[Cxx1z-constexpr-lambda-P0170R1] Support parsing of constexpr specifier (and its inference) on lambda expressions

Support the constexpr specifier on lambda expressions - and support its inference from the lambda call operator's body.

i.e.
  auto L = [] () constexpr { return 5; };
  static_assert(L() == 5); // OK
  auto Implicit = [] (auto a) { return a; };
  static_assert(Implicit(5) == 5); 

We do not support evaluation of lambda's within constant expressions just yet.

Implementation Strategy:
  - teach ParseLambdaExpressionAfterIntroducer to expect a constexpr specifier and mark the invented function call operator's declarator's decl-specifier with it; Have it emit fixits for multiple decl-specifiers (mutable or constexpr) in this location.
  - for cases where constexpr is not explicitly specified, have buildLambdaExpr check whether the invented function call operator satisfies the requirements of a constexpr function, by calling CheckConstexprFunctionDecl/Body.

Much obliged to Richard Smith for his patience and his care, in ensuring the code is clang-worthy.

llvm-svn: 264513

clang/include/clang/Basic/DiagnosticASTKinds.td

@@ -153,6 +153,12 @@ def warn_integer_constant_overflow : Warning<
   "overflow in expression; result is %0 with type %1">,
   InGroup<DiagGroup<"integer-overflow">>;
 
+// This is a temporary diagnostic, and shall be removed once our 
+// implementation is complete, and like the preceding constexpr notes belongs
+// in Sema.
+def note_unimplemented_constexpr_lambda_feature_ast : Note<
+    "unimplemented constexpr lambda feature: %0 (coming soon!)">;
+
 // inline asm related.
 let CategoryName = "Inline Assembly Issue" in {
   def err_asm_invalid_escape : Error<

clang/include/clang/Basic/DiagnosticParseKinds.td

@@ -765,11 +765,20 @@ def warn_cxx98_compat_lambda : Warning<
   InGroup<CXX98Compat>, DefaultIgnore;
 def err_lambda_missing_parens : Error<
   "lambda requires '()' before %select{'mutable'|return type|"
-  "attribute specifier}0">;
+  "attribute specifier|'constexpr'}0">;
+def err_lambda_decl_specifier_repeated : Error<
+  "%select{'mutable'|'constexpr'}0 cannot appear multiple times in a lambda declarator">;
 // C++1z lambda expressions
 def err_expected_star_this_capture : Error<
   "expected 'this' following '*' in lambda capture list">;
 
+// C++1z constexpr lambda expressions
+def warn_cxx14_compat_constexpr_on_lambda : Warning<
+  "constexpr on lambda expressions is incompatible with C++ standards before C++1z">,
+  InGroup<CXXPre1zCompat>, DefaultIgnore;
+def ext_constexpr_on_lambda_cxx1z : ExtWarn<
+  "'constexpr' on lambda expressions is a C++1z extension">, InGroup<CXX1z>;
+
 // Availability attribute
 def err_expected_version : Error<
   "expected a version of the form 'major[.minor[.subminor]]'">;

clang/include/clang/Sema/Sema.h

@@ -5013,7 +5013,8 @@ public:
                                        SourceRange IntroducerRange,
                                        TypeSourceInfo *MethodType,
                                        SourceLocation EndLoc,
-                                       ArrayRef<ParmVarDecl *> Params);
+                                       ArrayRef<ParmVarDecl *> Params, 
+                                       bool IsConstexprSpecified);
 
   /// \brief Endow the lambda scope info with the relevant properties.
   void buildLambdaScope(sema::LambdaScopeInfo *LSI, 

