Sema: Don't report ambiguities on an explicit public import

Silence errors on ambiguous implicitly public imports when a different import
is explicitly public. In this case all imports are public so the ambiguity is
not dangerous and this configuration can happen during adoption of
access-levels on imports. Keep the error when an explicit import is package,
internal or below.

rdar://129037503

Author: xymus

lib/Sema/ImportResolution.cpp

@@ -1074,6 +1074,13 @@ CheckInconsistentAccessLevelOnImport::evaluate(
       return;
 
     auto otherAccessLevel = otherImport->getAccessLevel();
+
+    // Only report ambiguities with non-public imports as bare imports are
+    // public when this diagnostic is active. Do not report ambiguities
+    // between implicitly vs explicitly public.
+    if (otherAccessLevel == AccessLevel::Public)
+      return;
+
     auto &diags = mod->getDiags();
     {
       InFlightDiagnostic error =

test/Sema/access-level-import-inconsistencies.swift

@@ -8,7 +8,7 @@
 /// Don't report any mismatch.
 
 // RUN: %empty-directory(%t)
-// RUN: split-file %s %t
+// RUN: split-file --leading-lines %s %t
 
 /// Build the library.
 // RUN: %target-swift-frontend -emit-module %t/Lib.swift -o %t
@@ -68,6 +68,14 @@ import Lib
 //--- ManyFiles_AmbiguitySwift6_FileB.swift
 internal import Lib
 
+/// Don't report an ambiguity between an explicitly vs implicitly public import.
+// RUN: %target-swift-frontend -typecheck %t/ManyFiles_AmbiguityPublicExplicitVsImplicit_File?.swift -I %t \
+// RUN:  -verify
+//--- ManyFiles_AmbiguityPublicExplicitVsImplicit_FileA.swift
+import Lib
+//--- ManyFiles_AmbiguityPublicExplicitVsImplicit_FileB.swift
+public import Lib // expected-warning {{public import of 'Lib' was not used in public declarations or inlinable code}}
+
 /// Don't report inconsistencies from a file generated by Xcode.
 // RUN: %target-swift-frontend -typecheck -I %t \
 // RUN    -primary-file %t/GeneratedAssetSymbols.swift \

test/Sema/access-level-import-inconsistent-same-file.swift

@@ -63,10 +63,7 @@ internal import Lib1 // expected-warning {{module 'Lib1' is imported as 'public'
 
 // There's no warning about "will be ignored" for a matching implicit access level.
 public import Lib2
-// expected-note @-1 {{imported 'public' here}}
 import Lib2
-// expected-error @-1 {{ambiguous implicit access level for import of 'Lib2'; it is imported as 'public' elsewhere}}
-// expected-note @-2 {{silence these warnings by adopting the upcoming feature 'InternalImportsByDefault'}}
 
 public func dummyAPI(t: Lib1.Type1, t2: Lib2.Type1) {}
 

test/Sema/superfluously-public-imports.swift

@@ -123,10 +123,8 @@ public struct Extended {
 
 //--- Client_Swift5.swift
 /// No diagnostics should be raised on the implicit access level.
-import UnusedImport // expected-error {{ambiguous implicit access level for import of 'UnusedImport'; it is imported as 'public' elsewhere}}
-// expected-note @-1 {{silence these warnings by adopting the upcoming feature 'InternalImportsByDefault'}}
+import UnusedImport
 public import UnusedImport // expected-warning {{public import of 'UnusedImport' was not used in public declarations or inlinable code}} {{1-7=internal}}
-// expected-note @-1 {{imported 'public' here}}
 
 //--- Client.swift
 public import DepUsedFromInlinableCode