[Parse] Support #if swift(<4.1)

include/swift/Basic/Version.h

@@ -152,6 +152,7 @@ class Version {
 };
 
 bool operator>=(const Version &lhs, const Version &rhs);
+bool operator<(const Version &lhs, const Version &rhs);
 bool operator==(const Version &lhs, const Version &rhs);
 inline bool operator!=(const Version &lhs, const Version &rhs) {
   return !(lhs == rhs);

lib/Basic/Version.cpp

@@ -384,6 +384,11 @@ bool operator>=(const class Version &lhs,
   return true;
 }
 
+bool operator<(const class Version &lhs,
+               const class Version &rhs) {
+    return !(lhs >= rhs);
+}
+
 bool operator==(const class Version &lhs,
                 const class Version &rhs) {
   auto n = std::max(lhs.size(), rhs.size());

lib/Parse/ParseIfConfig.cpp

@@ -282,10 +282,10 @@ class ValidateIfConfigCondition :
       auto PUE = dyn_cast<PrefixUnaryExpr>(Arg);
       llvm::Optional<StringRef> PrefixName = PUE ?
         getDeclRefStr(PUE->getFn(), DeclRefKind::PrefixOperator) : None;
-      if (!PrefixName || *PrefixName != ">=") {
+      if (!PrefixName || (*PrefixName != ">=" && *PrefixName != "<")) {
         D.diagnose(Arg->getLoc(),
                    diag::unsupported_platform_condition_argument,
-                   "a unary comparison, such as '>=2.2'");
+                   "a unary comparison, such as '>=2.2' or '<4.1'");
         return nullptr;
       }
       auto versionString = extractExprSource(Ctx.SourceMgr, PUE->getArg());
@@ -447,11 +447,16 @@ class EvaluateIfConfigCondition :
       return thisVersion >= Val;
     } else if (KindName == "swift") {
       auto PUE = cast<PrefixUnaryExpr>(Arg);
+      auto PrefixName = getDeclRefStr(PUE->getFn());
       auto Str = extractExprSource(Ctx.SourceMgr, PUE->getArg());
       auto Val = version::Version::parseVersionString(
           Str, SourceLoc(), nullptr).getValue();
       auto thisVersion = Ctx.LangOpts.EffectiveLanguageVersion;
-      return thisVersion >= Val;
+      if (PrefixName == ">=") {
+        return thisVersion >= Val;
+      } else if (PrefixName == "<") {
+        return thisVersion < Val;
+      }
     } else if (KindName == "canImport") {
       auto Str = extractExprSource(Ctx.SourceMgr, Arg);
       return Ctx.canImportModule({ Ctx.getIdentifier(Str) , E->getLoc()  });

test/Parse/ConditionalCompilation/language_version_explicit.swift

@@ -37,3 +37,46 @@
   let c = 1
 #endif
 // NOTE: ...the next time the version goes up.
+
+#if swift(<4)
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#else
+  let a = 1
+#endif
+
+#if !swift(<4)
+  let b = 1
+#else
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#endif
+
+#if swift(<4.0)
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#else
+  let c = 1
+#endif
+
+#if swift(<4.0.0)
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#else
+  let d = 1
+#endif
+
+#if swift(<4.0.1)
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#else
+  let e = 1
+#endif
+
+#if swift(<99)
+  let f = 1
+#else
+  // This shouldn't emit any diagnostics.
+  asdf asdf asdf asdf
+#endif
+