clang/lib/AST/ExprConstant.cpp

@@ -36,6 +36,7 @@
 #include "clang/AST/APValue.h"
 #include "clang/AST/ASTContext.h"
 #include "clang/AST/ASTDiagnostic.h"
+#include "clang/AST/ASTLambda.h"
 #include "clang/AST/CharUnits.h"
 #include "clang/AST/Expr.h"
 #include "clang/AST/RecordLayout.h"
@@ -2045,7 +2046,22 @@ static bool evaluateVarDeclInit(EvalInfo &Info, const Expr *E,
   // If this is a local variable, dig out its value.
   if (Frame) {
     Result = Frame->getTemporary(VD);
-    assert(Result && "missing value for local variable");
+    if (!Result) {
+      // Assume variables referenced within a lambda's call operator that were
+      // not declared within the call operator are captures and during checking
+      // of a potential constant expression, assume they are unknown constant
+      // expressions.
+      assert(isLambdaCallOperator(Frame->Callee) &&
+             (VD->getDeclContext() != Frame->Callee || VD->isInitCapture()) &&
+             "missing value for local variable");
+      if (Info.checkingPotentialConstantExpression())
+        return false;
+      // FIXME: implement capture evaluation during constant expr evaluation.
+      Info.Diag(E->getLocStart(),
+           diag::note_unimplemented_constexpr_lambda_feature_ast)
+          << "captures not currently allowed";
+      return false;
+    }
     return true;
   }
 

clang/lib/Parse/ParseExprCXX.cpp

@@ -1050,6 +1050,58 @@ bool Parser::TryParseLambdaIntroducer(LambdaIntroducer &Intro) {
   return false;
 }
 
+static void
+tryConsumeMutableOrConstexprToken(Parser &P, SourceLocation &MutableLoc,
+                                  SourceLocation &ConstexprLoc,
+                                  SourceLocation &DeclEndLoc) {
+  assert(MutableLoc.isInvalid());
+  assert(ConstexprLoc.isInvalid());
+  // Consume constexpr-opt mutable-opt in any sequence, and set the DeclEndLoc
+  // to the final of those locations. Emit an error if we have multiple
+  // copies of those keywords and recover.
+
+  while (true) {
+    switch (P.getCurToken().getKind()) {
+    case tok::kw_mutable: {
+      if (MutableLoc.isValid()) {
+        P.Diag(P.getCurToken().getLocation(),
+               diag::err_lambda_decl_specifier_repeated)
+            << 0 << FixItHint::CreateRemoval(P.getCurToken().getLocation());
+      }
+      MutableLoc = P.ConsumeToken();
+      DeclEndLoc = MutableLoc;
+      break /*switch*/;
+    }
+    case tok::kw_constexpr:
+      if (ConstexprLoc.isValid()) {
+        P.Diag(P.getCurToken().getLocation(),
+               diag::err_lambda_decl_specifier_repeated)
+            << 1 << FixItHint::CreateRemoval(P.getCurToken().getLocation());
+      }
+      ConstexprLoc = P.ConsumeToken();
+      DeclEndLoc = ConstexprLoc;
+      break /*switch*/;
+    default:
+      return;
+    }
+  }
+}
+
+static void
+addConstexprToLambdaDeclSpecifier(Parser &P, SourceLocation ConstexprLoc,
+                                  DeclSpec &DS) {
+  if (ConstexprLoc.isValid()) {
+    P.Diag(ConstexprLoc, !P.getLangOpts().CPlusPlus1z
+                             ? diag::ext_constexpr_on_lambda_cxx1z
+                             : diag::warn_cxx14_compat_constexpr_on_lambda);
+    const char *PrevSpec = nullptr;
+    unsigned DiagID = 0;
+    DS.SetConstexprSpec(ConstexprLoc, PrevSpec, DiagID);
+    assert(PrevSpec == nullptr && DiagID == 0 &&
+           "Constexpr cannot have been set previously!");
+  }
+}
+
 /// ParseLambdaExpressionAfterIntroducer - Parse the rest of a lambda
 /// expression.
 ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
@@ -1108,10 +1160,13 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
     // compatible with MSVC.
     MaybeParseMicrosoftDeclSpecs(Attr, &DeclEndLoc);
 
-    // Parse 'mutable'[opt].
+    // Parse mutable-opt and/or constexpr-opt, and update the DeclEndLoc.
     SourceLocation MutableLoc;
-    if (TryConsumeToken(tok::kw_mutable, MutableLoc))
-      DeclEndLoc = MutableLoc;
+    SourceLocation ConstexprLoc;
+    tryConsumeMutableOrConstexprToken(*this, MutableLoc, ConstexprLoc,
+                                      DeclEndLoc);
+    
+    addConstexprToLambdaDeclSpecifier(*this, ConstexprLoc, DS);
 
