[clang] 615d812 - [clang][ExprConstant] Improve error message of compound assignment against uninitialized object

[Takuya Shimizu via cfe-commits]

Author: Takuya Shimizu
Date: 2023-08-25T16:08:07+09:00
New Revision: 615d812696ee8827d6d78ad106d6022ff2778137

URL: https://github.com/llvm/llvm-project/commit/615d812696ee8827d6d78ad106d6022ff2778137
DIFF: https://github.com/llvm/llvm-project/commit/615d812696ee8827d6d78ad106d6022ff2778137.diff

LOG: [clang][ExprConstant] Improve error message of compound assignment against uninitialized object

BEFORE this patch, compound assignment operator against uninitialized object such as uninit += 1 was diagnosed as subexpression not valid
This patch clarifies the reason for the error by saying that uninit is an uninitialized object.

Fixes https://github.com/llvm/llvm-project/issues/51536

Reviewed By: shafik, tbaeder
Differential Revision: https://reviews.llvm.org/D157855

Added: 
    

Modified: 
    clang/docs/ReleaseNotes.rst
    clang/lib/AST/ExprConstant.cpp
    clang/test/SemaCXX/constant-expression-cxx14.cpp

Removed: 
    


################################################################################
diff  --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index a7c39bad615406..f0e601fc2df88b 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -140,6 +140,9 @@ Improvements to Clang's diagnostics
   tautologies like ``x && !x`` and ``!x || x`` in expressions. This also
   makes ``-Winfinite-recursion`` diagnose more cases.
   (`#56035: <https://github.com/llvm/llvm-project/issues/56035>`_).
+- Clang constexpr evaluator now diagnoses compound assignment operators against
+  uninitialized variables as a read of uninitialized object.
+  (`#51536 <https://github.com/llvm/llvm-project/issues/51536>_`)
 
 Bug Fixes in This Version
 -------------------------

diff  --git a/clang/lib/AST/ExprConstant.cpp b/clang/lib/AST/ExprConstant.cpp
index 51fdef708dde5a..d77c5d3f84a8d7 100644
--- a/clang/lib/AST/ExprConstant.cpp
+++ b/clang/lib/AST/ExprConstant.cpp
@@ -4442,6 +4442,10 @@ struct CompoundAssignSubobjectHandler {
       return foundPointer(Subobj, SubobjType);
     case APValue::Vector:
       return foundVector(Subobj, SubobjType);
+    case APValue::Indeterminate:
+      Info.FFDiag(E, diag::note_constexpr_access_uninit)
+          << /*read of=*/0 << /*uninitialized object=*/1;
+      return false;
     default:
       // FIXME: can this happen?
       Info.FFDiag(E);

diff  --git a/clang/test/SemaCXX/constant-expression-cxx14.cpp b/clang/test/SemaCXX/constant-expression-cxx14.cpp
index a43ee74e3df56c..273d7ff3a208e2 100644
--- a/clang/test/SemaCXX/constant-expression-cxx14.cpp
+++ b/clang/test/SemaCXX/constant-expression-cxx14.cpp
@@ -1275,3 +1275,33 @@ namespace TemporaryWithBadPointer {
     (dbt2.wp = nullptr, 0)
   };
 }
+
+namespace UninitCompoundAssign {
+constexpr int scalar(int a) {
+  int sum; // cxx14-warning {{uninitialized variable in a constexpr function is a C++20 extension}}
+  sum += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(scalar(3), ""); // expected-error {{constant expression}} \
+                              // expected-note {{in call to 'scalar(3)'}}
+
+constexpr int array(int a) {
+  int arr[3]; // cxx14-warning {{uninitialized variable in a constexpr function is a C++20 extension}}
+  arr[1] += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(array(3), ""); // expected-error {{constant expression}} \
+                             // expected-note {{in call to 'array(3)'}}
+
+struct Foo {
+  int val; // cxx14-note{{member not initialized by constructor}}
+  constexpr Foo() {} // cxx14-warning {{constexpr constructor that does not initialize all members is a C++20 extension}}
+};
+constexpr int field(int a) {
+  Foo f;
+  f.val += a; // expected-note {{read of uninitialized object}};
+  return 0;
+}
+static_assert(field(3), ""); // expected-error {{constant expression}} \
+                             // expected-note {{in call to 'field(3)'}}
+}