[Frontend] Don't skip function body when the return type is dependent on the template parameter.

[hokein]

Summary

Otherwise clang will provide an unexpected diagnostic error.

[rsmith]

lib/Sema/SemaDecl.cpp

12645

This is a lot broader than necessary; what we should do is to look for a deduced type in the return type. isUndeducedType doesn't quite do that for a template because we "deduce" the auto as a dependent type early.

[ilya-biryukov]

lib/Sema/SemaDecl.cpp

12645

My change from some time ago (D44480) seem to do exactly that. Could you take a look?

[hokein]

Abandon this, since @ilya-biryukov has fixed it in D44480.

lib/Sema/SemaDecl.cpp

 }
 
 bool Sema::canSkipFunctionBody(Decl *D) {
   // We cannot skip the body of a function (or function template) which is
   // constexpr, since we may need to evaluate its body in order to parse the
   // rest of the file.
   // We cannot skip the body of a function with an undeduced return type,
   // because any callers of that function need to know the type.
+  // We cannot skip the body of a function template with an 'auto' return type
+  // which is dependent on a template parameter, we need to see the function
+  // body before using it.
   if (const FunctionDecl *FD = D->getAsFunction())
-    if (FD->isConstexpr() || FD->getReturnType()->isUndeducedType())
+    if (FD->isConstexpr() || FD->getReturnType()->isUndeducedType() ||
+        FD->getReturnType()->isInstantiationDependentType())
       return false;
   return Consumer.shouldSkipFunctionBody(D);
 }
 
 Decl *Sema::ActOnSkippedFunctionBody(Decl *Decl) {
   if (!Decl)
     return nullptr;
   if (FunctionDecl *FD = Decl->getAsFunction())

test/Index/skipped-auto-fn-templates.cpp

+// RUN: env CINDEXTEST_SKIP_FUNCTION_BODIES=1 c-index-test -test-load-source all %s 2>&1 \
+// RUN: | FileCheck %s
+
+template <typename T>
+auto foo(T a) {
+  return a;
+}
+
+int b = foo(0);
+// CHECK-NOT: error: function 'foo<int>' with deduced return type cannot be used before it is defined