[clang-tools-extra] [clangd] Handle template template parameters in TargetFinder::VisitDeducedTemplateSpecializationType (PR #228601)

[Nathan Ridge via cfe-commits]

https://github.com/HighCommander4 updated https://github.com/llvm/llvm-project/pull/228601

>From 210a5db3cca9cd0a0ba27b1d43a47ec990ff2350 Mon Sep 17 00:00:00 2001
From: Nathan Ridge <zeratul976 at hotmail.com>
Date: Fri, 2 Oct 2026 15:21:43 -0400
Subject: [PATCH] [clangd] Handle template template parameters in
 TargetFinder::VisitDeducedTemplateSpecializationType

Fixes https://github.com/clangd/clangd/issues/1870
---
 clang-tools-extra/clangd/FindTarget.cpp       | 20 +++++++++++------
 .../clangd/unittests/FindTargetTests.cpp      | 22 +++++++++++++++++++
 2 files changed, 35 insertions(+), 7 deletions(-)

diff --git a/clang-tools-extra/clangd/FindTarget.cpp b/clang-tools-extra/clangd/FindTarget.cpp
index 142776d0dfaa7..a3379d1337d62 100644
--- a/clang-tools-extra/clangd/FindTarget.cpp
+++ b/clang-tools-extra/clangd/FindTarget.cpp
@@ -397,14 +397,20 @@ struct TargetFinder {
         if (const auto *USD = DTST->getTemplateName().getAsUsingShadowDecl())
           Outer.add(USD, Flags);
 
-        // FIXME: This is a workaround for https://llvm.org/PR42914,
-        // which is causing DTST->getDeducedType() to be empty. We
-        // fall back to the template pattern and miss the instantiation
-        // even when it's known in principle. Once that bug is fixed,
-        // the following code can be removed (the existing handling in
-        // VisitDeducedType() is sufficient).
-        if (auto *TD = DTST->getTemplateName().getAsTemplateDecl())
+        // Template template parameters have no templated decl, so they must
+        // refer to the parameter itself.
+        if (const auto *Parm =
+                DTST->getTemplateName().getAsTemplateTemplateParmDecl()) {
+          Outer.add(Parm, Flags);
+        } else if (auto *TD = DTST->getTemplateName().getAsTemplateDecl()) {
+          // FIXME: This is a workaround for https://llvm.org/PR42914,
+          // which is causing DTST->getDeducedType() to be empty. We
+          // fall back to the template pattern and miss the instantiation
+          // even when it's known in principle. Once that bug is fixed,
+          // the following code can be removed (the existing handling in
+          // VisitDeducedType() is sufficient).
           Outer.add(TD->getTemplatedDecl(), Flags | Rel::TemplatePattern);
+        }
       }
       void VisitDependentNameType(const DependentNameType *DNT) {
         if (Outer.Resolver) {
diff --git a/clang-tools-extra/clangd/unittests/FindTargetTests.cpp b/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
index 0c0ef1262141f..94837803cd64c 100644
--- a/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
+++ b/clang-tools-extra/clangd/unittests/FindTargetTests.cpp
@@ -593,6 +593,16 @@ TEST_F(TargetDeclTest, ClassTemplate) {
   EXPECT_DECLS("DeducedTemplateSpecializationTypeLoc",
                {"struct Test", Rel::TemplatePattern});
 
+  Code = R"cpp(
+    // Deduced specialization of a template template parameter
+    template <template<typename> class X>
+    void foo() {
+      [[X]] a;
+    }
+  )cpp";
+  EXPECT_DECLS("DeducedTemplateSpecializationTypeLoc",
+               "template <typename> class X");
+
   Code = R"cpp(
     // Deduction guide
     template <typename T>
@@ -604,6 +614,18 @@ TEST_F(TargetDeclTest, ClassTemplate) {
     [[Test]](I, I) -> Test<typename I::type>;
   )cpp";
   EXPECT_DECLS("CXXDeductionGuideDecl", {"template <typename T> struct Test"});
+
+  Flags.push_back("-std=c++26"); // for pack indexing
+
+  Code = R"cpp(
+    // Deduced specialization of an indexed template template parameter pack
+    template <template <typename> class... X>
+    void foo() {
+      [[X]]...[0] a(1);
+    }
+  )cpp";
+  EXPECT_DECLS("DeducedTemplateSpecializationTypeLoc",
+               "template <typename> class ...X");
 }
 
 TEST_F(TargetDeclTest, Concept) {