[PATCH] D115031: [AST] Print NTTP args as string-literals when possible

[Zhihao Yuan via Phabricator via cfe-commits]

lichray marked 4 inline comments as done.
lichray added inline comments.


================
Comment at: clang/include/clang/Basic/CharInfo.h:199-200
+    return "\\t";
+  case '\v':
+    return "\\v";
+  }
----------------
aaron.ballman wrote:
> lichray wrote:
> > aaron.ballman wrote:
> > > We're also missing `\?` right?
> > `?` does not seem to need `"escaping?"`
> It's the only simple escape sequence we're not handling here: http://eel.is/c++draft/lex.literal#nt:simple-escape-sequence-char
> 
> (You need to escape `?` thanks to trigraphs. Consider a string literal like `"This does what now??!"`.)
> It's the only simple escape sequence we're not handling here: http://eel.is/c++draft/lex.literal#nt:simple-escape-sequence-char
> 
> (You need to escape `?` thanks to trigraphs. Consider a string literal like `"This does what now??!"`.)

Hmm, I think we're safe here
```
~/devel/llvm-project> ./build/bin/clang++ -fsyntax-only -std=c++14 c.cc
c.cc:1:50: warning: trigraph converted to '|' character [-Wtrigraphs]
void foo() { char const s[] = "This does what now??!"; }
                                                 ^
1 warning generated.
~/devel/llvm-project> ./build/bin/clang++ -fsyntax-only -std=c++20 c.cc
c.cc:1:50: warning: trigraph ignored [-Wtrigraphs]
void foo() { char const s[] = "This does what now??!"; }
                                                 ^
1 warning generated.
```
For these reasons,
1. I don't want users to copy diagnosis messages, paste them into a program, and get something different. This has been prevented by the warnings.
2. We support trigraph up to C++14, way behind the version (C++20) we can use string literals in NTTP. So it should be mostly fine.
3. In C++14 we can use the characters in NTTP, but that one has a different diagnosis:

```
template <char C> class ASCII {};

void Foo(ASCII<'?'>);
void Foo(ASCII<'??!'>);
void test_pascal() {
  ASCII<'!'> a;
  Foo(a);
}
```

```
c.cc:4:17: warning: trigraph converted to '|' character [-Wtrigraphs]
void Foo(ASCII<'??!'>);
                ^
c.cc:7:3: error: no matching function for call to 'Foo'
  Foo(a);
  ^~~
c.cc:3:6: note: candidate function not viable: no known conversion from 'ASCII<'!' aka 33>' to 'ASCII<'?' aka 63>' for 1st argument
void Foo(ASCII<'?'>);
     ^
c.cc:4:6: note: candidate function not viable: no known conversion from 'ASCII<'!' aka 33>' to 'ASCII<'|' aka 124>' for 1st argument
void Foo(ASCII<'??!'>);
     ^
1 warning and 1 error generated.
```
Looks nice to me even if we don't escape.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D115031/new/

https://reviews.llvm.org/D115031