Parse: Only diagnose renamed @_inlineable and @_versioned in -swift-version 4.2

Normally we don't gate *warnings* on -swift-version flags, but
SwiftNIO makes use of @_inlineable / @_versioned and wants to
continue building with Swift 4.1 until 4.2 is released.

Fixes <https://bugs.swift.org/browse/SR-7578>,
<rdar://problem/40717640>.

Author: slavapestov

lib/Parse/ParseDecl.cpp

@@ -1537,15 +1537,17 @@ bool Parser::parseDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc) {
   // over to the alternate parsing path.
   DeclAttrKind DK = DeclAttribute::getAttrKindFromString(Tok.getText());
 
-  auto checkInvalidAttrName = [&](StringRef invalidName, StringRef correctName,
-                              DeclAttrKind kind, Diag<StringRef, StringRef> diag) {
+  auto checkInvalidAttrName = [&](StringRef invalidName,
+                                  StringRef correctName,
+                                  DeclAttrKind kind,
+                                  Optional<Diag<StringRef, StringRef>> diag = None) {
     if (DK == DAK_Count && Tok.getText() == invalidName) {
-      // We renamed @availability to @available, so if we see the former,
-      // treat it as the latter and emit a Fix-It.
       DK = kind;
-      
-      diagnose(Tok, diag, invalidName, correctName)
+
+      if (diag) {
+        diagnose(Tok, *diag, invalidName, correctName)
             .fixItReplace(Tok.getLoc(), correctName);
+      }
     }
   };
 
@@ -1556,14 +1558,17 @@ bool Parser::parseDeclAttribute(DeclAttributes &Attributes, SourceLoc AtLoc) {
   // Check if attr is inlineable, and suggest inlinable instead
   checkInvalidAttrName("inlineable", "inlinable", DAK_Inlinable, diag::attr_name_close_match);
 
-  // In Swift 5 and above, these become hard errors. Otherwise, emit a
-  // warning for compatibility.
-  if (!Context.isSwiftVersionAtLeast(5)) {
+  // In Swift 5 and above, these become hard errors. In Swift 4.2, emit a
+  // warning for compatibility. Otherwise, don't diagnose at all.
+  if (Context.isSwiftVersionAtLeast(5)) {
+    checkInvalidAttrName("_versioned", "usableFromInline", DAK_UsableFromInline, diag::attr_renamed);
+    checkInvalidAttrName("_inlineable", "inlinable", DAK_Inlinable, diag::attr_renamed);
+  } else if (Context.isSwiftVersionAtLeast(4, 2)) {
     checkInvalidAttrName("_versioned", "usableFromInline", DAK_UsableFromInline, diag::attr_renamed_warning);
     checkInvalidAttrName("_inlineable", "inlinable", DAK_Inlinable, diag::attr_renamed_warning);
   } else {
-    checkInvalidAttrName("_versioned", "usableFromInline", DAK_UsableFromInline, diag::attr_renamed);
-    checkInvalidAttrName("_inlineable", "inlinable", DAK_Inlinable, diag::attr_renamed);
+    checkInvalidAttrName("_versioned", "usableFromInline", DAK_UsableFromInline);
+    checkInvalidAttrName("_inlineable", "inlinable", DAK_Inlinable);
   }
 
   if (DK == DAK_Count && Tok.getText() == "warn_unused_result") {

test/Compatibility/attr_inlinable_old_spelling_4.swift

@@ -0,0 +1,7 @@
+// RUN: %target-typecheck-verify-swift -swift-version 3
+// RUN: %target-typecheck-verify-swift -swift-version 4
+
+// In -swift-version 4 mode, the old spelling is silently parsed as the new spelling.
+
+@_inlineable public func oldInlinableFunction() {}
+@_versioned func oldVersionedFunction() {}

test/Compatibility/attr_inlinable_old_spelling.swift renamed to test/Compatibility/attr_inlinable_old_spelling_42.swift

@@ -1,8 +1,6 @@
-// RUN: %target-typecheck-verify-swift -swift-version 3
-// RUN: %target-typecheck-verify-swift -swift-version 4
 // RUN: %target-typecheck-verify-swift -swift-version 4.2
 
-// Until -swift-version 5 mode, the old spelling only produces a warning.
+// In -swift-version 4.2 mode, the old spelling produces a warning.
 
 @_inlineable public func oldInlinableFunction() {}
 // expected-warning@-1 {{'@_inlineable' has been renamed to '@inlinable'}}{{2-13=inlinable}}