[clang] [Clang][Sema] Fix malformed AST for anonymous class access in template. (PR #90842)
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llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: None (martinboehme)
<details>
<summary>Changes</summary>
# Observed erroneous behavior
Prior to this change, a `MemberExpr` that accesses an anonymous class might have
a prvalue as its base (even though C++ mandates that the base of a `MemberExpr`
must be a glvalue), if the code containing the `MemberExpr` was in a template.
Here's an example on [godbolt](https://godbolt.org/z/Gz1Mer9oz) (that is
essentially identical to the new test this patch adds).
This example sets up a struct containing an anonymous struct:
```cxx
struct S {
struct {
int i;
};
};
```
It then accesses the member `i` using the expression `S().i`.
When we do this in a non-template function, we get the following AST:
```
`-ExprWithCleanups <col:10, col:14> 'int'
`-ImplicitCastExpr <col:10, col:14> 'int' <LValueToRValue>
`-MemberExpr <col:10, col:14> 'int' xvalue .i 0xbdcb3c0
`-MemberExpr <col:10, col:14> 'S::(anonymous struct at line:2:3)' xvalue .S::(anonymous struct at line:2:3) 0xbdcb488
`-MaterializeTemporaryExpr <col:10, col:12> 'S' xvalue
`-CXXTemporaryObjectExpr <col:10, col:12> 'S' 'void () noexcept' zeroing
```
As expected, the AST contains a `MaterializeTemporarExpr` to materialize the
prvalue `S()` before accessing its members.
When we perform this access in a function template (that doesn't actually even
use its template parameter), the AST for the template itself looks the same as
above. However, the AST for an instantiation of the template looks different:
```
`-ExprWithCleanups <col:10, col:14> 'int'
`-ImplicitCastExpr <col:10, col:14> 'int' <LValueToRValue>
`-MemberExpr <col:10, col:14> 'int' xvalue .i 0xbdcb3c0
`-MaterializeTemporaryExpr <col:10, col:14> 'S::(anonymous struct at line:2:3)' xvalue
`-MemberExpr <col:10, col:14> 'S::(anonymous struct at line:2:3)' .S::(anonymous struct at line:2:3) 0xbdcb488
`-CXXTemporaryObjectExpr <col:10, col:12> 'S' 'void () noexcept' zeroing
```
Note how the inner `MemberExpr` (the one accessing the anonymous struct) acts on
a prvalue.
Interestingly, this does not appear to cause any problems for CodeGen, probably
because CodeGen is set up to deal with `MemberExpr`s on rvalues in C. However,
it does cause issues in the dataflow framework, which only supports C++ today
and expects the base of a `MemberExpr` to be a glvalue.
Beyond the issues with the dataflow framework, I think this issue should be
fixed because it goes contrary to what the C++ standard mandates, and the AST
produced for the non-template case indicates that we want to follow the C++
rules here.
# Reasons for erroneous behavior
Here's why we're getting this malformed AST.
First of all, `TreeTransform` [strips any
`MaterializeTemporaryExpr`s](https://github.com/llvm/llvm-project/blob/cd132dcbeb0fc79fd657bd5e0a8e9244c3fb5da6/clang/lib/Sema/TreeTransform.h#L14853)
from the AST.
It is therefore up to
[`TreeTransform::RebuildMemberExpr()`](https://github.com/llvm/llvm-project/blob/cd132dcbeb0fc79fd657bd5e0a8e9244c3fb5da6/clang/lib/Sema/TreeTransform.h#L2853)
to recreate a `MaterializeTemporaryExpr` if needed. In the [general
case](https://github.com/llvm/llvm-project/blob/cd132dcbeb0fc79fd657bd5e0a8e9244c3fb5da6/clang/lib/Sema/TreeTransform.h#L2915),
it does this: It calls `Sema::BuildMemberReferenceExpr()`, which ensures that
the base is a glvalue by
[materializing a
temporary](https://github.com/llvm/llvm-project/blob/cd132dcbeb0fc79fd657bd5e0a8e9244c3fb5da6/clang/lib/Sema/SemaExprMember.cpp#L1016)
if needed. However, when `TreeTransform::RebuildMemberExpr()` encounters an
anonymous class, it [calls
`Sema::BuildFieldReferenceExpr()`](https://github.com/llvm/llvm-project/blob/cd132dcbeb0fc79fd657bd5e0a8e9244c3fb5da6/clang/lib/Sema/TreeTransform.h#L2880),
which, unlike `Sema::BuildMemberReferenceExpr()`, does not make sure that the
base is a glvalue.
