[backported/clang-7][Sema] Don't skip function bodies with 'auto' without trailing return type

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* https://reviews.llvm.org/D44480
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Summary:
Skipping them was clearly not intentional. It's impossible to
guarantee correctness if the bodies are skipped.
Also adds a test case for r327504, now that it does not produce
invalid errors that made the test fail.

Reviewers: aaron.ballman, sammccall, rsmith

Reviewed By: rsmith

Subscribers: rayglover-ibm, rwols, cfe-commits

Differential Revision: https://reviews.llvm.org/D44480

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@333538 91177308-0d34-0410-b5e6-96231b3b80d8

Task-number: QTCREATORBUG-20515
Change-Id: I5d60a094a967dc0e1e401256a1d70093084909f5
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

3 files changed, 78 insertions, 2 deletions

diff --git a/lib/Sema/SemaDecl.cpp b/lib/Sema/SemaDecl.cpp
index 7ca48c34e5..520f01a91b 100644
--- a/lib/Sema/SemaDecl.cpp
+++ b/lib/Sema/SemaDecl.cpp
@@ -12172,9 +12172,15 @@ bool Sema::canSkipFunctionBody(Decl *D) {
   // rest of the file.
   // We cannot skip the body of a function with an undeduced return type,
   // because any callers of that function need to know the type.
-  if (const FunctionDecl *FD = D->getAsFunction())
-    if (FD->isConstexpr() || FD->getReturnType()->isUndeducedType())
+  if (const FunctionDecl *FD = D->getAsFunction()) {
+    if (FD->isConstexpr())
       return false;
+    // We can't simply call Type::isUndeducedType here, because inside template
+    // auto can be deduced to a dependent type, which is not considered
+    // "undeduced".
+    if (FD->getReturnType()->getContainedDeducedType())
+      return false;
+  }
   return Consumer.shouldSkipFunctionBody(D);
 }
 
diff --git a/test/CodeCompletion/crash-skipped-bodies-template-inst.cpp b/test/CodeCompletion/crash-skipped-bodies-template-inst.cpp
new file mode 100644
index 0000000000..6161f100cb
--- /dev/null
+++ b/test/CodeCompletion/crash-skipped-bodies-template-inst.cpp
@@ -0,0 +1,27 @@
+// RUN: %clang_cc1 -fsyntax-only -code-completion-at=%s:24:5 %s -o - 2>&1 | FileCheck %s
+template <class T>
+auto make_func() {
+  struct impl {
+    impl* func() {
+      int x;
+      if (x = 10) {}
+      // Check that body of this function is actually skipped.
+      // CHECK-NOT: crash-skipped-bodies-template-inst.cpp:7:{{[0-9]+}}: warning: using the result of an assignment as a condition without parentheses
+      return this;
+    }
+  };
+
+  int x;
+  if (x = 10) {}
+  // Check that this function is not skipped.
+  // CHECK: crash-skipped-bodies-template-inst.cpp:15:9: warning: using the result of an assignment as a condition without parentheses
+  return impl();
+}
+
+void foo() {
+  []() {
+    make_func<int>();
+    m
+    // CHECK: COMPLETION: make_func : [#auto#]make_func<<#class T#>>()
+  };
+}
diff --git a/test/CodeCompletion/skip-auto-funcs.cpp b/test/CodeCompletion/skip-auto-funcs.cpp
new file mode 100644
index 0000000000..b20f95bdf2
--- /dev/null
+++ b/test/CodeCompletion/skip-auto-funcs.cpp
@@ -0,0 +1,43 @@
+// We run clang in completion mode to force skipping of function bodies and
+// check if the function bodies were skipped by observing the warnings that
+// clang produces.
+// RUN: not %clang_cc1 -fsyntax-only -code-completion-at=%s:42:1 %s -o - 2>&1 | FileCheck %s
+template <class T>
+auto not_skipped() {
+  int x;
+  if (x = 10) {}
+  // Check that this function is not skipped.
+  // CHECK: 8:9: warning: using the result of an assignment as a condition without parentheses
+  return 1;
+}
+
+template <class T>
+auto lambda_not_skipped = []() {
+  int x;
+  if (x = 10) {}
+  // Check that this function is not skipped.
+  // CHECK: 17:9: warning: using the result of an assignment as a condition without parentheses
+  return 1;
+}
+
+template <class T>
+auto skipped() -> T {
+  int x;
+  if (x = 10) {}
+  // Check that this function is skipped.
+  // CHECK-NOT: 26:9: warning: using the result of an assignment as a condition without parentheses
+  return 1;
+};
+
+auto lambda_skipped = []() -> int {
+  int x;
+  if (x = 10) {}
+  // This could potentially be skipped, but it isn't at the moment.
+  // CHECK: 34:9: warning: using the result of an assignment as a condition without parentheses
+  return 1;
+};
+
+int test() {
+  int complete_in_this_function;
+  // CHECK: COMPLETION: complete_in_this_function
+}