[clang][HeuristicResolver] Default argument heuristic for template parameters #131074

HighCommander4 · 2025-03-13T05:25:14Z

Fixes clangd/clangd#1056

llvmbot · 2025-03-14T16:13:45Z

@llvm/pr-subscribers-clang-tools-extra

@llvm/pr-subscribers-clang

Author: Nathan Ridge (HighCommander4)

Changes

Fixes clangd/clangd#1056

Full diff: https://github.com/llvm/llvm-project/pull/131074.diff

3 Files Affected:

(modified) clang-tools-extra/clangd/unittests/XRefsTests.cpp (+1-1)
(modified) clang/lib/Sema/HeuristicResolver.cpp (+15)
(modified) clang/unittests/Sema/HeuristicResolverTest.cpp (+17)

diff --git a/clang-tools-extra/clangd/unittests/XRefsTests.cpp b/clang-tools-extra/clangd/unittests/XRefsTests.cpp
index e12d7691c58fb..693e965e78a96 100644
--- a/clang-tools-extra/clangd/unittests/XRefsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/XRefsTests.cpp
@@ -1091,7 +1091,7 @@ TEST(LocateSymbol, All) {
       )objc",
       R"cpp(
         struct PointerIntPairInfo {
-          static void *getPointer(void *Value);
+          static void *$decl[[getPointer]](void *Value);
         };
 
         template <typename Info = PointerIntPairInfo> struct PointerIntPair {
diff --git a/clang/lib/Sema/HeuristicResolver.cpp b/clang/lib/Sema/HeuristicResolver.cpp
index d377379c627db..feda9696b8e05 100644
--- a/clang/lib/Sema/HeuristicResolver.cpp
+++ b/clang/lib/Sema/HeuristicResolver.cpp
@@ -11,6 +11,7 @@
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 
 namespace clang {
@@ -125,6 +126,20 @@ TagDecl *HeuristicResolverImpl::resolveTypeToTagDecl(QualType QT) {
   if (!T)
     return nullptr;
 
+  // If T is the type of a template parameter, we can't get a useful TagDecl
+  // out of it. However, if the template parameter has a default argument,
+  // as a heuristic we can replace T with the default argument type.
+  if (const auto *TTPT = dyn_cast<TemplateTypeParmType>(T)) {
+    if (const auto *TTPD = TTPT->getDecl()) {
+      if (TTPD->hasDefaultArgument()) {
+        const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+        if (DefaultArg.getKind() == TemplateArgument::Type) {
+          T = DefaultArg.getAsType().getTypePtrOrNull();
+        }
+      }
+    }
+  }
+
   // Unwrap type sugar such as type aliases.
   T = T->getCanonicalTypeInternal().getTypePtr();
 
diff --git a/clang/unittests/Sema/HeuristicResolverTest.cpp b/clang/unittests/Sema/HeuristicResolverTest.cpp
index c7cfe7917c532..5e36108172702 100644
--- a/clang/unittests/Sema/HeuristicResolverTest.cpp
+++ b/clang/unittests/Sema/HeuristicResolverTest.cpp
@@ -410,6 +410,23 @@ TEST(HeuristicResolver, MemberExpr_HangIssue126536) {
       cxxDependentScopeMemberExpr(hasMemberName("foo")).bind("input"));
 }
 
+TEST(HeuristicResolver, MemberExpr_DefaultTemplateArgument) {
+  std::string Code = R"cpp(
+    struct Default {
+      void foo();
+    };
+    template <typename T = Default>
+    void bar(T t) {
+      t.foo();
+    }
+  )cpp";
+  // Test resolution of "foo" in "t.foo()".
+  expectResolution(
+      Code, &HeuristicResolver::resolveMemberExpr,
+      cxxDependentScopeMemberExpr(hasMemberName("foo")).bind("input"),
+      cxxMethodDecl(hasName("foo")).bind("output"));
+}
+
 TEST(HeuristicResolver, DeclRefExpr_StaticMethod) {
   std::string Code = R"cpp(
     template <typename T>

llvmbot · 2025-03-14T16:13:45Z

@llvm/pr-subscribers-clangd

Author: Nathan Ridge (HighCommander4)

