[cxx-interop] Diagnose missing type witness instead of crashing

rdar://157473792

Author: Gabor Horvath

lib/ClangImporter/ImportDecl.cpp

@@ -27,6 +27,7 @@
 #include "swift/AST/ConformanceLookup.h"
 #include "swift/AST/Decl.h"
 #include "swift/AST/DiagnosticsClangImporter.h"
+#include "swift/AST/DiagnosticsSema.h"
 #include "swift/AST/ExistentialLayout.h"
 #include "swift/AST/Expr.h"
 #include "swift/AST/GenericEnvironment.h"
@@ -70,9 +71,9 @@
 #include "clang/AST/Type.h"
 #include "clang/Basic/Specifiers.h"
 #include "clang/Basic/TargetInfo.h"
-#include "clang/Sema/SemaDiagnostic.h"
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Sema/Lookup.h"
+#include "clang/Sema/SemaDiagnostic.h"
 
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallBitVector.h"
@@ -10113,8 +10114,8 @@ void ClangImporter::Implementation::startedImportingEntity() {
 }
 
 /// Look up associated type requirements in the conforming type.
-static void finishTypeWitnesses(
-    NormalProtocolConformance *conformance) {
+static void finishTypeWitnesses(NormalProtocolConformance *conformance,
+                                ASTContext &ctx) {
   auto *dc = conformance->getDeclContext();
   auto nominal = dc->getSelfNominalTypeDecl();
 
@@ -10158,11 +10159,11 @@ static void finishTypeWitnesses(
     }
 
     if (!satisfied) {
-      ABORT([&](auto &out) {
-        out << "Cannot look up associated type for imported conformance:\n";
-        conformance->getType().dump(out);
-        assocType->dump(out);
-      });
+      // Avoid compiler crash due to missing witness.
+      conformance->setTypeWitness(assocType, ErrorType::get(ctx), assocType);
+      proto->diagnose(diag::type_does_not_conform, selfType,
+                      proto->getDeclaredType());
+      proto->diagnose(diag::no_witnesses_type, assocType);
     }
   }
 }
@@ -10210,7 +10211,7 @@ void ClangImporter::Implementation::finishNormalConformance(
   auto *proto = conformance->getProtocol();
   PrettyStackTraceConformance trace("completing import of", conformance);
 
-  finishTypeWitnesses(conformance);
+  finishTypeWitnesses(conformance, SwiftContext);
 
   // Imported conformances to @objc protocols also require additional
   // initialization to complete the requirement to witness mapping.

test/Interop/Cxx/class/conforms-to-missing-associated-type.swift

@@ -0,0 +1,31 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/include)
+// RUN: split-file %s %t
+//
+// RUN: %target-swift-frontend -typecheck -module-name a -cxx-interoperability-mode=default -I %t/include %t/a.swift -verify
+
+//--- include/module.modulemap
+module cxx {
+  header "header.h"
+  export *
+}
+
+//--- include/header.h
+struct S {
+  S() {}
+} __attribute__((swift_attr("conforms_to:a.P")));
+
+//--- a.swift
+import cxx
+// expected-error@+2{{type 'S' does not conform to protocol 'P'}}
+// expected-note@+1{{protocol requires nested type 'A'}}
+public protocol P {
+  associatedtype A
+  func foo(_: A)
+}
+extension P {
+  func foo(_: A) {}
+}
+func test(x : S) {
+    x.foo(0) // expected-error{{cannot convert value of type 'Int' to expected argument type 'S.A'}}
+}