[C++17] Fix class template argument deduction for default constructors without an initializer

Summary:
As the title says, this makes following code compile:

```
template<typename> struct Foo {};
Foo() -> Foo<void>;

Foo f; // ok
```

Thanks Nicolas Lesser for coining the fix.

Reviewers: rsmith, lichray

Reviewed By: rsmith, lichray

Subscribers: lichray, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D38216

llvm-svn: 328409

1 files changed, 14 insertions, 4 deletions

diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index 6319e4b34ce..d583646a38b 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -10396,12 +10396,22 @@ QualType Sema::deduceVarTypeFromInitializer(VarDecl *VDecl,
   // C++11 [dcl.spec.auto]p3
   if (!Init) {
     assert(VDecl && "no init for init capture deduction?");
-    Diag(VDecl->getLocation(), diag::err_auto_var_requires_init)
-      << VDecl->getDeclName() << Type;
-    return QualType();
+
+    // Except for class argument deduction, and then for an initializing
+    // declaration only, i.e. no static at class scope or extern.
+    if (!isa<DeducedTemplateSpecializationType>(Deduced) ||
+        VDecl->hasExternalStorage() ||
+        VDecl->isStaticDataMember()) {
+      Diag(VDecl->getLocation(), diag::err_auto_var_requires_init)
+        << VDecl->getDeclName() << Type;
+      return QualType();
+    }
   }
 
-  ArrayRef<Expr*> DeduceInits = Init;
+  ArrayRef<Expr*> DeduceInits;
+  if (Init)
+    DeduceInits = Init;
+
   if (DirectInit) {
     if (auto *PL = dyn_cast_or_null<ParenListExpr>(Init))
       DeduceInits = PL->exprs();