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

Fixes clangd/clangd#1056

Author: HighCommander4

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 {
@@ -247,6 +248,20 @@ QualType HeuristicResolverImpl::simplifyType(QualType Type, const Expr *E,
         }
       }
     }
+    if (const auto *TTPT = dyn_cast_if_present<TemplateTypeParmType>(T.Type)) {
+      // We can't do much useful with a template parameter (e.g. we cannot look
+      // up member names inside 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 *TTPD = TTPT->getDecl()) {
+        if (TTPD->hasDefaultArgument()) {
+          const auto &DefaultArg = TTPD->getDefaultArgument().getArgument();
+          if (DefaultArg.getKind() == TemplateArgument::Type) {
+            return {DefaultArg.getAsType()};
+          }
+        }
+      }
+    }
     return T;
   };
   // As an additional protection against infinite loops, bound the number of

clang/unittests/Sema/HeuristicResolverTest.cpp

@@ -410,6 +410,40 @@ 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, MemberExpr_DefaultTemplateArgument_Recursive) {
+  std::string Code = R"cpp(
+    struct Default {
+      void foo();
+    };
+    template <typename D = Default, typename T = D>
+    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>