diff --git a/llvm/include/llvm/Support/YAMLParser.h b/llvm/include/llvm/Support/YAMLParser.h
--- a/llvm/include/llvm/Support/YAMLParser.h
+++ b/llvm/include/llvm/Support/YAMLParser.h
@@ -78,6 +78,9 @@
 /// escaped, but emitted verbatim.
 std::string escape(StringRef Input, bool EscapePrintable = true);
 
+/// Parse \p S as a bool according to https://yaml.org/type/bool.html.
+llvm::Optional<bool> parseBool(StringRef S);
+
 /// This class represents a YAML stream potentially containing multiple
 ///        documents.
 class Stream {
diff --git a/llvm/include/llvm/Support/YAMLTraits.h b/llvm/include/llvm/Support/YAMLTraits.h
--- a/llvm/include/llvm/Support/YAMLTraits.h
+++ b/llvm/include/llvm/Support/YAMLTraits.h
@@ -638,6 +638,7 @@
 }
 
 inline bool isBool(StringRef S) {
+  // FIXME: using parseBool is causing multiple tests to fail.
   return S.equals("true") || S.equals("True") || S.equals("TRUE") ||
          S.equals("false") || S.equals("False") || S.equals("FALSE");
 }
diff --git a/llvm/lib/Support/YAMLParser.cpp b/llvm/lib/Support/YAMLParser.cpp
--- a/llvm/lib/Support/YAMLParser.cpp
+++ b/llvm/lib/Support/YAMLParser.cpp
@@ -746,6 +746,92 @@
   return EscapedInput;
 }
 
+llvm::Optional<bool> yaml::parseBool(StringRef S) {
+  switch (S.size()) {
+  case 1:
+    switch (S.front()) {
+    case 'y':
+    case 'Y':
+      return true;
+    case 'n':
+    case 'N':
+      return false;
+    default:
+      return None;
+    }
+  case 2:
+    switch (S.front()) {
+    case 'O':
+      if (S[1] == 'N') // ON
+        return true;
+      LLVM_FALLTHROUGH;
+    case 'o':
+      if (S[1] == 'n') //[Oo]n
+        return true;
+      return None;
+    case 'N':
+      if (S[1] == 'O') // NO
+        return false;
+      LLVM_FALLTHROUGH;
+    case 'n':
+      if (S[1] == 'o') //[Nn]o
+        return false;
+      return None;
+    default:
+      return None;
+    }
+  case 3:
+    switch (S.front()) {
+    case 'O':
+      if (S.drop_front() == "FF") // OFF
+        return false;
+      LLVM_FALLTHROUGH;
+    case 'o':
+      if (S.drop_front() == "ff") //[Oo]ff
+        return false;
+      return None;
+    case 'Y':
+      if (S.drop_front() == "ES") // YES
+        return true;
+      LLVM_FALLTHROUGH;
+    case 'y':
+      if (S.drop_front() == "es") //[Yy]es
+        return true;
+      return None;
+    default:
+      return None;
+    }
+  case 4:
+    switch (S.front()) {
+    case 'T':
+      if (S.drop_front() == "RUE") // TRUE
+        return true;
+      LLVM_FALLTHROUGH;
+    case 't':
+      if (S.drop_front() == "rue") //[Tt]rue
+        return true;
+      return None;
+    default:
+      return None;
+    }
+  case 5:
+    switch (S.front()) {
+    case 'F':
+      if (S.drop_front() == "ALSE") // FALSE
+        return false;
+      LLVM_FALLTHROUGH;
+    case 'f':
+      if (S.drop_front() == "alse") //[Ff]alse
+        return false;
+      return None;
+    default:
+      return None;
+    }
+  default:
+    return None;
+  }
+}
+
 Scanner::Scanner(StringRef Input, SourceMgr &sm, bool ShowColors,
                  std::error_code *EC)
     : SM(sm), ShowColors(ShowColors), EC(EC) {
diff --git a/llvm/lib/Support/YAMLTraits.cpp b/llvm/lib/Support/YAMLTraits.cpp
--- a/llvm/lib/Support/YAMLTraits.cpp
+++ b/llvm/lib/Support/YAMLTraits.cpp
@@ -885,11 +885,8 @@
 }
 
 StringRef ScalarTraits<bool>::input(StringRef Scalar, void *, bool &Val) {
-  if (Scalar.equals("true")) {
-    Val = true;
-    return StringRef();
-  } else if (Scalar.equals("false")) {
-    Val = false;
+  if (llvm::Optional<bool> Parsed = parseBool(Scalar)) {
+    Val = *Parsed;
     return StringRef();
   }
   return "invalid boolean";
diff --git a/llvm/unittests/Support/YAMLParserTest.cpp b/llvm/unittests/Support/YAMLParserTest.cpp
--- a/llvm/unittests/Support/YAMLParserTest.cpp
+++ b/llvm/unittests/Support/YAMLParserTest.cpp
@@ -342,4 +342,44 @@
   }
 }
 
+static void expectCanParseBool(StringRef S, bool Expected) {
+  llvm::Optional<bool> Parsed = yaml::parseBool(S);
+  EXPECT_TRUE(Parsed.hasValue());
+  EXPECT_EQ(*Parsed, Expected);
+}
+
+static void expectCannotParseBool(StringRef S) {
+  EXPECT_FALSE(yaml::parseBool(S).hasValue());
+}
+
+TEST(YAMLParser, ParsesBools) {
+  // Test true values.
+  expectCanParseBool("ON", true);
+  expectCanParseBool("On", true);
+  expectCanParseBool("on", true);
+  expectCanParseBool("TRUE", true);
+  expectCanParseBool("True", true);
+  expectCanParseBool("true", true);
+  expectCanParseBool("Y", true);
+  expectCanParseBool("y", true);
+  expectCanParseBool("YES", true);
+  expectCanParseBool("Yes", true);
+  expectCanParseBool("yes", true);
+  expectCannotParseBool("1");
+
+  // Test false values.
+  expectCanParseBool("FALSE", false);
+  expectCanParseBool("False", false);
+  expectCanParseBool("false", false);
+  expectCanParseBool("N", false);
+  expectCanParseBool("n", false);
+  expectCanParseBool("NO", false);
+  expectCanParseBool("No", false);
+  expectCanParseBool("no", false);
+  expectCanParseBool("OFF", false);
+  expectCanParseBool("Off", false);
+  expectCanParseBool("off", false);
+  expectCannotParseBool("0");
+}
+
 } // end namespace llvm