[clang] [Sema] -Wpointer-bool-conversion: suppress lambda function pointer conversion diagnostic during instantiation (PR #83497)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay created https://github.com/llvm/llvm-project/pull/83497

I have seen internal pieces of code that uses a template type parameter
to accept std::function, a closure type, or any callable type. The
diagnostic added in #83152 would require adaptation to the template,
which is difficult and also seems unnecessary.

```cpp
template <typename... Ts>
static bool IsFalse(const Ts&...) { return false; }

template <typename T, typename... Ts,
          typename = typename std::enable_if<std::is_constructible<bool, const T&>::value>::type>
static bool IsFalse(const Partial&, const T& p, const Ts&...) {
  return p ? false : true;
}

template <typename... Args>
void Init(Args&&... args) {
  if (IsFalse(absl::implicit_cast<const typename std::decay<Args>::type&>(
              args)...)) {
    // A callable object convertible to false is either a null pointer or a
    // null functor (e.g., a default-constructed std::function).
    empty_ = true;
  } else {
    empty_ = false;
    new (&factory_) Factory(std::forward<Args>(args)...);
  }
}
```


>From d7a168190f2fdf3b4f8ec1457400ad8e03bc3f3a Mon Sep 17 00:00:00 2001
From: Fangrui Song <i at maskray.me>
Date: Thu, 29 Feb 2024 14:40:00 -0800
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
 =?UTF-8?q?l=20version?=
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Created using spr 1.3.4
---
 clang/lib/Sema/SemaChecking.cpp             | 17 ++++++++++-------
 clang/test/SemaCXX/warn-bool-conversion.cpp | 13 +++++++++++++
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 690bdaa63d058b..3533b5c8e0d235 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -16586,13 +16586,16 @@ void Sema::DiagnoseAlwaysNonNullPointer(Expr *E,
   }
 
   // Complain if we are converting a lambda expression to a boolean value
-  if (const auto *MCallExpr = dyn_cast<CXXMemberCallExpr>(E)) {
-    if (const auto *MRecordDecl = MCallExpr->getRecordDecl();
-        MRecordDecl && MRecordDecl->isLambda()) {
-      Diag(E->getExprLoc(), diag::warn_impcast_pointer_to_bool)
-          << /*LambdaPointerConversionOperatorType=*/3
-          << MRecordDecl->getSourceRange() << Range << IsEqual;
-      return;
+  // outside of instantiation.
+  if (!inTemplateInstantiation()) {
+    if (const auto *MCallExpr = dyn_cast<CXXMemberCallExpr>(E)) {
+      if (const auto *MRecordDecl = MCallExpr->getRecordDecl();
+          MRecordDecl && MRecordDecl->isLambda()) {
+        Diag(E->getExprLoc(), diag::warn_impcast_pointer_to_bool)
+            << /*LambdaPointerConversionOperatorType=*/3
+            << MRecordDecl->getSourceRange() << Range << IsEqual;
+        return;
+      }
     }
   }
 
diff --git a/clang/test/SemaCXX/warn-bool-conversion.cpp b/clang/test/SemaCXX/warn-bool-conversion.cpp
index 9e8cf0e4f8944a..8b1bf80af79d26 100644
--- a/clang/test/SemaCXX/warn-bool-conversion.cpp
+++ b/clang/test/SemaCXX/warn-bool-conversion.cpp
@@ -92,6 +92,19 @@ void foo() {
   bool is_true = [](){ return true; };
   // expected-warning at -1{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
 }
+
+template <typename... Ts>
+static bool IsFalse(const Ts&...) { return false; }
+template <typename T>
+static bool IsFalse(const T& p) {
+  bool b;
+  b = f7; // expected-warning {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  return p ? false : true;
+}
+
+bool use_instantiation() {
+  return IsFalse([]() { return 0; });
+}
 #endif
 
 void bar() {