[clang] [clang] Disable missing definition warning on pure virtual functions (PR #74510)

[via cfe-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Charalampos Mitrodimas (charmitro)

<details>
<summary>Changes</summary>

Warning '-Wundefined-func-template' incorrectly indicates that no definition is available for a pure virtual function. However, a definition is not needed for a pure virtual function.

Fixes #<!-- -->74016

---
Full diff: https://github.com/llvm/llvm-project/pull/74510.diff


2 Files Affected:

- (modified) clang/lib/Sema/SemaTemplateInstantiateDecl.cpp (+2-1) 
- (added) clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp (+20) 


``````````diff
diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d768bb72e07c0..0850b2d64b632 100644
--- a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
+++ b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
@@ -4952,7 +4952,8 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
         std::make_pair(Function, PointOfInstantiation));
     } else if (TSK == TSK_ImplicitInstantiation) {
       if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
-          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
+          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc()) &&
+          !Function->isVirtualAsWritten()) {
         Diag(PointOfInstantiation, diag::warn_func_template_missing)
           << Function;
         Diag(PatternDecl->getLocation(), diag::note_forward_template_decl);
diff --git a/clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp b/clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp
new file mode 100644
index 0000000000000..7ae323343330e
--- /dev/null
+++ b/clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp
@@ -0,0 +1,20 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wundefined-func-template %s
+// expected-no-diagnostics
+
+namespace PR74016 {
+  template <typename T> class B {
+  public:
+    constexpr void foo(const T &) { bar(1); }
+    virtual constexpr void bar(unsigned int) = 0;
+  };
+
+  template <typename T> class D : public B<T> {
+  public:
+    constexpr void bar(unsigned int) override {}
+  };
+
+  void test() {
+    auto t = D<int>();
+    t.foo(0);
+  }
+};

``````````

</details>


https://github.com/llvm/llvm-project/pull/74510