"-swift-version 3" means Swift 3.1, not 3.0.

include/swift/Basic/Version.h

@@ -52,13 +52,16 @@ namespace version {
 /// a: [0 - 999]
 /// b: [0 - 999]
 class Version {
-  SmallVector<uint64_t, 5> Components;
+  SmallVector<unsigned, 5> Components;
 public:
   /// Create the empty compiler version - this always compares greater
   /// or equal to any other CompilerVersion, as in the case of building Swift
   /// from latest sources outside of a build/integration/release context.
   Version() = default;
 
+  /// Create a literal version from a list of components.
+  Version(std::initializer_list<unsigned> Values) : Components(Values) {}
+
   /// Create a version from a string in source code.
   ///
   /// Must include only groups of digits separated by a dot.
@@ -94,11 +97,15 @@ class Version {
   /// away any 5th component that might be in this version.
   operator clang::VersionTuple() const;
 
-  /// Return whether this version is a valid Swift language version number
-  /// to set the compiler to using -swift-version; this is not the same as
-  /// the set of Swift versions that have ever existed, just those that we
-  /// are attempting to maintain backward-compatibility support for.
-  bool isValidEffectiveLanguageVersion() const;
+  /// Returns the concrete version to use when \e this version is provided as
+  /// an argument to -swift-version.
+  ///
+  /// This is not the same as the set of Swift versions that have ever existed,
+  /// just those that we are attempting to maintain backward-compatibility
+  /// support for. It's also common for valid versions to produce a different
+  /// result; for example "-swift-version 3" at one point instructed the
+  /// compiler to act as if it is version 3.1.
+  Optional<Version> getEffectiveLanguageVersion() const;
 
   /// Whether this version is in the Swift 3 family
   bool isVersion3() const { return !empty() && Components[0] == 3; }
@@ -141,6 +148,10 @@ bool operator==(const Version &lhs, const Version &rhs);
 raw_ostream &operator<<(raw_ostream &os, const Version &version);
 
 /// Retrieves the numeric {major, minor} Swift version.
+///
+/// Note that this is the underlying version of the language, ignoring any
+/// -swift-version flags that may have been used in a particular invocation of
+/// the compiler.
 std::pair<unsigned, unsigned> getSwiftNumericVersion();
 
 /// Retrieves a string representing the complete Swift version, which includes

lib/Basic/Version.cpp

@@ -242,16 +242,11 @@ Version Version::getCurrentCompilerVersion() {
 }
 
 Version Version::getCurrentLanguageVersion() {
-#ifndef SWIFT_VERSION_STRING
-#error Swift language version is not set!
+#if SWIFT_VERSION_PATCHLEVEL
+  return {SWIFT_VERSION_MAJOR, SWIFT_VERSION_MINOR, SWIFT_VERSION_PATCHLEVEL};
+#else
+  return {SWIFT_VERSION_MAJOR, SWIFT_VERSION_MINOR};
 #endif
-  auto currentVersion = Version::parseVersionString(
-    SWIFT_VERSION_STRING, SourceLoc(), nullptr);
-  assert(currentVersion.hasValue() &&
-         "Embedded Swift language version couldn't be parsed: '"
-         SWIFT_VERSION_STRING
-         "'");
-  return currentVersion.getValue();
 }
 
 raw_ostream &operator<<(raw_ostream &os, const Version &version) {
@@ -304,18 +299,35 @@ Version::operator clang::VersionTuple() const
   }
 }
 
-bool Version::isValidEffectiveLanguageVersion() const {
-  for (auto verStr : getValidEffectiveVersions()) {
-    auto v = parseVersionString(verStr, SourceLoc(), nullptr);
-    assert(v.hasValue());
-    // In this case, use logical-equality _and_ precision-equality. We do not
-    // want to permit users requesting effective language versions more precise
-    // than our whitelist (eg. we permit 3 but not 3.0 or 3.0.0), since
-    // accepting such an argument promises more than we're able to deliver.
-    if (v == *this && v.getValue().size() == size())
-      return true;
+Optional<Version> Version::getEffectiveLanguageVersion() const {
+  switch (size()) {
+  case 0:
+    return None;
+  case 1:
+    break;
+  default:
+    // We do not want to permit users requesting more precise effective language
+    // versions since accepting such an argument promises more than we're able
+    // to deliver.
+    return None;
+  }
+
+  // FIXME: When we switch to Swift 4 by default, the "3" case should return
+  // a version newer than any released 3.x compiler (probably "3.2"), and the
+  // "4" case should start returning getCurrentLanguageVersion. We should
+  // also check for the presence of SWIFT_VERSION_PATCHLEVEL, and if that's
+  // set apply it to the "3" case, so that Swift 4.0.1 will automatically
+  // have a compatibility mode of 3.2.1.
+  switch (Components[0]) {
+  case 3:
+    static_assert(SWIFT_VERSION_MAJOR == 3,
+                  "getCurrentLanguageVersion is no longer correct here");
+    return Version::getCurrentLanguageVersion();
+  case 4:
+    return Version{4, 0};
+  default:
+    return None;
   }
-  return false;
 }
 
 Version Version::asMajorVersion() const {

lib/Frontend/CompilerInvocation.cpp

@@ -802,7 +802,7 @@ static void diagnoseSwiftVersion(Optional<version::Version> &vers, Arg *verArg,
 
