Fixes bug so that %v with modifiers doesn't compile. %v is not in…

…tended to work with modifiers because the meaning of modifiers is type-dependent and `%v` is intended to be used in situations where the type is not important. Please continue using if `%s` if you require format modifiers.

PiperOrigin-RevId: 472534916
Change-Id: I5838761b2b40cbc4344077f23d44b1e634e5bae3

absl/strings/internal/str_format/checker.h

@@ -212,6 +212,10 @@ class ConvParser {
   constexpr ConvParser ParseConversion() const {
     char first_char = GetChar(format_, 0);
 
+    if (first_char == 'v' && *(format_.data() - 1) != '%') {
+      return SetError(true);
+    }
+
     if (is_positional_) {
       return VerifyPositional({ConsumeFront(format_), arg_position_},
                               first_char);

absl/strings/str_format_test.cc

@@ -291,8 +291,6 @@ TEST_F(FormatEntryPointTest, FormatStreamed) {
 
 TEST_F(FormatEntryPointTest, FormatStreamedWithV) {
   EXPECT_EQ("123", StrFormat("%v", FormatStreamed(123)));
-  EXPECT_EQ("  123", StrFormat("%5v", FormatStreamed(123)));
-  EXPECT_EQ("123  ", StrFormat("%-5v", FormatStreamed(123)));
   EXPECT_EQ("X", StrFormat("%v", FormatStreamed(streamed_test::X())));
   EXPECT_EQ("123", StrFormat("%v", FormatStreamed(StreamFormat("%d", 123))));
 }