Sema: Respect -fdelayed-template-parsing when parsing constexpr functions

Functions declared as constexpr must have their parsing delayed in
-fdelayed-template-parsing mode so as not to upset later template
instantiation.

N.B. My reading of the standard makes it seem like delayed template
parsing is at odds with constexpr.  We may want to make refinements in
other places in clang to make constexpr play nicer with this feature.

This fixes PR17334.

llvm-svn: 191484

clang/lib/Sema/SemaExpr.cpp

@@ -11010,7 +11010,8 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func) {
     // However, they cannot be referenced if they are deleted, and they are
     // deleted whenever the implicit definition of the special member would
     // fail.
-    if (!Func->isConstexpr() || Func->getBody())
+    if (!(Func->isConstexpr() && !getLangOpts().DelayedTemplateParsing) ||
+        Func->getBody())
       return;
     CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(Func);
     if (!Func->isImplicitlyInstantiable() && (!MD || MD->isUserProvided()))
@@ -11101,13 +11102,14 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func) {
       }
     }
 
-    if (!AlreadyInstantiated || Func->isConstexpr()) {
+    if (!AlreadyInstantiated ||
+        (Func->isConstexpr() && !getLangOpts().DelayedTemplateParsing)) {
       if (isa<CXXRecordDecl>(Func->getDeclContext()) &&
           cast<CXXRecordDecl>(Func->getDeclContext())->isLocalClass() &&
           ActiveTemplateInstantiations.size())
         PendingLocalImplicitInstantiations.push_back(
             std::make_pair(Func, PointOfInstantiation));
-      else if (Func->isConstexpr())
+      else if (Func->isConstexpr() && !getLangOpts().DelayedTemplateParsing)
         // Do not defer instantiations of constexpr functions, to avoid the
         // expression evaluator needing to call back into Sema if it sees a
         // call to such a function.

clang/test/Parser/DelayedTemplateParsing.cpp

@@ -102,3 +102,15 @@ namespace rdar11700604 {
   };
 }
 
+namespace PR17334 {
+
+template <typename = void> struct ArrayRef {
+  constexpr ArrayRef() {}
+};
+template <typename = void> void CreateConstInBoundsGEP2_32() {
+  ArrayRef<> IdxList;
+}
+void LLVMBuildStructGEP() { CreateConstInBoundsGEP2_32(); }
+
+}
+