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

[Charalampos Mitrodimas via cfe-commits]

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

>From 0be0d65a74cf7a6a06150362b68adff122a3a39b Mon Sep 17 00:00:00 2001
From: Charalampos Mitrodimas <charmitro at posteo.net>
Date: Tue, 5 Dec 2023 11:46:56 +0200
Subject: [PATCH] [clang] Disable missing definition warning on pure virtual
 functions

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

Signed-off-by: Charalampos Mitrodimas <charmitro at posteo.net>
---
 .../lib/Sema/SemaTemplateInstantiateDecl.cpp  |  3 ++-
 .../instantiate-pure-virtual-function.cpp     | 20 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp

diff --git a/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp b/clang/lib/Sema/SemaTemplateInstantiateDecl.cpp
index d768bb72e07c0..e6704c6f80808 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() && !Function->isPure()) {
         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..46aaf70c7efa4
--- /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 GH74016 {
+  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);
+  }
+};