# Proposed fix
I considered several possible ways to fix this issue:
- Add logic to `Sema::BuildFieldReferenceExpr()` that materializes a temporary
if needed. This appears to work, but it feels like the fix is in the wrong
place:
- AFAIU, other callers of `Sema::BuildFieldReferenceExpr()` don't need this
logic.
- The issue is caused by `TreeTransform` removing the
`MaterializeTemporaryExpr`, so it seems the fix should also be in
`TreeTransform`
- Materialize the temporary directly in `TreeTransform::RebuildMemberExpr()` if
needed (within the case that deals with anonymous classes).
This would work, too, but it would duplicate logic that already exists in
`Sema::BuildMemberReferenceExpr()` (which we leverage for the general case).
- Use `Sema::BuildMemberReferenceExpr()` instead of
`Sema::BuildFieldReferenceExpr()` for the anonymous class case, so that it
also uses the existing logic for materializing the temporary.
This is the option I've decided to go with here. There's a slight wrinkle in
that we create a `LookupResult` that claims we looked up the unnamed field
for the anonymous class -- even though we would obviously never be able to
look up an unnamed field. I think this is defensible and still better than
the other alternatives, but I would welcome feedback on this from others who
know the code better.
---
Full diff: https://github.com/llvm/llvm-project/pull/90842.diff
3 Files Affected:
- (modified) clang/lib/Sema/TreeTransform.h (+14-3)
- (added) clang/test/AST/ast-dump-anonymous-class.cpp (+49)
- (modified) clang/unittests/Analysis/FlowSensitive/TransferTest.cpp (+35)
``````````diff
diff --git a/clang/lib/Sema/TreeTransform.h b/clang/lib/Sema/TreeTransform.h
index f47bc219e6fa32..9c800c5705c7c6 100644
--- a/clang/lib/Sema/TreeTransform.h
+++ b/clang/lib/Sema/TreeTransform.h
@@ -2876,10 +2876,21 @@ class TreeTransform {
return ExprError();
Base = BaseResult.get();
+ // We want to use `BuildMemberReferenceExpr()` so we can use its logic
+ // that materializes `Base` into a temporary if it's a prvalue.
+ // To do so, we need to create a `LookupResult` for `Member`, even though
+ // it's an unnamed field (that we could never actually have looked up).
+ // This small hack seems preferable to duplicating the logic for
+ // materializing the temporary.
+ LookupResult R(getSema(), MemberNameInfo, Sema::LookupMemberName);
+ R.addDecl(Member);
+ R.resolveKind();
+
CXXScopeSpec EmptySS;
- return getSema().BuildFieldReferenceExpr(
- Base, isArrow, OpLoc, EmptySS, cast<FieldDecl>(Member),
- DeclAccessPair::make(FoundDecl, FoundDecl->getAccess()), MemberNameInfo);
+ return getSema().BuildMemberReferenceExpr(
+ Base, Base->getType(), OpLoc, isArrow, EmptySS, TemplateKWLoc,
+ FirstQualifierInScope, R, ExplicitTemplateArgs,
+ /*S*/ nullptr);
}
CXXScopeSpec SS;
diff --git a/clang/test/AST/ast-dump-anonymous-class.cpp b/clang/test/AST/ast-dump-anonymous-class.cpp
new file mode 100644
index 00000000000000..393c084c913d15
--- /dev/null
+++ b/clang/test/AST/ast-dump-anonymous-class.cpp
@@ -0,0 +1,49 @@
+// RUN: %clang_cc1 -std=c++17 -triple x86_64-unknown-unknown -ast-dump %s \
+// RUN: | FileCheck -strict-whitespace %s
+
+struct S {
+ struct {
+ int i;
+ };
+};
+
+int accessInRegularFunction() {
+ return S().i;
+ // CHECK: FunctionDecl {{.*}} accessInRegularFunction 'int ()'
+ // CHECK: | `-ReturnStmt {{.*}}
+ // CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'int'
+ // CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'int' <LValueToRValue>
+ // CHECK-NEXT: | `-MemberExpr {{.*}} 'int' xvalue .i
+ // CHECK-NEXT: | `-MemberExpr {{.*}} 'S::(anonymous struct at {{.*}})
+ // CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'S' xvalue
+ // CHECK-NEXT: | `-CXXTemporaryObjectExpr {{.*}} 'S' 'void () noexcept' zeroing
+}
+
+// AST should look the same in a function template with an unused template
+// parameter.
