1 files changed, 11 insertions, 1 deletions
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();