Fix and simplify handling of return type for (generic) lambda conversion function to function pointer.

Previously, we would:
 * compute the type of the conversion function and static invoker as a
   side-effect of template argument deduction for a conversion
 * re-compute the type as part of deduced return type deduction when building
   the conversion function itself

Neither of these turns out to be quite correct. There are other ways to reach a
declaration of the conversion function than in a conversion (such as an
explicit call or friend declaration), and performing auto deduction causes the
function type to be rebuilt in the context of the lambda closure type (which is
different from the context in which it originally appeared, resulting in
spurious substitution failures for constructs that are valid in one context but
not the other, such as the use of an enclosing class's "this" pointer).

This patch switches us to use a different strategy: as before, we use the
declared type of the operator() to form the type of the conversion function and
invoker, but we now populate that type as part of return type deduction for the
conversion function. And the invoker is now treated as simply being an
implementation detail of building the conversion function, and isn't given
special treatment by template argument deduction for the conversion function
any more.

llvm-svn: 321683

1 files changed, 24 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index eb0770a952c..ab68e7e671d 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -3729,6 +3729,30 @@ TemplateDeclInstantiator::InitMethodInstantiation(CXXMethodDecl *New,
   return false;
 }
 
+/// Instantiate (or find existing instantiation of) a function template with a
+/// given set of template arguments.
+///
+/// Usually this should not be used, and template argument deduction should be
+/// used in its place.
+FunctionDecl *
+Sema::InstantiateFunctionDeclaration(FunctionTemplateDecl *FTD,
+                                     const TemplateArgumentList *Args,
+                                     SourceLocation Loc) {
+  FunctionDecl *FD = FTD->getTemplatedDecl();
+
+  sema::TemplateDeductionInfo Info(Loc);
+  InstantiatingTemplate Inst(
+      *this, Loc, FTD, Args->asArray(),
+      CodeSynthesisContext::ExplicitTemplateArgumentSubstitution, Info);
+  if (Inst.isInvalid())
+    return nullptr;
+
+  ContextRAII SavedContext(*this, FD);
+  MultiLevelTemplateArgumentList MArgs(*Args);
+
+  return cast_or_null<FunctionDecl>(SubstDecl(FD, FD->getParent(), MArgs));
+}
+
 /// In the MS ABI, we need to instantiate default arguments of dllexported
 /// default constructors along with the constructor definition. This allows IR
 /// gen to emit a constructor closure which calls the default constructor with