+template <class>
+int accessInFunctionTemplate() {
+ return S().i;
+ // CHECK: FunctionDecl {{.*}} accessInFunctionTemplate 'int ()'
+ // CHECK: | `-ReturnStmt {{.*}}
+ // CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'int'
+ // CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'int' <LValueToRValue>
+ // CHECK-NEXT: | `-MemberExpr {{.*}} 'int' xvalue .i
+ // CHECK-NEXT: | `-MemberExpr {{.*}} 'S::(anonymous struct at {{.*}})
+ // CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'S' xvalue
+ // CHECK-NEXT: | `-CXXTemporaryObjectExpr {{.*}} 'S' 'void () noexcept' zeroing
+}
+
+// AST should look the same in an instantiation of the function template.
+// This is a regression test: The AST used to contain the
+// `MaterializeTemporaryExpr` in the wrong place, causing a `MemberExpr` to have
+// a prvalue base (which is not allowed in C++).
+template int accessInFunctionTemplate<int>();
+ // CHECK: FunctionDecl {{.*}} accessInFunctionTemplate 'int ()' explicit_instantiation_definition
+ // CHECK: `-ReturnStmt {{.*}}
+ // CHECK-NEXT: `-ExprWithCleanups {{.*}} 'int'
+ // CHECK-NEXT: `-ImplicitCastExpr {{.*}} 'int' <LValueToRValue>
+ // CHECK-NEXT: `-MemberExpr {{.*}} 'int' xvalue .i
+ // CHECK-NEXT: `-MemberExpr {{.*}} 'S::(anonymous struct at {{.*}})
+ // CHECK-NEXT: `-MaterializeTemporaryExpr {{.*}} 'S' xvalue
+ // CHECK-NEXT: `-CXXTemporaryObjectExpr {{.*}} 'S' 'void () noexcept' zeroing
diff --git a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
index 301bec32c0cf1d..3f68adaf1c62d2 100644
--- a/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
+++ b/clang/unittests/Analysis/FlowSensitive/TransferTest.cpp
@@ -27,6 +27,14 @@
#include <string>
#include <utility>
+namespace clang {
+namespace dataflow {
+namespace {
+AST_MATCHER(FunctionDecl, isTemplated) { return Node.isTemplated(); }
+} // namespace
+} // namespace dataflow
+} // namespace clang
+
namespace {
using namespace clang;
@@ -7205,4 +7213,31 @@ TEST(TransferTest, ConditionalRelation) {
});
}
+// This is a crash repro.
+// We used to crash while transferring `S().i` because Clang contained a bug
+// causing the AST to be malformed.
+TEST(TransferTest, AnonymousUnionMemberExprInTemplate) {
+ using ast_matchers::functionDecl;
+ using ast_matchers::hasName;
+ using ast_matchers::unless;
+
+ std::string Code = R"cc(
+ struct S {
+ struct {
+ int i;
+ };
+ };
+
+ template <class>
+ void target() {
+ S().i;
+ }
+
+ template void target<int>();
+ )cc";
+ auto Matcher = functionDecl(hasName("target"), unless(isTemplated()));
+ ASSERT_THAT_ERROR(checkDataflowWithNoopAnalysis(Code, Matcher),
+ llvm::Succeeded());
+}
+
} // namespace
``````````
</details>
https://github.com/llvm/llvm-project/pull/90842
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