[clang] [clang][analyzer] Support `fgetc` in StreamChecker (PR #72627)

[Balázs Kéri via cfe-commits]

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@@ -259,14 +283,33 @@ void error_indeterminate_clearerr(void) {
   fclose(F);
 }
 
+void error_indeterminate_fgetc(void) {
+  FILE *F = fopen("file", "r+");
+  if (!F)
+    return;
+  int rc = fseek(F, 0, SEEK_SET);
+  if (rc) {
+    if (feof(F)) {
+      clang_analyzer_warnIfReached(); // no warning
+      fgetc(F);                       // no warning
+    } else if (ferror(F)) {
+      fgetc(F); // expected-warning {{might be 'indeterminate'}}
+    } else {
+      fgetc(F); // expected-warning {{might be 'indeterminate'}}
+    }
+  }
+  fclose(F);
+}
+
 void error_indeterminate_fputc(void) {
   FILE *F = fopen("file", "r+");
   if (!F)
     return;
   int rc = fseek(F, 0, SEEK_SET);
   if (rc) {
     if (feof(F)) {
----------------
balazske wrote:

The branch `feof(F)` is not needed at all here. There is a previous test case that tells that `feof(F)` is never true when fseek to a 0 position is called.

https://github.com/llvm/llvm-project/pull/72627