The intellisense for the stm32 has been improved in the next release. If you click F12 or GoTo on the strstr() you will see that the method is wrapped in a special gcc EXTFUNC() call that then adds the code of the funition "inline".

Before the next release you have intellisense workaround the problem by adding a "__vm\vs-intellisense-fix.h" to the project folder. The .h does NOT actually have to be included in the project or referenced in your code. 

After adding the vs-intellisense-fix.h then past the following code into it and click build to force an intellisense refresh.

#pragma once

_PTR 	 memchr(const _PTR, int, size_t);
_PTR 	 memchr(const _PTR, int, size_t) {}
int 	  memcmp(const _PTR, const _PTR, size_t);
_PTR 	  memcpy(_PTR __restrict, const _PTR __restrict, size_t);
_PTR	  memmove(_PTR, const _PTR, size_t);
_PTR	  memset(_PTR, int, size_t);
_PTR	  memset(_PTR, int, size_t) {}
char* strcat(char* __restrict, const char* __restrict);
char* strcat(char* __restrict, const char* __restrict) {}
char* strchr(const char*, int);
char* strchr(const char*, int) {};
int	  strcmp(const char*, const char*);
int	  strcmp(const char*, const char*) {}
int	  strcoll(const char*, const char*);
int	  strcoll(const char*, const char*) {}
char* strcpy(char* __restrict, const char* __restrict);
char* strcpy(char* __restrict, const char* __restrict) {}
size_t	  strcspn(const char*, const char*);
size_t	  strcspn(const char*, const char*) {}
char* strerror(int);
char* strerror(int) {}
size_t	  strlen(const char*);
size_t	  strlen(const char*) {}
char* strncat(char* __restrict, const char* __restrict, size_t);
char* strncat(char* __restrict, const char* __restrict, size_t) {}
int	  strncmp(const char*, const char*, size_t);
int	  strncmp(const char*, const char*, size_t) {}
char* strncpy(char* __restrict, const char* __restrict, size_t);
char* strncpy(char* __restrict, const char* __restrict, size_t) {}
char* strpbrk(const char*, const char*);
char* strpbrk(const char*, const char*) {}
char* strrchr(const char*, int);
char* strrchr(const char*, int) {}
size_t	  strspn(const char*, const char*);
size_t	  strspn(const char*, const char*) {}
char 	* strstr (const char *, const char *);
char 	* strstr(const char *, const char *){}
#ifndef _REENT_ONLY
char* strtok(char* __restrict, const char* __restrict);
char* strtok(char* __restrict, const char* __restrict) {}
#endif
size_t	  strxfrm(char* __restrict, const char* __restrict, size_t);
size_t	  strxfrm(char* __restrict, const char* __restrict, size_t) {}

#if __POSIX_VISIBLE >= 200809
int	 strcoll_l(const char*, const char*, locale_t);
int	 strcoll_l(const char*, const char*, locale_t) {}
char* strerror_l(int, locale_t);
char* strerror_l(int, locale_t) {}
size_t	 strxfrm_l(char* __restrict, const char* __restrict, size_t, locale_t);
size_t	 strxfrm_l(char* __restrict, const char* __restrict, size_t, locale_t) {}
#endif

#if __MISC_VISIBLE || __POSIX_VISIBLE
char* strtok_r(char* __restrict, const char* __restrict, char** __restrict);
char* strtok_r(char* __restrict, const char* __restrict, char** __restrict) {}
#endif

#if __BSD_VISIBLE
int	  timingsafe_bcmp(const void*, const void*, size_t);
int	  timingsafe_bcmp(const void*, const void*, size_t) {}
int	  timingsafe_memcmp(const void*, const void*, size_t);
int	  timingsafe_memcmp(const void*, const void*, size_t) {}
#endif

#if __MISC_VISIBLE || __POSIX_VISIBLE
//_PTR	  memccpy (_PTR __restrict, const _PTR __restrict, int, size_t);
#endif

#if __GNU_VISIBLE
_PTR	  mempcpy (_PTR, const _PTR, size_t);
_PTR	  memmem (const _PTR, size_t, const _PTR, size_t);
_PTR 	  memrchr (const _PTR, int, size_t);
_PTR 	  rawmemchr (const _PTR, int);
#endif

#if __POSIX_VISIBLE >= 200809
char* stpcpy(char* __restrict, const char* __restrict);
char* stpcpy(char* __restrict, const char* __restrict) {}
char* stpncpy(char* __restrict, const char* __restrict, size_t);
char* stpncpy(char* __restrict, const char* __restrict, size_t) {}
#endif

#if __GNU_VISIBLE
char* strcasestr (const char*, const char*);
char* strchrnul (const char*, int);
#endif

#if __MISC_VISIBLE || __POSIX_VISIBLE >= 200809 || __XSI_VISIBLE >= 4
char* strdup(const char*);
char* strdup(const char*) {}
#endif
char* _strdup_r(struct _reent*, const char*);
char* _strdup_r(struct _reent*, const char*) {}
#if __POSIX_VISIBLE >= 200809
char* strndup(const char*, size_t);
char* strndup(const char*, size_t) {}
#endif
char* _strndup_r(struct _reent*, const char*, size_t);
char* _strndup_r(struct _reent*, const char*, size_t) {}

 

This is an area (intellisense) that will improve when we move from VC++ proejcts to clang.