[clang] [Sema] Enable nodiscard warnings for function pointers

A call through a function pointer has no associated FunctionDecl,
but it still might have a nodiscard return type. Ensure there is
a codepath to emit the nodiscard warning in this case.

Fixes #142453

Author: halbi2

clang/lib/AST/Expr.cpp

@@ -2794,6 +2794,16 @@ bool Expr::isUnusedResultAWarning(const Expr *&WarnE, SourceLocation &Loc,
         return true;
       }
     }
+    if (CE->hasUnusedResultAttr(Ctx)) {
+      WarnE = this;
+      Loc = getBeginLoc();
+      R1 = getSourceRange();
+
+      if (unsigned NumArgs = CE->getNumArgs())
+        R2 = SourceRange(CE->getArg(0)->getBeginLoc(),
+                         CE->getArg(NumArgs - 1)->getEndLoc());
+      return true;
+    }
     return false;
   }
 

clang/test/SemaCXX/warn-unused-result.cpp

@@ -481,9 +481,9 @@ void test() {
   p_return_warnunusedresult(); // expected-warning {{ignoring return value of type 'WarnUnusedResult' declared with 'gnu::warn_unused_result' attribute}}
 
   // Function pointer expression return values
-  pp_return_nodiscard()(); // no warning: TODO
+  pp_return_nodiscard()(); // expected-warning {{ignoring return value of type 'NoDiscard' declared with 'nodiscard' attribute}}
   pp_return_warnunused()(); // no warning
-  pp_return_warnunusedresult()(); // no warning: TODO
+  pp_return_warnunusedresult()(); // expected-warning {{ignoring return value of type 'WarnUnusedResult' declared with 'gnu::warn_unused_result' attribute}}
 
   // From a template
   from_a_template<NoDiscard>(); // expected-warning {{ignoring return value of type 'NoDiscard' declared with 'nodiscard' attribute}}