[Sema] Do not show unused parameter warnings when body is skipped

[ilya-biryukov]

Summary

Without the function body, we cannot determine is parameter was used.

[sammccall]

Haha, fun!

lib/Sema/SemaDecl.cpp

if (FD) {
 
     // MSVC permits the use of pure specifier (=0) on function definition,
     // defined at class scope, warn about this non-standard construct.
     if (getLangOpts().MicrosoftExt && FD->isPure() && FD->isCanonicalDecl())
       Diag(FD->getLocation(), diag::ext_pure_function_definition);
 
     if (!FD->isInvalidDecl()) {
       // Don't diagnose unused parameters of defaulted or deleted functions.
-      if (!FD->isDeleted() && !FD->isDefaulted())
+      if (!FD->isDeleted() && !FD->isDefaulted() && !FD->hasSkippedBody())
         DiagnoseUnusedParameters(FD->parameters());
       DiagnoseSizeOfParametersAndReturnValue(FD->parameters(),
                                              FD->getReturnType(), FD);
 
       // If this is a structor, we need a vtable.
       if (CXXConstructorDecl *Constructor = dyn_cast<CXXConstructorDecl>(FD))
         MarkVTableUsed(FD->getLocation(), Constructor->getParent());
       else if (CXXDestructorDecl *Destructor = dyn_cast<CXXDestructorDecl>(FD))
if (FD) {
     }
 
     assert((FD == getCurFunctionDecl() || getCurLambda()->CallOperator == FD) &&
            "Function parsing confused");
   } else if (ObjCMethodDecl *MD = dyn_cast_or_null<ObjCMethodDecl>(dcl)) {
     assert(MD == getCurMethodDecl() && "Method parsing confused");
     MD->setBody(Body);
     if (!MD->isInvalidDecl()) {
+      if (!MD->hasSkippedBody())
         DiagnoseUnusedParameters(MD->parameters());
       DiagnoseSizeOfParametersAndReturnValue(MD->parameters(),
                                              MD->getReturnType(), MD);
 
       if (Body)
         computeNRVO(Body, getCurFunction());
     }
     if (getCurFunction()->ObjCShouldCallSuper) {
       Diag(MD->getEndLoc(), diag::warn_objc_missing_super_call)

test/Index/skipped-bodies-unused.cpp

+// RUN: env CINDEXTEST_SKIP_FUNCTION_BODIES=1 c-index-test -test-load-source all %s -Wunused-parameter 2>&1 \
+// RUN: | FileCheck %s
+
+// No 'unused parameter' warnings should be shown when skipping the function bodies.
+inline int foo(int used, int unused) {
+    used = 100;
+}
+// CHECK-NOT: warning: unused parameter