[flang][msvc] Avoid ReferenceVariantBase ctor ambiguity. NFC.

Msvc reports the following error when a ReferenceVariantBase is constructed using an r-value reference or instantiated as std::vector template parameter.  The error message is:
```
PFTBuilder.h(59,1): error C2665: 'std::variant<...>::variant': none of the 2 overloads could convert all the argument types
variant(1248,1): message : could be 'std::variant<...>::variant(std::variant<...> &&) noexcept(false)'
variant(1248,1): message : or       'std::variant<...>::variant(const std::variant<...> &) noexcept(false)'
PFTBuilder.h(59,1): message : while trying to match the argument list '(common::Reference<lower::pft::ReferenceVariantBase<false,...>>)'
```

Work around the ambiguity by only taking `common::Reference` arguments in the constructor. That is, conversion to common::Reference has to be done be the caller instead of being done inside the ctor. Unfortunately, with this change clang/gcc (but not msvc) insist on that the ReferenceVariantBase is stored in a `std::initializer_list`-initialized variable before being used, like being passed to a function or returned.

This patch is part of the series to make flang compilable with MS Visual Studio <http://lists.llvm.org/pipermail/flang-dev/2020-July/000448.html>.

Reviewed By: DavidTruby

Differential Revision: https://reviews.llvm.org/D88109

diff --git a/flang/include/flang/Lower/PFTBuilder.h b/flang/include/flang/Lower/PFTBuilder.h
index 3230d36..044e608 100644 (file)
--- a/flang/include/flang/Lower/PFTBuilder.h
+++ b/flang/include/flang/Lower/PFTBuilder.h
@@ -55,8 +55,9 @@ public:
   using Ref = common::Reference<BaseType<B>>;
 
   ReferenceVariantBase() = delete;
-  template <typename B>
-  ReferenceVariantBase(B &b) : u{Ref<B>{b}} {}
+  ReferenceVariantBase(std::variant<Ref<A>...> b) : u(b) {}
+  template <typename T>
+  ReferenceVariantBase(Ref<T> b) : u(b) {}
 
   template <typename B>
   constexpr BaseType<B> &get() const {

diff --git a/flang/lib/Lower/PFTBuilder.cpp b/flang/lib/Lower/PFTBuilder.cpp
index 7195086..349f76e 100644 (file)
--- a/flang/lib/Lower/PFTBuilder.cpp
+++ b/flang/lib/Lower/PFTBuilder.cpp
@@ -64,8 +64,11 @@ struct UnwrapStmt<parser::UnlabeledStatement<A>> {
 class PFTBuilder {
 public:
   PFTBuilder(const semantics::SemanticsContext &semanticsContext)
-      : pgm{std::make_unique<lower::pft::Program>()},
-        parentVariantStack{*pgm.get()}, semanticsContext{semanticsContext} {}
+      : pgm{std::make_unique<lower::pft::Program>()}, semanticsContext{
+                                                          semanticsContext} {
+    lower::pft::ParentVariant parent{*pgm.get()};
+    parentVariantStack.push_back(parent);
+  }
 
   /// Get the result
   std::unique_ptr<lower::pft::Program> result() { return std::move(pgm); }
@@ -905,11 +908,15 @@ private:
 template <typename A, typename T>
 static lower::pft::FunctionLikeUnit::FunctionStatement
 getFunctionStmt(const T &func) {
-  return std::get<parser::Statement<A>>(func.t);
+  lower::pft::FunctionLikeUnit::FunctionStatement result{
+      std::get<parser::Statement<A>>(func.t)};
+  return result;
 }
 template <typename A, typename T>
 static lower::pft::ModuleLikeUnit::ModuleStatement getModuleStmt(const T &mod) {
-  return std::get<parser::Statement<A>>(mod.t);
+  lower::pft::ModuleLikeUnit::ModuleStatement result{
+      std::get<parser::Statement<A>>(mod.t)};
+  return result;
 }
 
 static const semantics::Symbol *getSymbol(
@@ -1078,7 +1085,8 @@ Fortran::lower::pft::FunctionLikeUnit::FunctionLikeUnit(
   const auto &ps{
       std::get<std::optional<parser::Statement<parser::ProgramStmt>>>(func.t)};
   if (ps.has_value()) {
-    beginStmt = ps.value();
+    FunctionStatement begin{ps.value()};
+    beginStmt = begin;
     symbol = getSymbol(beginStmt);
     processSymbolTable(*symbol->scope());
   } else {