clang-format 18 idempotent regression with #define within an initializer

[JohnC32]

Running clang-format 18 twice produces different results when we have #define statements within an initializer. Running with clang-format 17.0.5 shows no difference on repeated invocations.

Given idempotent_define.cpp

int foo(MyType aFooSizeKind) {
    static const AType0123456789 table{
#define FOO_SIZE_KIND_2_OP_ROW(row) {GET_SYM_SIZE_KIND(row), GET_OP(row)}

        COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_BINOP_SIZE_LIST),
        COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_UNARY_SIZE_LIST)};

    return table.at(aFooSizeKind);
}

using _clang-format:

---
BasedOnStyle: Google
IndentWidth: 4
ColumnLimit: 100
...

clang-format 18.1.1 yields the following differences on successive runs. Notice that define flips between being on a single line and multiple lines.

/path/to/llvm-18.1.1/bin/clang-format -style=file idempotent_define.cpp >out.18.1.1.a.cpp 2>&1
/path/to/llvm-18.1.1/bin/clang-format -style=file out.18.1.1.a.cpp >out.18.1.1.b.cpp 2>&1
/path/to/llvm-18.1.1/bin/clang-format -style=file out.18.1.1.b.cpp >out.18.1.1.c.cpp 2>&1

diff -u out.18.1.1.a.cpp out.18.1.1.b.cpp
--- out.18.1.1.a.cpp	2024-03-25 14:02:26.681362489 -0400
+++ out.18.1.1.b.cpp	2024-03-25 14:02:26.697362606 -0400
@@ -1,7 +1,6 @@
 int foo(MyType aFooSizeKind) {
     static const AType0123456789 table{
-#define FOO_SIZE_KIND_2_OP_ROW(row) \
-    { GET_SYM_SIZE_KIND(row), GET_OP(row) }
+#define FOO_SIZE_KIND_2_OP_ROW(row) {GET_SYM_SIZE_KIND(row), GET_OP(row)}
 
         COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_BINOP_SIZE_LIST),
         COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_UNARY_SIZE_LIST)};

diff -u out.18.1.1.b.cpp out.18.1.1.c.cpp
--- out.18.1.1.b.cpp	2024-03-25 14:02:26.697362606 -0400
+++ out.18.1.1.c.cpp	2024-03-25 14:02:26.715362737 -0400
@@ -1,6 +1,7 @@
 int foo(MyType aFooSizeKind) {
     static const AType0123456789 table{
-#define FOO_SIZE_KIND_2_OP_ROW(row) {GET_SYM_SIZE_KIND(row), GET_OP(row)}
+#define FOO_SIZE_KIND_2_OP_ROW(row) \
+    { GET_SYM_SIZE_KIND(row), GET_OP(row) }
 
         COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_BINOP_SIZE_LIST),
         COMMA_LIST_OF(FOO_SIZE_KIND_2_OP_ROW, FOOBAR12_UNARY_SIZE_LIST)};

diff -u out.18.1.1.a.cpp out.18.1.1.c.cpp

[owenca]

Bisected to 063e3fe. @rymiel