Unexpected/Incorrect Path Conversion when Exporting Environment Variables

I recently updated my msys and noticed that some Makefile recipes started failing. Basically, whenever more than one path-like string is exported, a weird conversion kicks in that tries to split each single path as if it were a list of paths and then replaces the first forward slash /.

As far as I understand, make exports variables through the environment for subsequent invocations of tools using the export command, so this isn't the usual path mangling when using command-line arguments or at least I assume that make does not invoke a second executable and passes the exported data via command-line arguments to achieve this.

I reduced this down to the following minimal example:

ex
│   Makefile
└───dir
        Makefile

The Makefile contents are simple:

##########################################################################
# Makefile

.PHONY: foo
foo: export SINGLE += $(abspath file1)
foo: export DOUBLE += $(abspath file2) $(abspath file3)
foo:
	@echo "SHELL = $(SHELL)"
	@echo "SINGLE = $(SINGLE)"
	@echo "DOUBLE = $(DOUBLE)"
	+@cd dir && $(MAKE) bar && cd ..

##########################################################################
# dir/Makefile

.PHONY: bar
bar:
	@echo "SHELL = $(SHELL)"
	@echo "SINGLE = $(SINGLE)"
	@echo "DOUBLE = $(DOUBLE)"

The output with msys-2.0.dll version 3004.3.0.0 used to be:

$ mingw32-make foo
SHELL = C:/msys2/usr/bin/sh.exe
SINGLE = C:/tmp/ex/file1
DOUBLE = C:/tmp/ex/file2 C:/tmp/ex/file3
mingw32-make[1]: Entering directory 'C:/tmp/ex/dir'
SHELL = C:/msys2/usr/bin/sh.exe
SINGLE = C:/tmp/ex/file1
DOUBLE = C:/tmp/ex/file2 C:/tmp/ex/file3
mingw32-make[1]: Leaving directory 'C:/tmp/ex/dir'

However with msys-2.0.dll version 3004.6.0.0 the output is:

$ mingw32-make foo
SHELL = C:/msys2/usr/bin/sh.exe
SINGLE = C:/tmp/ex/file1
DOUBLE = C:/tmp/ex/file2 C:/tmp/ex/file3
mingw32-make[1]: Entering directory 'C:/tmp/ex/dir'
SHELL = C:/msys2/usr/bin/sh.exe
SINGLE = C:/tmp/ex/file1
DOUBLE = C;C:\msys2\ex\file2 C;C:\msys2\ex\file3        # <-- error on this line
mingw32-make[1]: Leaving directory 'C:/tmp/ex/dir'

Basically, the each path is treated as if the first colon : was actually a separator within a list of paths, then the first forward slash / is replaced by the msys2 base directory and then the paths are concatenated again using the windows separator for lists of paths, semicolon ;.

I strongly suspect this is related to #150, because as one can see, it does not happen with just a single path.
However, curiously this error does not occur when the exported list itself starts with a space, so using foo: export DOUBLE := $(DOUBLE) $(abspath file2) $(abspath file3) (DOUBLE is initially empty) does not trigger this behavior. This leads me to believe this might be a slightly different case.

I also noticed that the behavior does not occur when using SHELL = cmd, so it's definitely the invocation of sh.exe in line 3 of the foo recipe that's causing the issue. Unfortunately, MSYS2_ARG_CONV_EXCL does not have any effect on this.

I apologize for the somewhat lengthy issue ;)

Start by reproducing the minimal Makefile example with the two reported msys-2.0.dll versions and compare the nested mingw32-make invocation through sh.exe. The payload names no implementation files or tests; done means exported values containing multiple paths remain unchanged across the nested invocation, with coverage for the reported case.