Changes

Fixes clangd/clangd#1056

Full diff: https://github.com/llvm/llvm-project/pull/131074.diff

3 Files Affected:

(modified) clang-tools-extra/clangd/unittests/XRefsTests.cpp (+1-1)
(modified) clang/lib/Sema/HeuristicResolver.cpp (+15)
(modified) clang/unittests/Sema/HeuristicResolverTest.cpp (+17)

diff --git a/clang-tools-extra/clangd/unittests/XRefsTests.cpp b/clang-tools-extra/clangd/unittests/XRefsTests.cpp
index e12d7691c58fb..693e965e78a96 100644
--- a/clang-tools-extra/clangd/unittests/XRefsTests.cpp
+++ b/clang-tools-extra/clangd/unittests/XRefsTests.cpp
@@ -1091,7 +1091,7 @@ TEST(LocateSymbol, All) {
       )objc",
       R"cpp(
         struct PointerIntPairInfo {
-          static void *getPointer(void *Value);
+          static void *$decl[[getPointer]](void *Value);
         };
 
         template <typename Info = PointerIntPairInfo> struct PointerIntPair {
diff --git a/clang/lib/Sema/HeuristicResolver.cpp b/clang/lib/Sema/HeuristicResolver.cpp
index d377379c627db..feda9696b8e05 100644
--- a/clang/lib/Sema/HeuristicResolver.cpp
+++ b/clang/lib/Sema/HeuristicResolver.cpp
@@ -11,6 +11,7 @@
 #include "clang/AST/CXXInheritance.h"
 #include "clang/AST/DeclTemplate.h"
 #include "clang/AST/ExprCXX.h"
+#include "clang/AST/TemplateBase.h"
 #include "clang/AST/Type.h"
 
 namespace clang {
@@ -125,6 +126,20 @@ TagDecl *HeuristicResolverImpl::resolveTypeToTagDecl(QualType QT) {
   if (!T)
     return nullptr;
 
+  // If T is the type of a template parameter, we can't get a useful TagDecl
+  // out of it. However, if the template parameter has a default argument,
+  // as a heuristic we can replace T with the default argument type.
+  if (const auto *TTPT = dyn_cast<TemplateTypeParmType>(T)) {
+    if (const auto *TTPD = TTPT->getDecl()) {
+      if (TTPD->hasDefaultArgument()) {
+        const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+        if (DefaultArg.getKind() == TemplateArgument::Type) {
+          T = DefaultArg.getAsType().getTypePtrOrNull();
+        }
+      }
+    }
+  }
+
   // Unwrap type sugar such as type aliases.
   T = T->getCanonicalTypeInternal().getTypePtr();
 
diff --git a/clang/unittests/Sema/HeuristicResolverTest.cpp b/clang/unittests/Sema/HeuristicResolverTest.cpp
index c7cfe7917c532..5e36108172702 100644
--- a/clang/unittests/Sema/HeuristicResolverTest.cpp
+++ b/clang/unittests/Sema/HeuristicResolverTest.cpp
@@ -410,6 +410,23 @@ TEST(HeuristicResolver, MemberExpr_HangIssue126536) {
       cxxDependentScopeMemberExpr(hasMemberName("foo")).bind("input"));
 }
 
+TEST(HeuristicResolver, MemberExpr_DefaultTemplateArgument) {
+  std::string Code = R"cpp(
+    struct Default {
+      void foo();
+    };
+    template <typename T = Default>
+    void bar(T t) {
+      t.foo();
+    }
+  )cpp";
+  // Test resolution of "foo" in "t.foo()".
+  expectResolution(
+      Code, &HeuristicResolver::resolveMemberExpr,
+      cxxDependentScopeMemberExpr(hasMemberName("foo")).bind("input"),
+      cxxMethodDecl(hasName("foo")).bind("output"));
+}
+
 TEST(HeuristicResolver, DeclRefExpr_StaticMethod) {
   std::string Code = R"cpp(
     template <typename T>

hokein · 2025-03-19T11:53:53Z

clang/lib/Sema/HeuristicResolver.cpp

+      if (TTPD->hasDefaultArgument()) {
+        const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+        if (DefaultArg.getKind() == TemplateArgument::Type) {
+          T = DefaultArg.getAsType().getTypePtrOrNull();


the code doesn't look safe -- getTypePtrOrNull could return a nullptr, then we have a null-reference on Line144. Move this new code block before line 126?

Good catch -- fixed.

Do we traverse the default arguments recursively? I'm thinking a case like

template <typename W = Waldo, typename T = W> void bar(T t) { t.foo(); // Can we resolve it to Waldo::foo()? }

Also there are some usages for template template parameters, e.g.

template <class E, class A, template <class, class> class V = std::vector> void foo(V<E, A>) { V.push_back(42); // Would be great to resolve it to std::vector<>::push_back() }

However it depends on the implementation complexity - there's no necessity to handle these in this patch

Thanks for the added testcases!

