[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 5582cfacf106a7c00912fcef0d116d6fa58e064b 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     | 48 +++++++++++++++++++
 2 files changed, 50 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..a034de0697b2b 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->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..09164e8a79f96
--- /dev/null
+++ b/clang/test/SemaTemplate/instantiate-pure-virtual-function.cpp
@@ -0,0 +1,48 @@
+// 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); }
+    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);
+  }
+};
+
+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 at -1 {{forward declaration of template entity is here}}
+    // expected-note at -2 {{forward declaration of template entity is here}}
+    // expected-note at -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 at -1 {{instantiation of function 'non_pure_virtual_function::B<int>::bar' required here, but no definition is available}}
+// expected-warning at -2 {{instantiation of function 'non_pure_virtual_function::B<int>::bar' required here, but no definition is available}}
+// expected-note at -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 at -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 at -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 at -6 {{used here}}
+
+  public:
+    constexpr void bar(unsigned int) override { }
+  };
+
+  void test() {
+    auto t = D<int>();
+    t.foo(0);
+  }
+};