[Clang][Sema]: Diagnose lambda to bool implicit casts (#83152)

Adds diagnostics for lambda expressions being cast to boolean values,
which results in the expression always evaluating to true.
Earlier, Clang allowed compilation of such erroneous programs, but now
emits a warning through `-Wpointer-bool-conversion`.

Fixes #82512

clang/docs/ReleaseNotes.rst

@@ -192,6 +192,9 @@ Improvements to Clang's diagnostics
 
 - Clang now diagnoses declarative nested name specifiers that name alias templates.
 
+- Clang now diagnoses lambda function expressions being implicitly cast to boolean values, under ``-Wpointer-bool-conversion``.
+  Fixes `#82512 <https://github.com/llvm/llvm-project/issues/82512>`_.
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -4127,8 +4127,8 @@ def ext_ms_impcast_fn_obj : ExtWarn<
   "Microsoft extension">, InGroup<MicrosoftCast>;
 
 def warn_impcast_pointer_to_bool : Warning<
-    "address of%select{| function| array}0 '%1' will always evaluate to "
-    "'true'">,
+    "address of %select{'%1'|function '%1'|array '%1'|lambda function pointer "
+    "conversion operator}0 will always evaluate to 'true'">,
     InGroup<PointerBoolConversion>;
 def warn_cast_nonnull_to_bool : Warning<
     "nonnull %select{function call|parameter}0 '%1' will evaluate to "

clang/lib/Sema/SemaChecking.cpp

@@ -16544,6 +16544,17 @@ 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;
+    }
+  }
+
   // Expect to find a single Decl.  Skip anything more complicated.
   ValueDecl *D = nullptr;
   if (DeclRefExpr *R = dyn_cast<DeclRefExpr>(E)) {

clang/test/CXX/drs/dr18xx.cpp

@@ -282,6 +282,7 @@ namespace dr1837 { // dr1837: 3.3
   struct A {
     int f();
     bool b = [] {
+      // since-cxx11-warning@-1 {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
       struct Local {
         static_assert(sizeof(this->f()) == sizeof(int), "");
       };

clang/test/CXX/expr/expr.prim/expr.prim.lambda/blocks.mm

@@ -65,10 +65,10 @@ void nesting() {
 
 namespace overloading {
   void bool_conversion() {
-    if ([](){}) {
+    if ([](){}) { // expected-warning{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
     }
 
-    bool b = []{};
+    bool b = []{}; // expected-warning{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
     b = (bool)[]{};
   }
 
@@ -108,8 +108,9 @@ void call_with_lambda() {
     using decltype(a)::operator id<void(*)()>; // expected-note {{here}}
   } extern d;
 
-  bool r1 = c;
-  bool r2 = d; // expected-error {{private}}
+  bool r1 = c; // expected-warning{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  bool r2 = d; // expected-error {{private}} \
+                  expected-warning{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
 }
 
 namespace PR13117 {

clang/test/SemaCXX/warn-bool-conversion.cpp

@@ -81,6 +81,18 @@ struct S2 {
 
 bool f5();
 bool f6(int);
+#if __cplusplus >= 201103L
+auto f7 = []{};
+auto f8 = [](){};
+
+void foo() {
+  bool b;
+  b = f7; // expected-warning {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  b = f8; // expected-warning {{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+  bool is_true = [](){ return true; };
+  // expected-warning@-1{{address of lambda function pointer conversion operator will always evaluate to 'true'}}
+}
+#endif
 
 void bar() {
   bool b;