[flang] Small edits to C++17 usage notes

Original-commit: flang-compiler/f18@bdf62ac90020c84516400085b75109979d599dd7
Reviewed-on: https://github.com/flang-compiler/f18/pull/306

diff --git a/flang/documentation/C++17.md b/flang/documentation/C++17.md
index 7c26c9e..607ab55 100644 (file)
--- a/flang/documentation/C++17.md
+++ b/flang/documentation/C++17.md
@@ -4,10 +4,10 @@ Copyright (c) 2018, NVIDIA CORPORATION.  All rights reserved.
 
 ## C++14/17 features used in f18
 
-The C++ dialect used in this project is a subset of the
+The C++ dialect used in this project constitutes a subset of the
 standard C++ programming language and library features.
-We want it to be compatible with the LLVM C++ language
-subset in use at the time that we integrate with that
+We want our dialect to be compatible with the LLVM C++ language
+subset that will be in use at the time that we integrate with that
 project.
 We also want to maximize portability, future-proofing,
 compile-time error checking, and use of best practices.
@@ -20,11 +20,19 @@ We have chosen to use some features of the recent C++17
 language standard in f18.
 The most important of these are:
 * sum types (discriminated unions) in the form of `std::variant`
+* `using` template parameter packs
+* generic lambdas with `auto` argument types
 * product types in the form of `std::tuple`
 * `std::optional`
 
+(`std::tuple` is actually a C++11 feature, but I include it
+in this list because it's not particularly well known.)
+
 ### Sum types
 
+First, some background information to explain the need for sum types
+in f18.
+
 Fortran is notoriously problematic to lex and parse, as tokenization
 depends on the state of the partial parse;
 the language has no reserved words in the sense that C++ does.
@@ -85,7 +93,8 @@ of the possibilities, and f18 will fail to build if one is missing.
 
 Were we unable to use `std::variant` directly, we would likely
 have chosen to implement a local `SumType` replacement; in the
-absence of C++17's ability of `using` a template parameter pack,
+absence of C++17's abilities of `using` a template parameter pack
+and allowing `auto` arguments in anonymous lambda functions,
 it would be less convenient to use.
 
 The other options for polymorphism in C++ at the level of C++11