[Clang] correct error message when assigning to const reference captured in lambda #105647
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@llvm/pr-subscribers-clang Author: None (nfrmtk) ChangesFixes:#98772 Full diff: https://github.com/llvm/llvm-project/pull/105647.diff 2 Files Affected:
diff --git a/clang/lib/Sema/SemaExpr.cpp b/clang/lib/Sema/SemaExpr.cpp
index c67183df335dd5..9c256fd1ed1a41 100644
--- a/clang/lib/Sema/SemaExpr.cpp
+++ b/clang/lib/Sema/SemaExpr.cpp
@@ -13140,6 +13140,8 @@ static NonConstCaptureKind isReferenceToNonConstCapture(Sema &S, Expr *E) {
VarDecl *var = dyn_cast<VarDecl>(DRE->getDecl());
if (!var) return NCCK_None;
if (var->getType().isConstQualified()) return NCCK_None;
+ if (var->getType()->isReferenceType())
+ return NCCK_None;
assert(var->hasLocalStorage() && "capture added 'const' to non-local?");
// Decide whether the first capture was for a block or a lambda.
diff --git a/clang/test/SemaCXX/lambda-expressions.cpp b/clang/test/SemaCXX/lambda-expressions.cpp
index acf8d014a9896b..612c6db950f273 100644
--- a/clang/test/SemaCXX/lambda-expressions.cpp
+++ b/clang/test/SemaCXX/lambda-expressions.cpp
@@ -189,6 +189,11 @@ namespace ModifyingCapture {
[=] {
n = 1; // expected-error {{cannot assign to a variable captured by copy in a non-mutable lambda}}
};
+ const int cn = 0;
+ // cxx03-cxx11-warning@+1 {{initialized lambda captures are a C++14 extension}}
+ [&cnr = cn]{ // expected-note {{variable 'cnr' declared const here}}
+ cnr = 1; // expected-error {{cannot assign to variable 'cnr' with const-qualified type 'const int &'}}
+ };
}
}
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Note emitted for this line is technically correct, because according to https://eel.is/c++draft/expr.prim.lambda#capture-6, this is where we deduce type of cnr to be const int&. But I believe it would be significantly more helpful if we point out to a const in the declaration of cn, because that's the real reason we can't perform the assignment.
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Hello.
I've thought of this too.
IMO the change shouldn't be limited to lambdas, but to diagnosing attempts to mutate objects behind const references in general.
I think, the most precise (if this word makes any sense in the context) solution is to point to the first place where user added const to type. For example
int a = 5;
const auto& ar = a;
auto& arr = ar;
auto& arrr = arr;
arrr = 2;
Should make note pointing to second line(currently points to fifth).
But I'm not sure if this is common case and will be helpful to the final user.
So, I'm confused and need some help.
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I'm not sure we want to go this far and look through explicit declarations with deduced type.
CC @cor3ntin
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ping @cor3ntin |
1 similar comment
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ping @cor3ntin |
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@nfrmtk I'm so sorry this fell off my radar. Feel free pinging every week |
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can you fix the merge conflict and ping me so that I can merge? thanks a lot! |
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@cor3ntin ping |
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| [=] { | ||
| n = 1; // expected-error {{cannot assign to a variable captured by copy in a non-mutable lambda}} | ||
| }; | ||
| const int cn = 0; |
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I think we should have also tested a constexpr variable case and maybe a auto & assigned from cn as well just to cover a more complete test set.
5 participants
Fixes:#98772