[cfe-dev] A disappearing CXXBindTemporaryExpr.
Artem Dergachev via cfe-dev
cfe-dev at lists.llvm.org
Mon Mar 26 19:58:50 PDT 2018
I've a C++ AST question.
I've got a piece of code that doesn't contain CXXBindTemporaryExpr in
its AST, but would have contained it if only i explicitly unwrapped a
template that contains it *or* changed a field visibility in a certain
class from private to public. I want to understand if it's intentional
and if so what's the language feature that provides such behavior. A few
hours of debugging Sema didn't quite help me (but i'm not super good
with it).
The code looks like this and compiles under -std=c++14:
```
class A {
public:
~A();
};
class B {
A a;
};
template <typename T> void foo(T) {
B i;
i = {};
}
int main() {
foo(1);
return 0;
}
```
AST for foo():
```
|-FunctionTemplateDecl 0x7fd5a8800f50 <line:10:1, line:13:1> line:10:28 foo
| |-TemplateTypeParmDecl 0x7fd5a8800ce8 <col:11, col:20> col:20
referenced typename depth 0 index 0 T
| |-FunctionDecl 0x7fd5a8800eb0 <col:23, line:13:1> line:10:28 foo 'void
(T)'
| | |-ParmVarDecl 0x7fd5a8800db0 <col:32> col:33 'T'
| | `-CompoundStmt 0x7fd5a8801740 <col:35, line:13:1>
| | |-DeclStmt 0x7fd5a8801320 <line:11:3, col:6>
| | | `-VarDecl 0x7fd5a8800ff8 <col:3, col:5> col:5 referenced i 'B'
callinit
| | | `-CXXConstructExpr 0x7fd5a88012f0 <col:5> 'B' 'void () noexcept'
| | `-ExprWithCleanups 0x7fd5a8801728 <line:12:3, col:8> 'B' lvalue
| | `-CXXOperatorCallExpr 0x7fd5a88016e0 <col:3, col:8> 'B' lvalue
| | |-ImplicitCastExpr 0x7fd5a88016c8 <col:5> 'B &(*)(B &&)
noexcept' <FunctionToPointerDecay>
| | | `-DeclRefExpr 0x7fd5a8801648 <col:5> 'B &(B &&) noexcept'
lvalue CXXMethod 0x7fd5a785de28 'operator=' 'B &(B &&) noexcept'
| | |-DeclRefExpr 0x7fd5a8801338 <col:3> 'B' lvalue Var
0x7fd5a8800ff8 'i' 'B'
| | `-MaterializeTemporaryExpr 0x7fd5a88015b0 <col:7, col:8> 'B'
xvalue
| | `-CXXBindTemporaryExpr 0x7fd5a8801590 <col:7, col:8> 'B'
(CXXTemporary 0x7fd5a8801588)
| | `-CXXConstructExpr 0x7fd5a8801558 <col:7, col:8> 'B' 'void
() noexcept' list zeroing
| `-FunctionDecl 0x7fd5a8801b10 <line:10:23, line:13:1> line:10:28 used
foo 'void (int)'
| |-TemplateArgument type 'int'
| |-ParmVarDecl 0x7fd5a8801a08 <col:32> col:33 'int':'int'
| `-CompoundStmt 0x7fd5a8802e28 <col:35, line:13:1>
| |-DeclStmt 0x7fd5a88029f8 <line:11:3, col:6>
| | `-VarDecl 0x7fd5a8802958 <col:3, col:5> col:5 used i 'B' callinit
| | `-CXXConstructExpr 0x7fd5a88029b8 <col:5> 'B' 'void () noexcept'
| `-ExprWithCleanups 0x7fd5a8802e10 <line:12:3, col:8> 'B' lvalue
| `-CXXOperatorCallExpr 0x7fd5a8802dc8 <col:3, col:8> 'B' lvalue
| |-ImplicitCastExpr 0x7fd5a8802db0 <col:5> 'B &(*)(B &&)
noexcept' <FunctionToPointerDecay>
| | `-DeclRefExpr 0x7fd5a8802d88 <col:5> 'B &(B &&) noexcept'
lvalue CXXMethod 0x7fd5a785de28 'operator=' 'B &(B &&) noexcept'
| |-DeclRefExpr 0x7fd5a8802d48 <col:3> 'B' lvalue Var
0x7fd5a8802958 'i' 'B'
| `-MaterializeTemporaryExpr 0x7fd5a8802d70 <col:7, col:8> 'B'
xvalue
| `-CXXConstructExpr 0x7fd5a8801558 <col:7, col:8> 'B' 'void
() noexcept' list zeroing
```
As you see, there's a CXXBindTemporaryExpr in the template, but it's not
there in the instantiation. I expected the CXXBindTemporaryExpr to be
there whenever the object has a non-trivial destructor, but in this case
it's suddenly missing.
However, if i replace "template <typename T> void foo(T)" with "void
foo(int)" (to which it should resolve anyway), CXXBindTemporaryExpr
would be present.
