P0859R0: List-initialization is potentially-constant-evaluated and

triggers instantiation of constexpr functions.

We mostly implemented this since Clang 6, but missed the template
instantiation case.

We do not implement the '&cast-expression' special case. It appears to
be a mistake / oversight. I've mailed CWG to see if we can remove it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@343064 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Sema/TreeTransform.h

@@ -3386,6 +3386,11 @@ ExprResult TreeTransform<Derived>::TransformInitializer(Expr *Init,
   if (Construct && Construct->isStdInitListInitialization())
     return TransformInitializer(Construct->getArg(0), NotCopyInit);
 
+  // Enter a list-init context if this was list initialization.
+  EnterExpressionEvaluationContext Context(
+      getSema(), EnterExpressionEvaluationContext::InitList,
+      Construct->isListInitialization());
+
   SmallVector<Expr*, 8> NewArgs;
   bool ArgChanged = false;
   if (getDerived().TransformExprs(Construct->getArgs(), Construct->getNumArgs(),
@@ -9549,6 +9554,9 @@ TreeTransform<Derived>::TransformInitListExpr(InitListExpr *E) {
 
   bool InitChanged = false;
 
+  EnterExpressionEvaluationContext Context(
+      getSema(), EnterExpressionEvaluationContext::InitList);
+
   SmallVector<Expr*, 4> Inits;
   if (getDerived().TransformExprs(E->getInits(), E->getNumInits(), false,
                                   Inits, &InitChanged))
@@ -10780,9 +10788,14 @@ TreeTransform<Derived>::TransformCXXConstructExpr(CXXConstructExpr *E) {
 
   bool ArgumentChanged = false;
   SmallVector<Expr*, 8> Args;
-  if (getDerived().TransformExprs(E->getArgs(), E->getNumArgs(), true, Args,
-                                  &ArgumentChanged))
-    return ExprError();
+  {
+    EnterExpressionEvaluationContext Context(
+        getSema(), EnterExpressionEvaluationContext::InitList,
+        E->isListInitialization());
+    if (getDerived().TransformExprs(E->getArgs(), E->getNumArgs(), true, Args,
+                                    &ArgumentChanged))
+      return ExprError();
+  }
 
   if (!getDerived().AlwaysRebuild() &&
       T == E->getType() &&
@@ -10865,9 +10878,14 @@ TreeTransform<Derived>::TransformCXXTemporaryObjectExpr(
   bool ArgumentChanged = false;
   SmallVector<Expr*, 8> Args;
   Args.reserve(E->getNumArgs());
-  if (TransformExprs(E->getArgs(), E->getNumArgs(), true, Args,
-                     &ArgumentChanged))
-    return ExprError();
+  {
+    EnterExpressionEvaluationContext Context(
+        getSema(), EnterExpressionEvaluationContext::InitList,
+        E->isListInitialization());
+    if (TransformExprs(E->getArgs(), E->getNumArgs(), true, Args,
+                       &ArgumentChanged))
+      return ExprError();
+  }
 
   if (!getDerived().AlwaysRebuild() &&
       T == E->getTypeSourceInfo() &&
@@ -11159,9 +11177,14 @@ TreeTransform<Derived>::TransformCXXUnresolvedConstructExpr(
   bool ArgumentChanged = false;
   SmallVector<Expr*, 8> Args;
   Args.reserve(E->arg_size());
-  if (getDerived().TransformExprs(E->arg_begin(), E->arg_size(), true, Args,
-                                  &ArgumentChanged))
-    return ExprError();
+  {
+    EnterExpressionEvaluationContext Context(
+        getSema(), EnterExpressionEvaluationContext::InitList,
+        E->isListInitialization());
+    if (getDerived().TransformExprs(E->arg_begin(), E->arg_size(), true, Args,
+                                    &ArgumentChanged))
+      return ExprError();
+  }
 
   if (!getDerived().AlwaysRebuild() &&
       T == E->getTypeSourceInfo() &&

test/CXX/expr/expr.const/p6.cpp

@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -std=c++17 -verify %s
+
+template<typename T> int not_constexpr() { return T::error; }
+template<typename T> constexpr int is_constexpr() { return T::error; } // expected-error {{'::'}}
+
+template<typename T> int not_constexpr_var = T::error;
+template<typename T> constexpr int is_constexpr_var = T::error; // expected-error {{'::'}}
+template<typename T> const int is_const_var = T::error; // expected-error {{'::'}}
+template<typename T> const volatile int is_const_volatile_var = T::error;
+template<typename T> T is_dependent_var = T::error; // expected-error {{'::'}}
+template<typename T> int &is_reference_var = T::error; // expected-error {{'::'}}
+template<typename T> float is_float_var = T::error;
+
+void test() {
+  // Do not instantiate functions referenced in unevaluated operands...
+  (void)sizeof(not_constexpr<long>());
+  (void)sizeof(is_constexpr<long>());
+  (void)sizeof(not_constexpr_var<long>);
+  (void)sizeof(is_constexpr_var<long>);
+  (void)sizeof(is_const_var<long>);
+  (void)sizeof(is_const_volatile_var<long>);
+  (void)sizeof(is_dependent_var<long>);
+  (void)sizeof(is_dependent_var<const long>);
+  (void)sizeof(is_reference_var<long>);
+  (void)sizeof(is_float_var<long>);
+
+  // ... but do if they are potentially constant evaluated, and refer to
+  // constexpr functions or to variables usable in constant expressions.
+  (void)sizeof(int{not_constexpr<int>()});
+  (void)sizeof(int{is_constexpr<int>()}); // expected-note {{instantiation of}}
+  (void)sizeof(int{not_constexpr_var<int>});
+  (void)sizeof(int{is_constexpr_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_const_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_const_volatile_var<int>});
+  (void)sizeof(int{is_dependent_var<int>});
+  (void)sizeof(int{is_dependent_var<const int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_reference_var<int>}); // expected-note {{instantiation of}}
+  (void)sizeof(int{is_float_var<int>}); // expected-error {{cannot be narrowed}} expected-note {{cast}}
+}

test/CXX/temp/temp.spec/temp.inst/p7.cpp

@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -verify -std=c++17 %s
+
+template<typename T> constexpr int f() { return T::value; } // expected-error {{'::'}}
+template<bool B, typename T> void g(decltype(B ? f<T>() : 0));
+template<bool B, typename T> void g(...);
+template<bool B, typename T> void h(decltype(int{B ? f<T>() : 0})); // expected-note {{instantiation of}}
+template<bool B, typename T> void h(...);
+void x() {
+  g<false, int>(0); // ok
+  g<true, int>(0); // ok
+  h<false, int>(0); // expected-note {{while substituting}}
+}

www/cxx_status.html

@@ -207,7 +207,7 @@ <h2 id="cxx11">C++11 implementation status</h2>
       <tr>
         <!-- from Albuquerque 2017 -->
         <td><a href="http://wg21.link/p0859r0">P0859R0</a> (<a href="#dr">DR</a>)</td>
-        <td class="none" align="center">No</td>
+        <td class="svn" align="center">SVN</td>
       </tr>
     <tr>
       <td>Alignment support</td>