   // Check for an unneeded minor version, otherwise just list valid versions
   if (vers.hasValue() && !vers.getValue().empty() &&
-      vers.getValue().asMajorVersion().isValidEffectiveLanguageVersion()) {
+      vers.getValue().asMajorVersion().getEffectiveLanguageVersion()) {
     diags.diagnose(SourceLoc(), diag::note_swift_version_major,
                    vers.getValue()[0]);
   } else {
@@ -822,12 +822,15 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   if (auto A = Args.getLastArg(OPT_swift_version)) {
     auto vers = version::Version::parseVersionString(
       A->getValue(), SourceLoc(), &Diags);
-    if (vers.hasValue() &&
-        vers.getValue().isValidEffectiveLanguageVersion()) {
-      Opts.EffectiveLanguageVersion = vers.getValue();
-    } else {
-      diagnoseSwiftVersion(vers, A, Args, Diags);
+    bool isValid = false;
+    if (vers.hasValue()) {
+      if (auto effectiveVers = vers.getValue().getEffectiveLanguageVersion()) {
+        Opts.EffectiveLanguageVersion = effectiveVers.getValue();
+        isValid = true;
+      }
     }
+    if (!isValid)
+      diagnoseSwiftVersion(vers, A, Args, Diags);
   }
 
   Opts.AttachCommentsToDecls |= Args.hasArg(OPT_dump_api_path);

test/Driver/swift-version-default.swift

@@ -11,8 +11,26 @@ asdf // expected-error {{use of unresolved identifier}}
 jkl
 #endif
 
+#if swift(>=3.1)
+asdf // expected-error {{use of unresolved identifier}}
+#else
+jkl
+#endif
+
 #if swift(>=4)
 aoeu
 #else
 htn // expected-error {{use of unresolved identifier}}
 #endif
+
+#if swift(>=4.1)
+aoeu
+#else
+htn // expected-error {{use of unresolved identifier}}
+#endif
+
+#if swift(>=5)
+aoeu
+#else
+htn // expected-error {{use of unresolved identifier}}
+#endif

test/Driver/swift-version.swift

@@ -1,4 +1,3 @@
-// RUN: %target-swiftc_driver -swift-version 3 %s
 // RUN: not %target-swiftc_driver -swift-version foo %s 2>&1 | %FileCheck --check-prefix BAD %s
 // RUN: not %target-swiftc_driver -swift-version 1 %s 2>&1 | %FileCheck --check-prefix BAD %s
 // RUN: not %target-swiftc_driver -swift-version 2 %s 2>&1 | %FileCheck --check-prefix BAD %s
@@ -9,10 +8,50 @@
 // RUN: not %target-swiftc_driver -swift-version 3.3 %s 2>&1 | %FileCheck --check-prefix FIXIT_3 %s
 // RUN: not %target-swiftc_driver -swift-version 4.3 %s 2>&1 | %FileCheck --check-prefix FIXIT_4 %s
 
+// RUN: not %target-swiftc_driver -swift-version 3 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_3 %s
+// RUN: not %target-swiftc_driver -swift-version 4 -typecheck %s 2>&1 | %FileCheck --check-prefix ERROR_4 %s
+
 // BAD: invalid value
 // BAD: note: valid arguments to '-swift-version' are '3', '4'
 
 // FIXIT_3: use major version, as in '-swift-version 3'
 // FIXIT_4: use major version, as in '-swift-version 4'
 
-let x = 1
+
+#if swift(>=3)
+asdf
+// ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+#else
+jkl
+#endif
+
+#if swift(>=3.1)
+asdf
+// ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+#else
+jkl
+#endif
+
+#if swift(>=4)
+asdf // ERROR_4: [[@LINE]]:1: error: {{use of unresolved identifier}}
+#else
+jkl // ERROR_3: [[@LINE]]:1: error: {{use of unresolved identifier}}
+#endif
+
+#if swift(>=4.1)
+asdf
+#else
+jkl
+// ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+#endif
+
+#if swift(>=5)
+asdf
+#else
+jkl
+// ERROR_3: [[@LINE-1]]:1: error: {{use of unresolved identifier}}
+// ERROR_4: [[@LINE-2]]:1: error: {{use of unresolved identifier}}
+#endif

tools/swift-api-digester/swift-api-digester.cpp

@@ -3491,15 +3491,19 @@ static int prepareForDump(const char *Main,
   options::ModuleCachePath;
 
   if (!options::SwiftVersion.empty()) {
-    if (auto Version = version::Version::
-        parseVersionString(options::SwiftVersion, SourceLoc(), nullptr)) {
-      if (Version.getValue().isValidEffectiveLanguageVersion())
-        InitInvok.getLangOptions().EffectiveLanguageVersion = Version.getValue();
-      else {
-        llvm::errs() << "Unsupported Swift Version.\n";
-        return 1;
+    using version::Version;
+    bool isValid = false;
+    if (auto Version = Version::parseVersionString(options::SwiftVersion,
+                                                   SourceLoc(), nullptr)) {
+      if (auto Effective = Version.getValue().getEffectiveLanguageVersion()) {
+        InitInvok.getLangOptions().EffectiveLanguageVersion = *Effective;
+        isValid = true;
       }
     }
+    if (!isValid) {
+      llvm::errs() << "Unsupported Swift Version.\n";
+      return 1;
+    }
   }
 
   if (!options::ResourceDir.empty()) {

tools/swift-ide-test/swift-ide-test.cpp

@@ -2957,7 +2957,8 @@ int main(int argc, char *argv[]) {
     if (auto swiftVersion =
           version::Version::parseVersionString(options::SwiftVersion,
                                                SourceLoc(), nullptr)) {
-      InitInvok.getLangOptions().EffectiveLanguageVersion = *swiftVersion;
+      if (auto actual = swiftVersion.getValue().getEffectiveLanguageVersion())
+        InitInvok.getLangOptions().EffectiveLanguageVersion = actual.getValue();
     }
   }
   InitInvok.getClangImporterOptions().ModuleCachePath =