *make_s() { return new S; }
+consteval T *null_t() { return nullptr; }
+consteval T *make_t() { return new T; } // #alloc
+
+void func() {
+  (void)typeid(*null_s());
+  (void)typeid(*make_s());
+  (void)typeid(*null_t()); // expected-warning {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
+  (void)typeid(*make_t()); // expected-error {{call to consteval function 'unevaluated::make_t' is not a constant expression}} \
+                              expected-note {{pointer to heap-allocated object is not a constant expression}} \
+                              expected-note@#alloc {{heap allocation performed here}} \
+                              expected-warning {{expression with side effects will be evaluated despite being used as an operand to 'typeid'}}
+}
+
+} // namespace unevaluated
+
+#endif

clang/test/SemaCXX/cxx2a-consteval.cpp

@@ -594,3 +594,21 @@ void test() {
 }
 
 } // namespace special_ctor
+
+namespace unevaluated {
+
+template <typename T, typename U> struct is_same { static const bool value = false; };
+template <typename T> struct is_same<T, T> { static const bool value = true; };
+
+long f(); // expected-note {{declared here}}
+auto consteval g(auto a) {
+  return a;
+}
+
+auto e = g(f()); // expected-error {{is not a constant expression}}
+                 // expected-note@-1 {{non-constexpr function 'f' cannot be used in a constant expression}}
+
+using T = decltype(g(f()));
+static_assert(is_same<long, T>::value);
+
+} // namespace unevaluated

clang/www/cxx_status.html

@@ -1105,10 +1105,11 @@ <h2 id="cxx20">C++20 implementation status</h2>
     <tr>
       <td rowspan=2>Immediate functions (<tt>consteval</tt>)</td>
       <td><a href="https://wg21.link/p1073r3">P1073R3</a></td>
-      <td rowspan=2 class="none" align="center">No</td>
+      <td class="partial" align="center">Partial</td>
     </tr>
       <tr> <!-- from Prague -->
         <td><a href="https://wg21.link/p1937r2">P1937R2</a></td>
+        <td class="unreleased" align="center">Clang 14</td>
       </tr>
     <tr>
       <td><tt>std::is_constant_evaluated</tt></td>