Moreover, if instead of unrolling the template i simply add "public:"
before "A a;", the mysterious CXXBindTemporaryExpr would also be present.
WHY I've a C++ AST question.
I've got a piece of code that doesn't contain CXXBindTemporaryExpr in
its AST, but would have contained it if only i explicitly unwrapped a
template that contains it *or* changed a field visibility in a certain
class from private to public. I want to understand if it's intentional
and if so what's the language feature that provides such behavior. A few
hours of debugging Sema didn't quite help me (but i'm not super good
with it).
The code looks like this and compiles under -std=c++14:
```
class A {
public:
~A();
};
class B {
A a;
};
template <typename T> void foo(T) {
B i;
i = {};
}
int main() {
foo(1);
return 0;
}
```
AST for foo():
```
|-FunctionTemplateDecl 0x7fd5a8800f50 <line:10:1, line:13:1> line:10:28 foo
| |-TemplateTypeParmDecl 0x7fd5a8800ce8 <col:11, col:20> col:20
referenced typename depth 0 index 0 T
| |-FunctionDecl 0x7fd5a8800eb0 <col:23, line:13:1> line:10:28 foo 'void
(T)'
| | |-ParmVarDecl 0x7fd5a8800db0 <col:32> col:33 'T'
| | `-CompoundStmt 0x7fd5a8801740 <col:35, line:13:1>
| | |-DeclStmt 0x7fd5a8801320 <line:11:3, col:6>
| | | `-VarDecl 0x7fd5a8800ff8 <col:3, col:5> col:5 referenced i 'B'
callinit
| | | `-CXXConstructExpr 0x7fd5a88012f0 <col:5> 'B' 'void () noexcept'
| | `-ExprWithCleanups 0x7fd5a8801728 <line:12:3, col:8> 'B' lvalue
| | `-CXXOperatorCallExpr 0x7fd5a88016e0 <col:3, col:8> 'B' lvalue
| | |-ImplicitCastExpr 0x7fd5a88016c8 <col:5> 'B &(*)(B &&)
noexcept' <FunctionToPointerDecay>
| | | `-DeclRefExpr 0x7fd5a8801648 <col:5> 'B &(B &&) noexcept'
lvalue CXXMethod 0x7fd5a785de28 'operator=' 'B &(B &&) noexcept'
| | |-DeclRefExpr 0x7fd5a8801338 <col:3> 'B' lvalue Var
0x7fd5a8800ff8 'i' 'B'
| | `-MaterializeTemporaryExpr 0x7fd5a88015b0 <col:7, col:8> 'B'
xvalue
| | `-CXXBindTemporaryExpr 0x7fd5a8801590 <col:7, col:8> 'B'
(CXXTemporary 0x7fd5a8801588)
| | `-CXXConstructExpr 0x7fd5a8801558 <col:7, col:8> 'B' 'void
() noexcept' list zeroing
| `-FunctionDecl 0x7fd5a8801b10 <line:10:23, line:13:1> line:10:28 used
foo 'void (int)'
| |-TemplateArgument type 'int'
| |-ParmVarDecl 0x7fd5a8801a08 <col:32> col:33 'int':'int'
| `-CompoundStmt 0x7fd5a8802e28 <col:35, line:13:1>
| |-DeclStmt 0x7fd5a88029f8 <line:11:3, col:6>
| | `-VarDecl 0x7fd5a8802958 <col:3, col:5> col:5 used i 'B' callinit
| | `-CXXConstructExpr 0x7fd5a88029b8 <col:5> 'B' 'void () noexcept'
| `-ExprWithCleanups 0x7fd5a8802e10 <line:12:3, col:8> 'B' lvalue
| `-CXXOperatorCallExpr 0x7fd5a8802dc8 <col:3, col:8> 'B' lvalue
| |-ImplicitCastExpr 0x7fd5a8802db0 <col:5> 'B &(*)(B &&)
noexcept' <FunctionToPointerDecay>
| | `-DeclRefExpr 0x7fd5a8802d88 <col:5> 'B &(B &&) noexcept'
lvalue CXXMethod 0x7fd5a785de28 'operator=' 'B &(B &&) noexcept'
| |-DeclRefExpr 0x7fd5a8802d48 <col:3> 'B' lvalue Var
0x7fd5a8802958 'i' 'B'
| `-MaterializeTemporaryExpr 0x7fd5a8802d70 <col:7, col:8> 'B'
xvalue
| `-CXXConstructExpr 0x7fd5a8801558 <col:7, col:8> 'B' 'void
() noexcept' list zeroing
```
As you see, there's a CXXBindTemporaryExpr in the template, but it's not
there in the instantiation. I expected the CXXBindTemporaryExpr to be
there whenever the object has a non-trivial destructor, but in this case
it's suddenly missing.
However, if i replace "template <typename T> void foo(T)" with "void
foo(int)" (to which it should resolve anyway), CXXBindTemporaryExpr
would be present.
Moreover, if instead of unrolling the template i simply add "public:"
before "A a;", the mysterious CXXBindTemporaryExpr would also be present.
WHY :)
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