     // Parse exception-specification[opt].
     ExceptionSpecificationType ESpecType = EST_None;
@@ -1169,7 +1224,8 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
                                            LParenLoc, FunLocalRangeEnd, D,
                                            TrailingReturnType),
                   Attr, DeclEndLoc);
-  } else if (Tok.isOneOf(tok::kw_mutable, tok::arrow, tok::kw___attribute) ||
+  } else if (Tok.isOneOf(tok::kw_mutable, tok::arrow, tok::kw___attribute,
+                         tok::kw_constexpr) ||
              (Tok.is(tok::l_square) && NextToken().is(tok::l_square))) {
     // It's common to forget that one needs '()' before 'mutable', an attribute
     // specifier, or the result type. Deal with this.
@@ -1179,6 +1235,7 @@ ExprResult Parser::ParseLambdaExpressionAfterIntroducer(
     case tok::arrow: TokKind = 1; break;
     case tok::kw___attribute:
     case tok::l_square: TokKind = 2; break;
+    case tok::kw_constexpr: TokKind = 3; break;
     default: llvm_unreachable("Unknown token kind");
     }
 

clang/lib/Sema/SemaLambda.cpp

@@ -355,7 +355,8 @@ CXXMethodDecl *Sema::startLambdaDefinition(CXXRecordDecl *Class,
                                            SourceRange IntroducerRange,
                                            TypeSourceInfo *MethodTypeInfo,
                                            SourceLocation EndLoc,
-                                           ArrayRef<ParmVarDecl *> Params) {
+                                           ArrayRef<ParmVarDecl *> Params,
+                                           const bool IsConstexprSpecified) {
   QualType MethodType = MethodTypeInfo->getType();
   TemplateParameterList *TemplateParams = 
             getGenericLambdaTemplateParameterList(getCurLambda(), *this);
@@ -392,7 +393,7 @@ CXXMethodDecl *Sema::startLambdaDefinition(CXXRecordDecl *Class,
                             MethodType, MethodTypeInfo,
                             SC_None,
                             /*isInline=*/true,
-                            /*isConstExpr=*/false,
+                            IsConstexprSpecified,
                             EndLoc);
   Method->setAccess(AS_public);
   
@@ -879,8 +880,9 @@ void Sema::ActOnStartOfLambdaDefinition(LambdaIntroducer &Intro,
   CXXRecordDecl *Class = createLambdaClosureType(Intro.Range, MethodTyInfo,
                                                  KnownDependent, Intro.Default);
 
-  CXXMethodDecl *Method = startLambdaDefinition(Class, Intro.Range,
-                                                MethodTyInfo, EndLoc, Params);
+  CXXMethodDecl *Method =
+      startLambdaDefinition(Class, Intro.Range, MethodTyInfo, EndLoc, Params,
+                            ParamInfo.getDeclSpec().isConstexprSpecified());
   if (ExplicitParams)
     CheckCXXDefaultArguments(Method);
   
@@ -1600,6 +1602,17 @@ ExprResult Sema::BuildLambdaExpr(SourceLocation StartLoc, SourceLocation EndLoc,
                                           CaptureInits, ArrayIndexVars, 
                                           ArrayIndexStarts, EndLoc,
                                           ContainsUnexpandedParameterPack);
+  // If the lambda expression's call operator is not explicitly marked constexpr
+  // and we are not in a dependent context, analyze the call operator to infer
+  // its constexpr-ness, supressing diagnostics while doing so.
+  if (getLangOpts().CPlusPlus1z && !CallOperator->isInvalidDecl() &&
+      !CallOperator->isConstexpr() &&
+      !Class->getDeclContext()->isDependentContext()) {
+    TentativeAnalysisScope DiagnosticScopeGuard(*this);
+    CallOperator->setConstexpr(
+        CheckConstexprFunctionDecl(CallOperator) &&
+        CheckConstexprFunctionBody(CallOperator, CallOperator->getBody()));
+  }
 
