Mbed-os: STM serial configured in Simplex causes runtime assert

[alkaes]

Description of defect

System halts on assert when declaring a serial object that one of the pins is declared as NC (according to official Mbed OS reference)
For example: simple declaration of: Serial tx_only_ser(PE_8, NC);
Hangs on assert at: https://github.com/ARMmbed/mbed-os/blob/bacf37894945a5c9b7619b709eeabeeb68d25d9c/targets/TARGET_STM/pinmap.c#L62
That is called from: https://github.com/ARMmbed/mbed-os/blob/bacf37894945a5c9b7619b709eeabeeb68d25d9c/targets/TARGET_STM/serial_api.c#L161

Target(s) affected by this defect ?

STM

Toolchain(s) (name and version) displaying this defect ?

IAR 8.40

What version of Mbed-os are you using (tag or sha) ?

mbed-os-5.15.0

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

mbed-cli

How is this defect reproduced ?

main()
{
 Serial tx_only_serial(PE_8, NC); // halts here
 tx_only_serial.putc('A');
}

[ciarmcom]

Internal Jira reference: https://jira.arm.com/browse/MBOTRIAGE-2487

[0xc0170]

cc @ARMmbed/team-st-mcd

[jeromecoutant]

Hi

@mprse @kjbracey-arm

I think issue came with static pinmap feature.
Before that pinmap_pinout was called for each uart pin, which is OK when pin is "NC".
Now serial init calls pin_function which goes to assert when pin is "NC".

[mprse]

Looking into this.

[mprse]

The patch can be found here: PR https://github.com/ARMmbed/mbed-os/pull/12186.

[0xc0170]

Fixed via 12186