Implement a rudimentary form of generic lambdas.

Specifically, the following features are not included in this commit:
  - any sort of capturing within generic lambdas 
  - nested lambdas
  - conversion operator for captureless lambdas
  - ensuring all visitors are generic lambda aware


As an example of what compiles:

template <class F1, class F2>
struct overload : F1, F2 {
    using F1::operator();
    using F2::operator();
    overload(F1 f1, F2 f2) : F1(f1), F2(f2) { }
  };

  auto Recursive = [](auto Self, auto h, auto ... rest) {
    return 1 + Self(Self, rest...);
  };
  auto Base = [](auto Self, auto h) {
      return 1;
  };
  overload<decltype(Base), decltype(Recursive)> O(Base, Recursive);
  int num_params =  O(O, 5, 3, "abc", 3.14, 'a');

Please see attached tests for more examples.

Some implementation notes:

  - Add a new Declarator context => LambdaExprParameterContext to 
    clang::Declarator to allow the use of 'auto' in declaring generic
    lambda parameters
    
  - Augment AutoType's constructor (similar to how variadic 
    template-type-parameters ala TemplateTypeParmDecl are implemented) to 
    accept an IsParameterPack to encode a generic lambda parameter pack.
  
  - Add various helpers to CXXRecordDecl to facilitate identifying
    and querying a closure class
  
  - LambdaScopeInfo (which maintains the current lambda's Sema state)
    was augmented to house the current depth of the template being
    parsed (id est the Parser calls Sema::RecordParsingTemplateParameterDepth)
    so that Sema::ActOnLambdaAutoParameter may use it to create the 
    appropriate list of corresponding TemplateTypeParmDecl for each
    auto parameter identified within the generic lambda (also stored
    within the current LambdaScopeInfo).  Additionally, 
    a TemplateParameterList data-member was added to hold the invented
    TemplateParameterList AST node which will be much more useful
    once we teach TreeTransform how to transform generic lambdas.
    
  - SemaLambda.h was added to hold some common lambda utility
    functions (this file is likely to grow ...)
    
  - Teach Sema::ActOnStartOfFunctionDef to check whether it
    is being called to instantiate a generic lambda's call
    operator, and if so, push an appropriately prepared
    LambdaScopeInfo object on the stack.
    
  - Teach Sema::ActOnStartOfLambdaDefinition to set the
    return type of a lambda without a trailing return type
    to 'auto' in C++1y mode, and teach the return type
    deduction machinery in SemaStmt.cpp to process either
    C++11 and C++14 lambda's correctly depending on the flag.    

  - various tests were added - but much more will be needed.

A greatful thanks to all reviewers including Eli Friedman,  
James Dennett and the ever illuminating Richard Smith.  And 
yet I am certain that I have allowed unidentified bugs to creep in; 
bugs, that I will do my best to slay, once identified!

Thanks!

llvm-svn: 188977

1 files changed, 57 insertions, 0 deletions

diff --git a/clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4-1y.cpp b/clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4-1y.cpp
index 79ee76f0258..f8461335b76 100644
--- a/clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4-1y.cpp
+++ b/clang/test/CXX/expr/expr.prim/expr.prim.lambda/p4-1y.cpp
@@ -7,3 +7,60 @@ int &d = [] (int &r) -> auto & { return r; } (a);
 int &e = [] (int &r) -> auto { return r; } (a); // expected-error {{cannot bind to a temporary}}
 int &f = [] (int r) -> decltype(auto) { return r; } (a); // expected-error {{cannot bind to a temporary}}
 int &g = [] (int r) -> decltype(auto) { return (r); } (a); // expected-warning {{reference to stack}}
+
+
+int test_explicit_auto_return() 
+{
+    struct X {};
+    auto L = [](auto F, auto a) { return F(a); };
+    auto M = [](auto a) -> auto { return a; }; // OK
+    auto MRef = [](auto b) -> auto& { return b; }; //expected-warning{{reference to stack}}
+    auto MPtr = [](auto c) -> auto* { return &c; }; //expected-warning{{address of stack}}
+    auto MDeclType = [](auto&& d) -> decltype(auto) { return static_cast<decltype(d)>(d); }; //OK
+    M(3);
+    
+    auto &&x = MDeclType(X{});
+    auto &&x1 = M(X{});
+    auto &&x2 = MRef(X{});//expected-note{{in instantiation of}}
+    auto &&x3 = MPtr(X{}); //expected-note{{in instantiation of}}
+    return 0;    
+}
+
+int test_implicit_auto_return() 
+{  
+  {
+    auto M = [](auto a) { return a; };
+    struct X {};
+    X x = M(X{});
+    
+  }
+}
+ 
+int test_multiple_returns()  {
+    auto M = [](auto a) { 
+      bool k;
+      if (k)
+        return a;
+      else
+        return 5; //expected-error{{deduced as 'int' here}}
+    }; 
+    M(3); // OK
+    M('a'); //expected-note{{in instantiation of}}
+  return 0;
+}
+int test_no_parameter_list()
+{
+  static int si = 0;
+    auto M = [] { return 5; }; // OK
+    auto M2 = [] -> auto&& { return si; }; // expected-error{{lambda requires '()'}}
+    M();
+}
+
+int test_conditional_in_return() {
+  auto Fac = [](auto f, auto n) { 
+    return n <= 0 ? n : f(f, n - 1) * n;
+  };
+  // FIXME: this test causes a recursive limit - need to error more gracefully.
+  //Fac(Fac, 3);
+
+}
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