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pkg-config --version
0.29.2

cat app.pc:
prefix=/opt/app
exec_prefix=${prefix}
libdir=${exec_prefix}/lib64
includedir=${exec_prefix}/include

Name: app
Description: app lib
Version: 0
Libs: -L ${libdir} -l app -Wl,-rpath=${libdir}
Cflags: -I${includedir}

pkg-config --cflags app
-I/opt/app/include

Changing the Cflags line to: Cflags: -I ${includedir} that is, with a space between -I and the path, causes the following, (arguments reordered):

pkg-config --cflags app
/opt/app/include -I

Considering that both POSIX and gcc specify -I<SPACE>path as valid command line arguments for the c99/gcc commands respectivly, I consider this to be a bug.

POSIX: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/c99.html

c99 [options...] pathname [[pathname] [-I directory]
       [-L directory] [-l library]]... 
...
-I  directory

GCC: https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html#Directory-Options

3.16 Options for Directory Search

These options specify directories to search for header files, for libraries and for parts of the compiler:

-I dir