[Sema] Reject value-initialization of function types

T in the expression T() must be a non-array complete object type or
the void type.  Function types are neither.

This fixes PR24798.

llvm-svn: 247535

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -5219,6 +5219,8 @@ def err_builtin_func_cast_more_than_one_arg : Error<
   "function-style cast to a builtin type can only take one argument">;
 def err_value_init_for_array_type : Error<
   "array types cannot be value-initialized">;
+def err_value_init_for_function_type : Error<
+  "function types cannot be value-initialized">;
 def warn_format_nonliteral_noargs : Warning<
   "format string is not a string literal (potentially insecure)">,
   InGroup<FormatSecurity>;

clang/lib/Sema/SemaExprCXX.cpp

@@ -1031,6 +1031,11 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
     return BuildCXXFunctionalCastExpr(TInfo, LParenLoc, Arg, RParenLoc);
   }
 
+  // C++14 [expr.type.conv]p2: The expression T(), where T is a
+  //   simple-type-specifier or typename-specifier for a non-array complete
+  //   object type or the (possibly cv-qualified) void type, creates a prvalue
+  //   of the specified type, whose value is that produced by value-initializing
+  //   an object of type T.
   QualType ElemTy = Ty;
   if (Ty->isArrayType()) {
     if (!ListInitialization)
@@ -1039,6 +1044,10 @@ Sema::BuildCXXTypeConstructExpr(TypeSourceInfo *TInfo,
     ElemTy = Context.getBaseElementType(Ty);
   }
 
+  if (!ListInitialization && Ty->isFunctionType())
+    return ExprError(Diag(TyBeginLoc, diag::err_value_init_for_function_type)
+                     << FullRange);
+
   if (!Ty->isVoidType() &&
       RequireCompleteType(TyBeginLoc, ElemTy,
                           diag::err_invalid_incomplete_type_use, FullRange))

clang/test/SemaCXX/type-convert-construct.cpp

@@ -5,6 +5,8 @@ void f() {
   int v2 = typeof(int)(1,2); // expected-error {{excess elements in scalar initializer}}
   typedef int arr[];
   int v3 = arr(); // expected-error {{array types cannot be value-initialized}}
+  typedef void fn_ty();
+  fn_ty(); // expected-error {{function types cannot be value-initialized}}
   int v4 = int();
   int v5 = int; // expected-error {{expected '(' for function-style cast or type construction}}
   typedef int T;