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

evaluation.

This reinstates r360559, reverted in r360580, with a fix to avoid
crashing if evaluation-for-overflow mode encounters a virtual call on an
object of a class with a virtual base class, and to generally not try to
resolve virtual function calls to objects whose (notional) vptrs are not
readable. (The standard rules are unclear here, but this seems like a
reasonable approach.)

llvm-svn: 360635

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();