   if (!CurContext->isDependentContext()) {
     switch (ExprEvalContexts.back().Context) {

clang/lib/Sema/TreeTransform.h

@@ -10054,7 +10054,9 @@ TreeTransform<Derived>::TransformLambdaExpr(LambdaExpr *E) {
   CXXMethodDecl *NewCallOperator = getSema().startLambdaDefinition(
       Class, E->getIntroducerRange(), NewCallOpTSI,
       E->getCallOperator()->getLocEnd(),
-      NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams());
+      NewCallOpTSI->getTypeLoc().castAs<FunctionProtoTypeLoc>().getParams(),
+      E->getCallOperator()->isConstexpr());
+
   LSI->CallOperator = NewCallOperator;
 
   getDerived().transformAttrs(E->getCallOperator(), NewCallOperator);

clang/test/Parser/cxx1z-constexpr-lambdas.cpp

@@ -0,0 +1,31 @@
+// RUN: %clang_cc1 -std=c++1z %s -verify 
+// RUN: %clang_cc1 -std=c++14 %s -verify 
+// RUN: %clang_cc1 -std=c++11 %s -verify 
+
+
+auto XL0 = [] constexpr { }; //expected-error{{requires '()'}} expected-error{{expected body}}
+auto XL1 = [] () mutable 
+                 mutable     //expected-error{{cannot appear multiple times}}
+                 mutable { }; //expected-error{{cannot appear multiple times}}
+
+#if __cplusplus > 201402L
+auto XL2 = [] () constexpr mutable constexpr { }; //expected-error{{cannot appear multiple times}}
+auto L = []() mutable constexpr { };
+auto L2 = []() constexpr { };
+auto L4 = []() constexpr mutable { }; 
+auto XL16 = [] () constexpr
+                  mutable
+                  constexpr   //expected-error{{cannot appear multiple times}}
+                  mutable     //expected-error{{cannot appear multiple times}}
+                  mutable     //expected-error{{cannot appear multiple times}}
+                  constexpr   //expected-error{{cannot appear multiple times}}
+                  constexpr   //expected-error{{cannot appear multiple times}}
+                  { };
+
+#else
+auto L = []() mutable constexpr {return 0; }; //expected-warning{{is a C++1z extension}}
+auto L2 = []() constexpr { return 0;};//expected-warning{{is a C++1z extension}}
+auto L4 = []() constexpr mutable { return 0; }; //expected-warning{{is a C++1z extension}}
+#endif
+
+

clang/test/SemaCXX/cxx1z-constexpr-lambdas.cpp

@@ -0,0 +1,36 @@
+// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks %s
+// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fdelayed-template-parsing %s 
+// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fms-extensions %s 
+// RUN: %clang_cc1 -std=c++1z -verify -fsyntax-only -fblocks -fdelayed-template-parsing -fms-extensions %s 
+
+namespace test_constexpr_checking {
+
+namespace ns1 {
+  struct NonLit { ~NonLit(); };  //expected-note{{not literal}}
+  auto L = [](NonLit NL) constexpr { }; //expected-error{{not a literal type}}
+} // end ns1
+
+namespace ns2 {
+  auto L = [](int I) constexpr { asm("non-constexpr");  }; //expected-error{{not allowed in constexpr function}}
+} // end ns1
+
+} // end ns test_constexpr_checking
+
+namespace test_constexpr_call {
+
+namespace ns1 {
+  auto L = [](int I) { return I; };
+  static_assert(L(3) == 3);
+} // end ns1
+namespace ns2 {
+  auto L = [](auto a) { return a; };
+  static_assert(L(3) == 3);
+  static_assert(L(3.14) == 3.14);
+}
+namespace ns3 {
+  auto L = [](auto a) { asm("non-constexpr"); return a; }; //expected-note{{declared here}}
+  constexpr int I =  //expected-error{{must be initialized by a constant expression}}
+      L(3); //expected-note{{non-constexpr function}}
+} 
+
+} // end ns test_constexpr_call