[YAML] Plain scalars can not begin with most indicators.

[hokein]

Summary

Discovered when clangd loads YAML symbols, some symbol documentations
start with indicators (e.g. "-"), but YAML prints them as plain scalars
(no quotes), which make the YAML parser fail to parse.

For these kind of strings, we need quotes.

[sammccall]

include/llvm/Support/YAMLTraits.h

524

Up to you, but I'd leave this comment out, or rephrase as a non-fixme - there's nothing *wrong* with the new behavior.

[hokein]

Remove fixme.

include/llvm/Support/YAMLTraits.h

 // (except for TAB #x9, LF #xA, and CR #xD which are allowed), DEL #x7F, the C1
 // control block #x80-#x9F (except for NEL #x85 which is allowed), the surrogate
 // block #xD800-#xDFFF, #xFFFE, and #xFFFF.
 inline QuotingType needsQuotes(StringRef S) {
   if (S.empty())
     return QuotingType::Single;
   if (isspace(S.front()) || isspace(S.back()))
     return QuotingType::Single;
-  if (S.front() == ',')
-    return QuotingType::Single;
   if (isNull(S))
     return QuotingType::Single;
   if (isBool(S))
     return QuotingType::Single;
   if (isNumeric(S))
     return QuotingType::Single;
 
+  // 7.3.3 Plain Style
+  // Plain scalars must not begin with most indicators, as this would cause
+  // ambiguity with other YAML constructs.
+  static constexpr char Indicators[] = R"(-?:\,[]{}#&*!|>'"%@`)";
+  if (S.find_first_of(Indicators) == 0)
+    return QuotingType::Single;
+
   QuotingType MaxQuotingNeeded = QuotingType::None;
   for (unsigned char C : S) {
     // Alphanum is safe.
     if (isAlnum(C))
       continue;
 
     switch (C) {
     // Safe scalar characters.

unittests/Support/YAMLIOTest.cpp

struct StringTypes {
   std::string stdstr4;
   std::string stdstr5;
   std::string stdstr6;
   std::string stdstr7;
   std::string stdstr8;
   std::string stdstr9;
   std::string stdstr10;
   std::string stdstr11;
+  std::string stdstr12;
 };
 
 namespace llvm {
 namespace yaml {
   template <>
   struct MappingTraits<StringTypes> {
     static void mapping(IO &io, StringTypes& st) {
       io.mapRequired("str1",      st.str1);
static void mapping(IO &io, StringTypes& st) {
       io.mapRequired("stdstr4",   st.stdstr4);
       io.mapRequired("stdstr5",   st.stdstr5);
       io.mapRequired("stdstr6",   st.stdstr6);
       io.mapRequired("stdstr7",   st.stdstr7);
       io.mapRequired("stdstr8",   st.stdstr8);
       io.mapRequired("stdstr9",   st.stdstr9);
       io.mapRequired("stdstr10",  st.stdstr10);
       io.mapRequired("stdstr11",  st.stdstr11);
+      io.mapRequired("stdstr12",  st.stdstr12);
     }
   };
 }
 }
 
 TEST(YAMLIO, TestReadWriteStringTypes) {
   std::string intermediate;
   {
std::string intermediate;
     map.stdstr4 = "@hhh";
     map.stdstr5 = "";
     map.stdstr6 = "0000000004000000";
     map.stdstr7 = "true";
     map.stdstr8 = "FALSE";
     map.stdstr9 = "~";
     map.stdstr10 = "0.2e20";
     map.stdstr11 = "0x30";
+    map.stdstr12 = "- match";
 
     llvm::raw_string_ostream ostr(intermediate);
     Output yout(ostr);
     yout << map;
   }
 
   llvm::StringRef flowOut(intermediate);
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'''aaa"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'\"bbb'"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'`ccc'"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'@ddd'"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("''\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'0000000004000000'\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'true'\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'FALSE'\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'~'\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'0.2e20'\n"));
   EXPECT_NE(llvm::StringRef::npos, flowOut.find("'0x30'\n"));
+  EXPECT_NE(llvm::StringRef::npos, flowOut.find("'- match'\n"));
   EXPECT_NE(std::string::npos, flowOut.find("'''eee"));
   EXPECT_NE(std::string::npos, flowOut.find("'\"fff'"));
   EXPECT_NE(std::string::npos, flowOut.find("'`ggg'"));
   EXPECT_NE(std::string::npos, flowOut.find("'@hhh'"));
   EXPECT_NE(std::string::npos, flowOut.find("''\n"));
   EXPECT_NE(std::string::npos, flowOut.find("'0000000004000000'\n"));
 
   {