Frontend: Downgrade 'already enabled' to a warning for experimental features.

If an upcoming feature was enabled by passing it via `-enable-experimental-feature`,
downgrade the `already enabled` diagnostic to a warning.

Resolves rdar://139087679.

Author: tshortli

lib/Frontend/CompilerInvocation.cpp

@@ -1015,13 +1015,16 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.EnableExperimentalStringProcessing = true;
   }
 
-  auto enableUpcomingFeature = [&Opts, &Diags](Feature feature) -> bool {
+  auto enableUpcomingFeature = [&Opts, &Diags](Feature feature,
+                                               bool downgradeDiag) -> bool {
     // Check if this feature was introduced already in this language version.
     if (auto firstVersion = getFeatureLanguageVersion(feature)) {
       if (Opts.isSwiftVersionAtLeast(*firstVersion)) {
-        Diags.diagnose(SourceLoc(), diag::error_upcoming_feature_on_by_default,
-                       getFeatureName(feature), *firstVersion);
-        return true;
+        Diags
+            .diagnose(SourceLoc(), diag::error_upcoming_feature_on_by_default,
+                      getFeatureName(feature), *firstVersion)
+            .limitBehaviorIf(downgradeDiag, DiagnosticBehavior::Warning);
+        return !downgradeDiag;
       }
     }
 
@@ -1062,7 +1065,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     // -enable-experimental-feature flag too since the feature may have
     // graduated from being experimental.
     if (auto feature = getUpcomingFeature(value)) {
-      if (enableUpcomingFeature(*feature))
+      if (enableUpcomingFeature(*feature, /*downgradeDiag=*/true))
         HadError = true;
     }
 
@@ -1087,7 +1090,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     if (!feature)
       continue;
 
-    if (enableUpcomingFeature(*feature))
+    if (enableUpcomingFeature(*feature, /*downgradeDiag=*/false))
       HadError = true;
   }
 

test/Frontend/upcoming_feature.swift

@@ -1,29 +1,30 @@
 // Make sure that hasFeature(ConciseMagicFile) evaluates true when provided
 // explicitly.
-// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile %s
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ConciseMagicFile
 
 // Make sure that hasFeature(ConciseMagicFile) evaluates true in Swift 6.
-// RUN: %target-swift-frontend -typecheck -swift-version 6 %s
+// RUN: %target-typecheck-verify-swift -swift-version 6
 
 // Make sure that hasFeature(ConciseMagicFile) is off prior to Swift 6
-// RUN: %target-typecheck-verify-swift %s
+// RUN: %target-typecheck-verify-swift -verify-additional-prefix swift5-
 
 // It's fine to provide a feature that we don't know about
-// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile -enable-upcoming-feature UnknownFeature %s
-// RUN: %target-swift-frontend -typecheck -enable-upcoming-feature UnknownFeature -enable-upcoming-feature ConciseMagicFile %s
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature ConciseMagicFile -enable-upcoming-feature UnknownFeature
+// RUN: %target-typecheck-verify-swift -enable-upcoming-feature UnknownFeature -enable-upcoming-feature ConciseMagicFile
 
 // For compatibility when a feature graduates, it's fine to refer to an
 // upcoming feature as an experimental feature.
-// RUN: %target-swift-frontend -typecheck -enable-experimental-feature ConciseMagicFile %s
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature ConciseMagicFile
 
 // It's not fine to provide a feature that's in the specified language version.
-// RUN: not %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s
-// RUN: not %target-swift-frontend -typecheck -enable-experimental-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s
+// RUN: not %target-swift-frontend -typecheck -enable-upcoming-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s --check-prefix=CHECK-ERROR
+// RUN: %target-swift-frontend -typecheck -enable-experimental-feature ConciseMagicFile -swift-version 6 %s 2>&1 | %FileCheck %s --check-prefix=CHECK-WARN
 
-// CHECK: error: upcoming feature 'ConciseMagicFile' is already enabled as of Swift version 6
+// CHECK-ERROR: error: upcoming feature 'ConciseMagicFile' is already enabled as of Swift version 6
+// CHECK-WARN: warning: upcoming feature 'ConciseMagicFile' is already enabled as of Swift version 6
 
 #if hasFeature(ConciseMagicFile)
 let x = 0
 #else
-let y = boom // expected-error{{'boom'}}
+let y = boom // expected-swift5-error{{'boom'}}
 #endif