[clang] Allow delayed function instantiation at TU end if initial instantiation fails

clang/include/clang/Sema/Sema.h

@@ -865,6 +865,8 @@ class Sema final : public SemaBase {
   /// checks.
   const TranslationUnitKind TUKind;
 
+  bool AtEndOfTU = false;
+
   /// Translation Unit Scope - useful to Objective-C actions that need
   /// to lookup file scope declarations in the "ordinary" C decl namespace.
   /// For example, user-defined classes, built-in "id" type, etc.
@@ -13523,7 +13525,9 @@ class Sema final : public SemaBase {
           S.PendingLocalImplicitInstantiations);
     }
 
-    void perform() { S.PerformPendingInstantiations(/*LocalOnly=*/true); }
+    void perform() {
+      S.PerformPendingInstantiations(/*LocalOnly=*/true);
+    }
 
     ~LocalEagerInstantiationScope() {
       assert(S.PendingLocalImplicitInstantiations.empty() &&
@@ -13571,7 +13575,7 @@ class Sema final : public SemaBase {
     void perform() {
       if (Enabled) {
         S.DefineUsedVTables();
-        S.PerformPendingInstantiations();
+        S.PerformPendingInstantiations(false);
       }
     }
 
@@ -13682,8 +13686,7 @@ class Sema final : public SemaBase {
   void InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
                                      FunctionDecl *Function,
                                      bool Recursive = false,
-                                     bool DefinitionRequired = false,
-                                     bool AtEndOfTU = false);
+                                     bool DefinitionRequired = false);
   VarTemplateSpecializationDecl *BuildVarTemplateInstantiation(
       VarTemplateDecl *VarTemplate, VarDecl *FromVar,
       const TemplateArgumentList *PartialSpecArgs,

clang/lib/Sema/Sema.cpp

@@ -1138,7 +1138,7 @@ void Sema::ActOnEndOfTranslationUnitFragment(TUFragmentKind Kind) {
 
   {
     llvm::TimeTraceScope TimeScope("PerformPendingInstantiations");
-    PerformPendingInstantiations();
+    PerformPendingInstantiations(/*LocalOnly=*/false);
   }
 
   emitDeferredDiags();
@@ -1160,6 +1160,7 @@ void Sema::ActOnEndOfTranslationUnitFragment(TUFragmentKind Kind) {
 void Sema::ActOnEndOfTranslationUnit() {
   assert(DelayedDiagnostics.getCurrentPool() == nullptr
          && "reached end of translation unit with a pool attached?");
+  AtEndOfTU = true;
 
   // If code completion is enabled, don't perform any end-of-translation-unit
   // work.

clang/lib/Sema/SemaExpr.cpp

@@ -21054,8 +21054,7 @@ ExprResult Sema::CheckPlaceholderExpr(Expr *E) {
           // precedes this use.
           InstantiateFunctionDefinition(E->getBeginLoc(), FD,
                                         /*Recursive=*/false,
-                                        /*DefinitionRequired=*/true,
-                                        /*AtEndOfTU=*/false);
+                                        /*DefinitionRequired=*/true);
         }
         // Produce a properly-typed reference to the function.
         CXXScopeSpec SS;

clang/lib/Sema/SemaTemplateInstantiateDecl.cpp

@@ -513,7 +513,7 @@ static void instantiateOMPDeclareVariantAttr(
           return;
         S.InstantiateFunctionDefinition(
             New->getLocation(), SubstFD, /* Recursive */ true,
-            /* DefinitionRequired */ false, /* AtEndOfTU */ false);
+            /* DefinitionRequired */ false);
         SubstFD->setInstantiationIsPending(!SubstFD->isDefined());
         E = DeclRefExpr::Create(S.Context, NestedNameSpecifierLoc(),
                                 SourceLocation(), SubstFD,
@@ -4932,8 +4932,7 @@ FunctionDecl *Sema::InstantiateFunctionDeclaration(
 void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
                                          FunctionDecl *Function,
                                          bool Recursive,
-                                         bool DefinitionRequired,
-                                         bool AtEndOfTU) {
+                                         bool DefinitionRequired) {
   if (Function->isInvalidDecl() || isa<CXXDeductionGuideDecl>(Function))
     return;
 
@@ -6470,15 +6469,17 @@ void Sema::PerformPendingInstantiations(bool LocalOnly) {
         getASTContext().forEachMultiversionedFunctionVersion(
             Function, [this, Inst, DefinitionRequired](FunctionDecl *CurFD) {
               InstantiateFunctionDefinition(/*FIXME:*/ Inst.second, CurFD, true,
-                                            DefinitionRequired, true);
+                                            DefinitionRequired);
               if (CurFD->isDefined())
                 CurFD->setInstantiationIsPending(false);
             });
       } else {
         InstantiateFunctionDefinition(/*FIXME:*/ Inst.second, Function, true,
-                                      DefinitionRequired, true);
+                                      DefinitionRequired);
         if (Function->isDefined())
           Function->setInstantiationIsPending(false);
+        else if (!AtEndOfTU)
+          LateParsedInstantiations.push_back(Inst);
       }
       // Definition of a PCH-ed template declaration may be available only in the TU.
       if (!LocalOnly && LangOpts.PCHInstantiateTemplates &&

clang/test/SemaTemplate/instantiate-function-delayed.cpp

@@ -0,0 +1,26 @@
+// RUN: %clang_cc1 -std=c++20 -verify %s
+// expected-no-diagnostics
+
+template <typename T>
+auto foo(T const& arg) -> T;
+
+template <typename Fp, typename Vis>
+auto dispatch(Fp fp, Vis vis) {
+  return fp(vis);
+}
+
+auto baz(int v) {
+  auto callable = []<typename Arg>(Arg const& arg) -> int {
+    return foo(arg);
+  };
+  return dispatch(callable, v);
+}
+
+template <typename T>
+auto foo(T const& arg) -> T {
+  return arg;
+}
+
+int main() {
+  return baz(5);
+}