[Sema] Make public imports of private modules an error

rdar://83731983

Author: xymus

include/swift/AST/DiagnosticsSema.def

@@ -2907,7 +2907,7 @@ WARNING(warn_implementation_only_conflict,none,
         (Identifier))
 NOTE(implementation_only_conflict_here,none,
      "imported as implementation-only here", ())
-WARNING(warn_public_import_of_private_module,none,
+ERROR(error_public_import_of_private_module,none,
         "private module %0 is imported publicly from the public module %1",
         (Identifier, Identifier))
 

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -1709,7 +1709,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
 
         auto &diags = ID->getASTContext().Diags;
         InFlightDiagnostic inFlight =
-            diags.diagnose(ID, diag::warn_public_import_of_private_module,
+            diags.diagnose(ID, diag::error_public_import_of_private_module,
                            target->getName(), importer->getName());
         if (ID->getAttrs().isEmpty()) {
            inFlight.fixItInsert(ID->getStartLoc(),

test/Sema/implementation-only-import-suggestion.swift

@@ -34,11 +34,11 @@
 // RUN:   -library-level other -D PUBLIC_IMPORTS
 #if PUBLIC_IMPORTS
 import PublicSwift
-import PrivateSwift // expected-warning{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
+import PrivateSwift // expected-error{{private module 'PrivateSwift' is imported publicly from the public module 'main'}}
 
 import PublicClang
-import PublicClang_Private // expected-warning{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
-import FullyPrivateClang // expected-warning{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
+import PublicClang_Private // expected-error{{private module 'PublicClang_Private' is imported publicly from the public module 'main'}}
+import FullyPrivateClang // expected-error{{private module 'FullyPrivateClang' is imported publicly from the public module 'main'}}
 import main // expected-warning{{'implementation-only-import-suggestion.swift' is part of module 'main'; ignoring import}}
 
 /// Expect no warnings with implementation-only imports.