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The current master branch at commit af5a895073c24637c094772b27526b94a12ec897
fails while building the interpreter.
The following is the traceback while running `make` after `./configure --with-pydebug`
gcc -c -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -I. -I./Include -DPy_BUILD_CORE -o Modules/main.o Modules/main.c
Modules/main.c:904:20: error: expected expression
 return SET_DECODE_ERROR("PYTHONEXECUTABLE environment "
 ^
Modules/main.c:46:5: note: expanded from macro 'SET_DECODE_ERROR'
 do { \
 ^
1 error generated.
make: *** [Modules/main.o] Error 1

I used git bisect to track down the commit that broke the build.
Seems like this is the SHA: ebac19dad6263141d5db0a2c923efe049dba99d2
ebac19dad6263141d5db0a2c923efe049dba99d2 is the first bad commit
commit ebac19dad6263141d5db0a2c923efe049dba99d2
Author: Victor Stinner <victor.stinner@gmail.com>
Date: Fri Dec 1 20:09:52 2017 +0100
 bpo-32030: Don't call _PyPathConfig_Fini() in Py_FinalizeEx() (#4667)
 Changes:
 * _PyPathConfig_Fini() cannot be called in Py_FinalizeEx().
 Py_Initialize() and Py_Finalize() can be called multiple times, but
 it must not "forget" parameters set by Py_SetProgramName(),
 Py_SetPath() or Py_SetPythonHome(), whereas _PyPathConfig_Fini()
 clear all these parameters.
 * config_get_program_name() and calculate_program_full_path() now
 also decode paths using Py_DecodeLocale() to use the
 surrogateescape error handler, rather than decoding using
 mbstowcs() which is strict.
 * Change _Py_CheckPython3() prototype: () => (void)
 * Truncate a few lines which were too long
:040000 040000 de4ec929ff3fcd3c6455d8dae4d16e47fdd32ad5 8b289341f1b5e68757ac3b111a7f11808f5de1ec M	Include
:040000 040000 ecddc93b37f74818d85f6d8e8ab0e0c57e893c03 c51971a3af2b6f163e625505265277410bb9eedd M	Modules
:040000 040000 b92d30ac6375b7dde865a878e701684dd400e742 7d5d8810407060f32c9cbce20dcfc6f9d425524f M	PC
:040000 040000 537f9e0e6a27b41178fe7b828a8da99c63cc2d9b b14940c66180023a3704ad3949367db3e82ca832 M	Python

I tried to resolve the issue by trying to figure out the syntax for the SET_DECODE_ERROR macro.
I tried to call a function from the macro, and eventually do the checking in that function.
Second way I tried was to replace statement for macro instead of using do-while for placing the if-else clause.
I was stuck at the issue that at the point when the compiler tries to replace the macro with actual code, `pymain` wasn't defined.
If someone could provide a pointer on this, it would be really helpful and I'll try to work on a patch.
One simplest possible solution would be not use macro and simply use the code at the 3 places where it is being used. But I'll defer this anti-DRY approach if we can have a better solution.

Even a simple use of ternary operator instead of using if-else inside do-while to handle the macro like:
#define SET_DECODE_ERROR(NAME, LEN) \
 pymain->err = ((LEN) == (size_t)-2) \
 ? _Py_INIT_USER_ERR("cannot decode " #NAME) \
 : _Py_INIT_NO_MEMORY()
, throws the error `undeclared identifier pymain`:
gcc -c -Wno-unused-result -Wsign-compare -g -O0 -Wall -Wstrict-prototypes -std=c99 -Wextra -Wno-unused-result -Wno-unused-parameter -Wno-missing-field-initializers -Werror=implicit-function-declaration -I. -I./Include -DPy_BUILD_CORE -o Modules/main.o Modules/main.c
Modules/main.c:899:20: error: use of undeclared identifier 'pymain'
 return SET_DECODE_ERROR("PYTHONEXECUTABLE environment "
 ^
Modules/main.c:46:5: note: expanded from macro 'SET_DECODE_ERROR'
 pymain->err = ((LEN) == (size_t)-2) \
 ^
1 error generated.
make: *** [Modules/main.o] Error 1
I'm still trying to figure out a solution for this.

New changeset 13badcbc60cdbfae1dba1683fd2fae9d70717143 by Serhiy Storchaka in branch 'master':
bpo-32197: Try to fix a compiler error on OS X introduced in bpo-32030. (#4681)
https://github.com/python/cpython/commit/13badcbc60cdbfae1dba1683fd2fae9d70717143

New changeset 31a8393cf6a74c870c3484dd68500619f6232c6d by Victor Stinner in branch 'master':
Revert "bpo-32197: Try to fix a compiler error on OS X introduced in bpo-32030. (#4681)" (#4694)
https://github.com/python/cpython/commit/31a8393cf6a74c870c3484dd68500619f6232c6d

Oh sorry, I missed the macOS failure and I wasn't available last week-end for quickly fix it. So thanks Serhiy for the revert ;-) I reapplied my changes with the following fix. test_embed randomly failed, but it's unrelated (I was able to reproduce the bug on master, with the revert):
https://bugs.python.org/issue20891#msg307550
The fix:
diff --git a/Modules/main.c b/Modules/main.c
index 84706e1e290..4095259b88c 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -901,8 +901,8 @@ config_get_program_name(_PyMainInterpreterConfig *config)
 size_t len;
 wchar_t* program_name = Py_DecodeLocale(p, &len);
 if (program_name == NULL) {
- return SET_DECODE_ERROR("PYTHONEXECUTABLE environment "
- "variable", len);
+ return DECODE_LOCALE_ERR("PYTHONEXECUTABLE environment "
+ "variable", (Py_ssize_t)len);
 }
 config->program_name = program_name;
 }
@@ -916,8 +916,8 @@ config_get_program_name(_PyMainInterpreterConfig *config)
 size_t len;
 wchar_t* program_name = Py_DecodeLocale(pyvenv_launcher, &len);
 if (program_name == NULL) {
- return SET_DECODE_ERROR("__PYVENV_LAUNCHER__ environment "
- "variable", len);
+ return DECODE_LOCALE_ERR("__PYVENV_LAUNCHER__ environment "
+ "variable", (Py_ssize_t)len);
 }
 config->program_name = program_name;
 }

I tested manually that compilation and tests succeed on macOS.

Hey Victor, thanks for the update.
I was confused about how the macro was replaced in the previous version.