git diff --ignore-space-change -- ignore whitespace prefers unmodified next line to modified one in patch

Commit Message

Dipl. Ing. Sergey Brester Dec. 21, 2020, 2:43 p.m. UTC

Steps to reproduce the issue:

  1. create two simple C files and diff both ignoring whitespaces:
```
printf "if (some_var1) {\n  do {\n    some-code\n  } while (1);\n}\n" > 
a.c
printf "if (some_var1) {\n  do {\n    if (some_var2) {\n      
some-code\n    }\n  } while (1);\n}\n" > b.c
git diff --ignore-space-change --no-index a.c b.c
```
     which would produce fully correct resulting patch:
```
```

I understand that using --ignore-space-change both lines are quasi 
"equal", and then first "change"
will be noticed as a new created line, but from point of view of the 
developer (and common sense)
it is very strange either, especially if one considers that whole code 
starting from line 6 is totally unmodified.

This causes that on some much complex nested blocks, this simple "first 
wins" logic (without a simple look-ahead),
confuses still more (because affects several unmodified blocks, braces 
etc), especially if some
conflicts should be resolved or a 3-way merge/review takes place.

This makes the usage of option "--ignore-space-change" pretty pointless, 
because the diff of larger nested blocks
is often error prune on braces, brackets etc, so the review is often 
needless complicated.


[System Info]
git version:
git version 2.28.0.windows.1
cpu: x86_64
built from commit: 77982caf269b7ee713a76da2bcf260c34d3bf7a7
sizeof-long: 4
sizeof-size_t: 8
shell-path: /bin/sh
uname: Windows 10.0 18363
compiler info: gnuc: 10.2
libc info: no libc information available
$SHELL (typically, interactive shell): <unset>

Patch

diff --git a/a.c b/b.c
--- a/a.c
+++ b/b.c
@@ -1,5 +1,7 @@ 
  if (some_var1) {
    do {
+    if (some_var2) {
        some-code
+    }
    } while (1);
  }
```
     but this behavior will change if line 5 would be equal line 6 
(containing only brace ...).

  2. now we'll modify that a bit (e. g. use while instead of do, to make 
the lines more similar
     (containing only braces):
```
printf "if (some_var1) {\n  while (1) {\n    some-code\n  }\n}\n" > a.c
printf "if (some_var1) {\n  while (1) {\n    if (some_var2) {\n      
some-code\n    }\n  }\n}\n" > b.c
git diff --ignore-space-change --no-index a.c b.c
```
     which then would produce this result:
```
diff --git a/a.c b/b.c
--- a/a.c
+++ b/b.c
@@ -1,5 +1,7 @@ 
  if (some_var1) {
    while (1) {
+    if (some_var2) {
        some-code
      }
+  }
  }
```
     As one can see, a line 6 instead of line 5 is marked as new (thereby 
line 6 was **completely unmodified**).
     In my opinion correct would be this variant (also similar patch of 
test in step 1):
```
diff --git a/a.c b/b.c
--- a/a.c
+++ b/b.c
@@ -1,5 +1,7 @@ 
  if (some_var1) {
    while (1) {
+    if (some_var2) {
        some-code
+    }
    }
  }