[Frontend] Correctly handle instantiating ctors with skipped bodies

[ilya-biryukov]

Summary

Previsouly clang tried instantiating member initializers even if ctor
body was skipped, this caused spurious errors (see the test).

[sepavloff]

LGTM.

Thanks!

lib/Sema/SemaTemplateInstantiateDecl.cpp

else {
     // Enter the scope of this instantiation. We don't use
     // PushDeclContext because we don't have a scope.
     Sema::ContextRAII savedContext(*this, Function);
 
     if (addInstantiatedParametersToScope(*this, Function, PatternDecl, Scope,
                                          TemplateArgs))
       return;
 
+    if (PatternDecl->hasSkippedBody()) {
+      ActOnSkippedFunctionBody(Function);
+    } else {
       if (CXXConstructorDecl *Ctor = dyn_cast<CXXConstructorDecl>(Function)) {
         // If this is a constructor, instantiate the member initializers.
         InstantiateMemInitializers(Ctor, cast<CXXConstructorDecl>(PatternDecl),
                                    TemplateArgs);
 
         // If this is an MS ABI dllexport default constructor, instantiate any
         // default arguments.
         if (Context.getTargetInfo().getCXXABI().isMicrosoft() &&
             Ctor->isDefaultConstructor()) {
           InstantiateDefaultCtorDefaultArgs(*this, Ctor);
         }
       }
 
-    if (PatternDecl->hasSkippedBody()) {
-      ActOnSkippedFunctionBody(Function);
-    } else {
       // Instantiate the function body.
       StmtResult Body = SubstStmt(Pattern, TemplateArgs);
 
       if (Body.isInvalid())
         Function->setInvalidDecl();
 
       // FIXME: finishing the function body while in an expression evaluation
       // context seems wrong. Investigate more.

test/Index/skipped-bodies-ctors.cpp

+// RUN: env CINDEXTEST_SKIP_FUNCTION_BODIES=1 c-index-test -test-load-source all %s 2>&1 \
+// RUN: | FileCheck --implicit-check-not "error:" %s
+
+
+template <class T>
+struct Foo {
+  template <class = int>
+  Foo(int &a) : a(a) {
+  }
+
+  int &a;
+};
+
+
+int bar = Foo<int>(bar).a + Foo<int>(bar).a;
+// CHECK-NOT: error: constructor for 'Foo<int>' must explicitly initialize the reference