[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 llvm#74016

Signed-off-by: Charalampos Mitrodimas <charmitro@posteo.net>

clang/docs/ReleaseNotes.rst

@@ -346,6 +346,10 @@ Improvements to Clang's time-trace
 
 Bug Fixes in This Version
 -------------------------
+- Clang's ``-Wundefined-func-template`` no longer warns on pure virtual
+  functions.
+  (`#74016 <https://github.com/llvm/llvm-project/issues/74016>`_)
+
 - Fixed missing warnings when comparing mismatched enumeration constants
   in C (`#29217 <https://github.com/llvm/llvm-project/issues/29217>`).
 

clang/lib/Sema/SemaExpr.cpp

@@ -18980,8 +18980,9 @@ void Sema::MarkFunctionReferenced(SourceLocation Loc, FunctionDecl *Func,
   // Note that we skip the implicit instantiation of templates that are only
   // used in unused default arguments or by recursive calls to themselves.
   // This is formally non-conforming, but seems reasonable in practice.
-  bool NeedDefinition = !IsRecursiveCall && (OdrUse == OdrUseContext::Used ||
-                                             NeededForConstantEvaluation);
+  bool NeedDefinition = !IsRecursiveCall &&
+                        (OdrUse == OdrUseContext::Used ||
+                         NeededForConstantEvaluation || !Func->isPureVirtual());
 
   // C++14 [temp.expl.spec]p6:
   //   If a template [...] is explicitly specialized then that specialization

clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp

@@ -0,0 +1,68 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wundefined-func-template %s
+
+namespace GH74016 {
+  template <typename T> class B {
+  public:
+    constexpr void foo(const T &) { bar(1); } // expected-warning {{instantiation of function 'GH74016::B<int>::bar' required here, but no definition is available}}
+    // expected-note@-1 {{add an explicit instantiation declaration to suppress this warning if 'GH74016::B<int>::bar' is explicitly instantiated in another translation unit}}
+    virtual constexpr void bar(unsigned int) = 0; // expected-note {{forward declaration of template entity is here}}
+  };
+
+  template <typename T> class D : public B<T> {
+  public:
+    constexpr void bar(unsigned int) override {}
+  };
+
+  void test() {
+    auto t = D<int>();
+    t.foo(0);
+  }
+};
+
+namespace call_pure_virtual_function_from_virtual {
+  template <typename T> class B {
+  public:
+    const void foo(const T &) { B::bar(1); } // expected-warning {{instantiation of function 'call_pure_virtual_function_from_virtual::B<int>::bar' required here, but no definition is available}}
+    // expected-note@-1 {{add an explicit instantiation declaration to suppress this warning if 'call_pure_virtual_function_from_virtual::B<int>::bar' is explicitly instantiated in another translation unit}}
+    virtual const void bar(unsigned int) = 0; // expected-note {{forward declaration of template entity is here}}
+  };
+
+  template <typename T> class D : public B<T> {
+  public:
+    const void bar(unsigned int) override {}
+  };
+
+  void test() {
+    auto t = D<int>();
+    t.foo(0); // expected-note {{in instantiation of member function 'call_pure_virtual_function_from_virtual::B<int>::foo' requested here}}
+  }
+};
+
+namespace non_pure_virtual_function {
+  template <typename T> class B {
+  public:
+    constexpr void foo(const T &) { bar(1); }
+
+    virtual constexpr void bar(unsigned int); // expected-warning {{inline function 'non_pure_virtual_function::B<int>::bar' is not defined}}
+    // expected-note@-1 {{forward declaration of template entity is here}}
+    // expected-note@-2 {{forward declaration of template entity is here}}
+    // expected-note@-3 {{forward declaration of template entity is here}}
+  };
+
+  template <typename T> class D : public B<T> { // expected-warning {{instantiation of function 'non_pure_virtual_function::B<int>::bar' required here, but no definition is available}}
+// expected-warning@-1 {{instantiation of function 'non_pure_virtual_function::B<int>::bar' required here, but no definition is available}}
+// expected-warning@-2 {{instantiation of function 'non_pure_virtual_function::B<int>::bar' required here, but no definition is available}}
+// expected-note@-3 {{add an explicit instantiation declaration to suppress this warning if 'non_pure_virtual_function::B<int>::bar' is explicitly instantiated in another translation unit}}
+// expected-note@-4 {{add an explicit instantiation declaration to suppress this warning if 'non_pure_virtual_function::B<int>::bar' is explicitly instantiated in another translation unit}}
+// expected-note@-5 {{add an explicit instantiation declaration to suppress this warning if 'non_pure_virtual_function::B<int>::bar' is explicitly instantiated in another translation unit}}
+// expected-note@-6 {{used here}}
+
+  public:
+    constexpr void bar(unsigned int) override { }
+  };
+
+  void test() {
+    auto t = D<int>();
+    t.foo(0);
+  }
+};