[c++20] P1064R0: Allow virtual function calls in constant expression

evaluation.

llvm-svn: 360559

2 files changed, 34 insertions, 16 deletions

diff --git a/clang/lib/Sema/SemaDeclCXX.cpp b/clang/lib/Sema/SemaDeclCXX.cpp
index afd50f1438e..1da2ca8ca5f 100644
--- a/clang/lib/Sema/SemaDeclCXX.cpp
+++ b/clang/lib/Sema/SemaDeclCXX.cpp
@@ -1596,6 +1596,9 @@ bool Sema::CheckConstexprFunctionDecl(const FunctionDecl *NewFD) {
     //  The definition of a constexpr constructor shall satisfy the following
     //  constraints:
     //  - the class shall not have any virtual base classes;
+    //
+    // FIXME: This only applies to constructors, not arbitrary member
+    // functions.
     const CXXRecordDecl *RD = MD->getParent();
     if (RD->getNumVBases()) {
       Diag(NewFD->getLocation(), diag::err_constexpr_virtual_base)
@@ -1612,21 +1615,25 @@ bool Sema::CheckConstexprFunctionDecl(const FunctionDecl *NewFD) {
     // C++11 [dcl.constexpr]p3:
     //  The definition of a constexpr function shall satisfy the following
     //  constraints:
-    // - it shall not be virtual;
+    // - it shall not be virtual; (removed in C++20)
     const CXXMethodDecl *Method = dyn_cast<CXXMethodDecl>(NewFD);
     if (Method && Method->isVirtual()) {
-      Method = Method->getCanonicalDecl();
-      Diag(Method->getLocation(), diag::err_constexpr_virtual);
-
-      // If it's not obvious why this function is virtual, find an overridden
-      // function which uses the 'virtual' keyword.
-      const CXXMethodDecl *WrittenVirtual = Method;
-      while (!WrittenVirtual->isVirtualAsWritten())
-        WrittenVirtual = *WrittenVirtual->begin_overridden_methods();
-      if (WrittenVirtual != Method)
-        Diag(WrittenVirtual->getLocation(),
-             diag::note_overridden_virtual_function);
-      return false;
+      if (getLangOpts().CPlusPlus2a) {
+        Diag(Method->getLocation(), diag::warn_cxx17_compat_constexpr_virtual);
+      } else {
+        Method = Method->getCanonicalDecl();
+        Diag(Method->getLocation(), diag::err_constexpr_virtual);
+
+        // If it's not obvious why this function is virtual, find an overridden
+        // function which uses the 'virtual' keyword.
+        const CXXMethodDecl *WrittenVirtual = Method;
+        while (!WrittenVirtual->isVirtualAsWritten())
+          WrittenVirtual = *WrittenVirtual->begin_overridden_methods();
+        if (WrittenVirtual != Method)
+          Diag(WrittenVirtual->getLocation(),
+               diag::note_overridden_virtual_function);
+        return false;
+      }
     }
 
     // - its return type shall be a literal type;
@@ -15197,7 +15204,8 @@ void Sema::MarkVirtualMemberExceptionSpecsNeeded(SourceLocation Loc,
 }
 
 void Sema::MarkVirtualMembersReferenced(SourceLocation Loc,
-                                        const CXXRecordDecl *RD) {
+                                        const CXXRecordDecl *RD,
+                                        bool ConstexprOnly) {
   // Mark all functions which will appear in RD's vtable as used.
   CXXFinalOverriderMap FinalOverriders;
   RD->getFinalOverriders(FinalOverriders);
@@ -15212,7 +15220,7 @@ void Sema::MarkVirtualMembersReferenced(SourceLocation Loc,
 
       // C++ [basic.def.odr]p2:
       //   [...] A virtual member function is used if it is not pure. [...]
-      if (!Overrider->isPure())
+      if (!Overrider->isPure() && (!ConstexprOnly || Overrider->isConstexpr()))
         MarkFunctionReferenced(Loc, Overrider);
     }
   }
diff --git a/clang/lib/Sema/SemaTemplateInstantiate.cpp b/clang/lib/Sema/SemaTemplateInstantiate.cpp
index edc281cad6e..55d1d94bd82 100644
--- a/clang/lib/Sema/SemaTemplateInstantiate.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiate.cpp
@@ -2082,6 +2082,7 @@ Sema::InstantiateClass(SourceLocation PointOfInstantiation,
   LateInstantiatedAttrVec LateAttrs;
   Instantiator.enableLateAttributeInstantiation(&LateAttrs);
 
+  bool MightHaveConstexprVirtualFunctions = false;
   for (auto *Member : Pattern->decls()) {
     // Don't instantiate members not belonging in this semantic context.
     // e.g. for:
@@ -2128,6 +2129,10 @@ Sema::InstantiateClass(SourceLocation PointOfInstantiation,
           Instantiation->setInvalidDecl();
           break;
         }
+      } else if (CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewMember)) {
+        if (MD->isConstexpr() && !MD->getFriendObjectKind() &&
+            (MD->isVirtualAsWritten() || Instantiation->getNumBases()))
+          MightHaveConstexprVirtualFunctions = true;
       }
 
       if (NewMember->isInvalidDecl())
@@ -2220,9 +2225,14 @@ Sema::InstantiateClass(SourceLocation PointOfInstantiation,
     Consumer.HandleTagDeclDefinition(Instantiation);
 
     // Always emit the vtable for an explicit instantiation definition
-    // of a polymorphic class template specialization.
+    // of a polymorphic class template specialization. Otherwise, eagerly
+    // instantiate only constexpr virtual functions in preparation for their use
+    // in constant evaluation.
     if (TSK == TSK_ExplicitInstantiationDefinition)
       MarkVTableUsed(PointOfInstantiation, Instantiation, true);
+    else if (MightHaveConstexprVirtualFunctions)
+      MarkVirtualMembersReferenced(PointOfInstantiation, Instantiation,
+                                   /*ConstexprOnly*/ true);
   }
 
   return Instantiation->isInvalidDecl();