[nullability] Fix false positives for functions with forward declarat…

…ions.

Since [this
patch](b4120db),
we erroneously used the `ParmVarDecl`s of the forward declaration when creating
initial values for the parameters; the body uses the `ParmVarDecl`s of the
definition, so it would not see these initial values.

PiperOrigin-RevId: 598560058
Change-Id: I28c0659e431a7bdf9154969bd3709fb466b7c76d

nullability/pointer_nullability_diagnosis.cc

@@ -441,7 +441,10 @@ diagnosePointerNullability(const FunctionDecl *Func) {
   for (const ParmVarDecl *Parm : Func->parameters())
     checkParmVarDeclWithPointerDefaultArg(Ctx, *Parm, Diags);
 
-  if (!Func->hasBody()) return Diags;
+  // If `Func` has multiple declarations in the current TU, make sure we use the
+  // one that defines the body of the function. (We retrieve this using the
+  // output parameter of `hasBody()`.)
+  if (!Func->hasBody(Func)) return Diags;
 
   auto Diagnoser = pointerNullabilityDiagnoser();
   const std::int64_t MaxSATIterations = 2'000'000;

nullability/test/basic.cc

@@ -247,5 +247,20 @@ TEST(PointerNullabilityTest, ArraySubscript) {
   )cc"));
 }
 
+TEST(PointerNullabilityTest, ForwardDeclaration) {
+  // Check that we handle a function with a forward declaration correctly. This
+  // is a regression test for a bug where we erroneously used the `ParmVarDecl`s
+  // of the first declaration when creating initial values for the parameters;
+  // the body uses the `ParmVarDecl`s of the second declaration, so it would not
+  // see these initial values.
+  EXPECT_TRUE(checkDiagnostics(R"cc(
+    void target(int* _Nullable p);
+    void target(int* _Nullable p) {
+      if (p == nullptr) return;
+      *p;
+    }
+  )cc"));
+}
+
 }  // namespace
 }  // namespace clang::tidy::nullability