[cxx-interop] Add diagnostics for explicit protocol conformance on the C++ side.

Author: zoecarver

include/swift/AST/DiagnosticsClangImporter.def

@@ -199,6 +199,16 @@ ERROR(foreign_reference_types_invalid_release,none,
       "specified release function '%0' is invalid. Release must have exactly "
       "one argument of type '%1'", (StringRef, StringRef))
 
+ERROR(cannot_find_conforms_to,none,
+      "specified protocol conformance could not be found '%0'.", (StringRef))
+
+ERROR(conforms_to_ambiguous,none,
+      "specified protocol conformance is ambiguous. "
+      "Found multiple protocols named '%0'.", (StringRef))
+
+ERROR(conforms_to_not_protocol,none,
+      "specified protocol conformance '%0' is not a protocol.", (StringRef))
+
 NOTE(unsupported_builtin_type, none, "built-in type '%0' not supported", (StringRef))
 NOTE(record_field_not_imported, none, "field %0 not imported", (const clang::NamedDecl*))
 NOTE(invoked_func_not_imported, none, "function %0 not imported", (const clang::NamedDecl*))

lib/ClangImporter/ImportDecl.cpp

@@ -2634,7 +2634,7 @@ namespace {
     void addExplicitProtocolConformances(NominalTypeDecl *decl) {
       auto clangDecl = decl->getClangDecl();
 
-      SmallVector<ValueDecl *, 2> results;
+      SmallVector<ValueDecl *, 1> results;
       auto conformsToAttr =
           llvm::find_if(clangDecl->getAttrs(), [](auto *attr) {
             if (auto swiftAttr = dyn_cast<clang::SwiftAttrAttr>(attr))
@@ -2653,11 +2653,24 @@ namespace {
         module.second->lookupValue(Impl.SwiftContext.getIdentifier(name),
                                    NLKind::UnqualifiedLookup, results);
       }
-      for (auto proto : results) {
-        if (auto protocol = dyn_cast<ProtocolDecl>(proto)) {
-          decl->getAttrs().add(
-              new (Impl.SwiftContext) SynthesizedProtocolAttr(protocol, &Impl, false));
-        }
+
+      if (results.empty()) {
+        HeaderLoc attrLoc((*conformsToAttr)->getLocation());
+        Impl.diagnose(attrLoc, diag::cannot_find_conforms_to, name);
+        return;
+      } else if (results.size() != 1) {
+        HeaderLoc attrLoc((*conformsToAttr)->getLocation());
+        Impl.diagnose(attrLoc, diag::conforms_to_ambiguous, name);
+        return;
+      }
+
+      auto result = results.front();
+      if (auto protocol = dyn_cast<ProtocolDecl>(result)) {
+        decl->getAttrs().add(
+            new (Impl.SwiftContext) SynthesizedProtocolAttr(protocol, &Impl, false));
+      } else {
+        HeaderLoc attrLoc((*conformsToAttr)->getLocation());
+        Impl.diagnose(attrLoc, diag::conforms_to_not_protocol, name);
       }
     }
 

test/Interop/Cxx/class/conforms-to-errors.swift

@@ -0,0 +1,30 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: not %target-swift-frontend -typecheck -I %t/Inputs  %t/test.swift  -enable-experimental-cxx-interop 2>&1 | %FileCheck %s
+
+//--- Inputs/module.modulemap
+module Test {
+    header "test.h"
+    requires cplusplus
+}
+
+//--- Inputs/test.h
+
+struct  __attribute__((swift_attr("conforms_to:X"))) CX {};
+struct  __attribute__((swift_attr("conforms_to:A"))) CA {};
+struct  __attribute__((swift_attr("conforms_to:B"))) CB {};
+
+//--- test.swift
+
+import Test
+
+struct B {}
+
+protocol A {}
+protocol A {}
+
+// CHECK: error: specified protocol conformance could not be found 'X'.
+// CHECK: error: specified protocol conformance is ambiguous. Found multiple protocols named 'A'.
+// CHECK: error: specified protocol conformance 'B' is not a protocol.
+
+func test(_ x: CX, _ a: CA, _ b: CB) {}