question about illegal utf-8 encoding in string literals

[Blower, Melanie]

Hello
I work for Intel on the Intel C++ compiler and we strive to be compatible with the gnu compiler.
We are processing a source file assuming utf-8 encoding and we see a string literal with illegal utf-8 encoding, such as an 8-bit character with the high bit set like 0xa3.
Testing shows that gcc is passes the illegal utf-8 character through without diagnostic message, as though it were an "extended ascii" character.
I don't see a way to enable warnings for this issue.
Please confirm that gcc handles illegal utf-8 encodings this way.
Thanks and regards, Melanie Blower