performTypeChecking: Defer verifyAllLoadedModules in WMO mode.

Suggested by Jordan Rose.

Defer per-file verification in whole-module mode. Verifying imports between
files could cause the importer to cache declarations without adding them to the
ASTContext. This happens when the importer registers a declaration without a
valid TypeChecker instance, as is the case during verification. A subsequent
file may require that declaration to be fully imported (e.g. to synthesized a
function body), but since it has already been cached, it will never be added to
the ASTContext. The solution is to skip verification and avoid caching
it. Instead, loaded modules are now verified in WMO mode once during
performWholeModuleTypeChecking.

Fixes <rdar:36801676> [Edge][swift 4.1] SIL verification failed: external
declarations of SILFunctions with shared visibility is not allowed.

Author: atrick

lib/Sema/TypeChecker.cpp

@@ -675,8 +675,18 @@ void swift::performTypeChecking(SourceFile &SF, TopLevelContext &TLC,
     verify(SF);
 
     // Verify imported modules.
+    //
+    // Skip per-file verification in whole-module mode. Verifying imports
+    // between files could cause the importer to cache declarations without
+    // adding them to the ASTContext. This happens when the importer registers a
+    // declaration without a valid TypeChecker instance, as is the case during
+    // verification. A subsequent file may require that declaration to be fully
+    // imported (e.g. to synthesized a function body), but since it has already
+    // been cached, it will never be added to the ASTContext. The solution is to
+    // skip verification and avoid caching it.
 #ifndef NDEBUG
-    if (SF.Kind != SourceFileKind::REPL &&
+    if (!(Options & TypeCheckingFlags::DelayWholeModuleChecking) &&
+        SF.Kind != SourceFileKind::REPL &&
         SF.Kind != SourceFileKind::SIL &&
         !Ctx.LangOpts.DebuggerSupport) {
       Ctx.verifyAllLoadedModules();
@@ -692,6 +702,16 @@ void swift::performWholeModuleTypeChecking(SourceFile &SF) {
   Ctx.diagnoseObjCMethodConflicts(SF);
   Ctx.diagnoseObjCUnsatisfiedOptReqConflicts(SF);
   Ctx.diagnoseUnintendedObjCMethodOverrides(SF);
+
+  // In whole-module mode, import verification is deferred until all files have
+  // been type checked. This avoids caching imported declarations when a valid
+  // type checker is not present. The same declaration may need to be fully
+  // imported by subsequent files.
+  if (SF.Kind != SourceFileKind::REPL &&
+      SF.Kind != SourceFileKind::SIL &&
+      !Ctx.LangOpts.DebuggerSupport) {
+    Ctx.verifyAllLoadedModules();
+  }
 }
 
 bool swift::performTypeLocChecking(ASTContext &Ctx, TypeLoc &T,