The recursive case made me realize that application of this heuristic is probably better done in simplifyType. I updated the patch and added the testcase.

For the template template parameter case, I agree that it would be nice to make this work as well, but I'd rather leave that to a follow-up patch.

zyn0217

Sorry for missing this. LGTM assuming @hokein is happy too.

zyn0217 · 2025-03-20T01:50:07Z

clang/lib/Sema/HeuristicResolver.cpp

+      if (TTPD->hasDefaultArgument()) {
+        const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+        if (DefaultArg.getKind() == TemplateArgument::Type) {
+          T = DefaultArg.getAsType().getTypePtrOrNull();


Do we traverse the default arguments recursively? I'm thinking a case like

template <typename W = Waldo, typename T = W> void bar(T t) { t.foo(); // Can we resolve it to Waldo::foo()? }

Also there are some usages for template template parameters, e.g.

template <class E, class A, template <class, class> class V = std::vector> void foo(V<E, A>) { V.push_back(42); // Would be great to resolve it to std::vector<>::push_back() }

However it depends on the implementation complexity - there's no necessity to handle these in this patch

…rameters

zyn0217

thanks

zyn0217 · 2025-03-21T17:23:06Z

@HighCommander4 did you merge it into a wrong branch?

HighCommander4 · 2025-03-21T17:24:38Z

@HighCommander4 did you merge it into a wrong branch?

Oops. I forgot that the patch was stacked on top of another patch for #130468.

I will resubmit and reland.

HighCommander4 · 2025-03-21T20:16:51Z

Resubmitted at #132465

…eDeclRefExpr as well (#132576) This is a follow-up to #131074. After moving the default argument heuristic to `simplifyType` in that patch, the heuristic no longer applied to the `DependentScopeDeclRefExpr` case, because that wasn't using `simplifyType`. This patch fixes that, with an added testcase.

…c in resolveDeclRefExpr as well (#132576) This is a follow-up to llvm/llvm-project#131074. After moving the default argument heuristic to `simplifyType` in that patch, the heuristic no longer applied to the `DependentScopeDeclRefExpr` case, because that wasn't using `simplifyType`. This patch fixes that, with an added testcase.

HighCommander4 force-pushed the users/HighCommander4/clangd-issue-1056 branch from 32ca27b to aeeb8f2 Compare March 14, 2025 06:13

HighCommander4 marked this pull request as ready for review March 14, 2025 16:13

HighCommander4 requested review from hokein and zyn0217 March 14, 2025 16:13

llvmbot added clang Clang issues not falling into any other category clang-tools-extra clangd clang:frontend Language frontend issues, e.g. anything involving "Sema" labels Mar 14, 2025

hokein reviewed Mar 19, 2025

View reviewed changes

HighCommander4 force-pushed the users/HighCommander4/clangd-issue-1056 branch from aeeb8f2 to 6f29edf Compare March 20, 2025 01:23

zyn0217 approved these changes Mar 20, 2025

View reviewed changes

HighCommander4 force-pushed the users/HighCommander4/clangd-issue-1056 branch from 6f29edf to 9530a21 Compare March 21, 2025 06:12

[clang][HeuristicResolver] Default argument heuristic for template pa…

556926d

…rameters

HighCommander4 force-pushed the users/HighCommander4/clangd-issue-1056 branch from 9530a21 to 556926d Compare March 21, 2025 06:52

zyn0217 approved these changes Mar 21, 2025

View reviewed changes

HighCommander4 merged this pull request into users/HighCommander4/issue-130468 Mar 21, 2025
11 checks passed

HighCommander4 deleted the users/HighCommander4/clangd-issue-1056 branch March 21, 2025 17:12

HighCommander4 mentioned this pull request Mar 21, 2025

[clang][HeuristicResolver] Default argument heuristic for template parameters #132465

Merged

HighCommander4 mentioned this pull request Mar 23, 2025

[clang][HeuristicResolver] Apply default argument heuristic in resolveDeclRefExpr as well #132576

Merged

HighCommander4 mentioned this pull request Sep 1, 2025

Default argument heuristic for template template parameters clangd/clangd#2478

Closed

[clang][HeuristicResolver] Default argument heuristic for template parameters #131074

[clang][HeuristicResolver] Default argument heuristic for template parameters #131074

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