[clangd] Avoid triggering linkage computation for decl with unstable linkage in SymbolRelevanceSignals::computeASTSignals()

[nridge]

Summary

Fixes https://github.com/clangd/clangd/issues/1427

[nridge]

clang-tools-extra/clangd/Quality.cpp

306

Would it be better to reuse this getSymbolID overload instead? (Would require plumbing in a SourceManager to this call site.)

[sammccall]

Thanks!

clang-tools-extra/clangd/Quality.cpp

306

Ah, that requirement is unfortunate. It'd be nice if it worked but not worth changing signature I think.

clang-tools-extra/clangd/Quality.cpp

 void SymbolRelevanceSignals::computeASTSignals(
     const CodeCompletionResult &SemaResult) {
   if (!MainFileSignals)
     return;
   if ((SemaResult.Kind != CodeCompletionResult::RK_Declaration) &&
       (SemaResult.Kind != CodeCompletionResult::RK_Pattern))
     return;
   if (const NamedDecl *ND = SemaResult.getDeclaration()) {
+    if (hasUnstableLinkage(ND))
+      return;
     auto ID = getSymbolID(ND);
     if (!ID)
       return;
     MainFileRefs =
         std::max(MainFileRefs, MainFileSignals->ReferencedSymbols.lookup(ID));
     if (const auto *NSD = dyn_cast<NamespaceDecl>(ND->getDeclContext())) {
       if (NSD->isAnonymousNamespace())
         return;

clang-tools-extra/clangd/unittests/CodeCompleteTests.cpp

auto Results = completions(R"cpp(
               }
               return n * ^(f)(n - 1);
             };
           })(10);
         }
     )cpp");
 }
 
+TEST(CompletionTest, Issue1427Crash) {
+    // Need to provide main file signals to ensure that the branch in
+    // SymbolRelevanceSignals::computeASTSignals() that tries to
+    // compute a symbol ID is taken.
+    ASTSignals MainFileSignals;
+    CodeCompleteOptions Opts;
+    Opts.MainFileSignals = &MainFileSignals;
+    completions(R"cpp(
+      auto f = []() {
+        1.0_^
+      };
+    )cpp",
+                {}, Opts);
+}
+
 TEST(CompletionTest, BacktrackCrashes) {
   // Sema calls code completion callbacks twice in these cases.
   auto Results = completions(R"cpp(
       namespace ns {
       struct FooBarBaz {};
       } // namespace ns
 
      int foo(ns::FooBar^