Workaround MSVC precision handling with "%a" conversion

Force precision to 13 in this case to guarentee roundtripping of double
precision hexfloat formatted numbers with MSVC.

Author: Chris Foster

README.md

@@ -204,10 +204,9 @@ Here's a list of known incompatibilities:
 
 * The `"%a"` and `"%A"` hexadecimal floating point conversions ignore precision
   as stream output of hexfloat (introduced in C++11) ignores precision, always
-  outputting the minimum number of digits required for exact representation
-  (MSVC incorrectly honors stream precision - the default precision of 6 digits
-  risks losing precision by truncation so to guarantee lossless roundtrip
-  conversion "%.13a" will output full precision padded with 0's as needed).
+  outputting the minimum number of digits required for exact representation.
+  MSVC incorrectly honors stream precision, so we force precision to 13 in this
+  case to guarentee lossless roundtrip conversion.
 * The precision for integer conversions cannot be supported by the iostreams
   state independently of the field width.  (Note: **this is only a
   problem for certain obscure integer conversions**; float conversions like

tinyformat.h

@@ -842,6 +842,12 @@ inline const char* streamStateFromFormat(std::ostream& out, bool& positionalMode
             out.setf(std::ios::uppercase);
             // Falls through
         case 'a':
+#           ifdef _MSC_VER
+            // Workaround https://developercommunity.visualstudio.com/content/problem/520472/hexfloat-stream-output-does-not-ignore-precision-a.html
+            // by always setting maximum precision on MSVC to avoid precision
+            // loss for doubles.
+            out.precision(13);
+#           endif
             out.setf(std::ios::fixed | std::ios::scientific, std::ios::floatfield);
             break;
         case 'G':

tinyformat_test.cpp

@@ -122,8 +122,13 @@ int unitTests()
     CHECK_EQUAL(tfm::format("%E", -1.23456E10), "-1.234560E+10");
     CHECK_EQUAL(tfm::format("%f", -9.8765), "-9.876500");
     CHECK_EQUAL(tfm::format("%F", 9.8765), "9.876500");
+#   ifndef _MSC_VER
     CHECK_EQUAL(tfm::format("%a", -1.671111047267913818359375), "-0x1.abcdefp+0");
-    CHECK_EQUAL(tfm::format("%A", 1.671111047267913818359375), "0X1.ABCDEFP+0");
+    CHECK_EQUAL(tfm::format("%A",  1.671111047267913818359375),  "0X1.ABCDEFP+0");
+#   else
+    CHECK_EQUAL(tfm::format("%a", -1.671111047267913818359375), "-0x1.abcdef0000000p+0");
+    CHECK_EQUAL(tfm::format("%A",  1.671111047267913818359375),  "0X1.ABCDEF0000000P+0");
+#   endif
     CHECK_EQUAL(tfm::format("%g", 10), "10");
     CHECK_EQUAL(tfm::format("%G", 100), "100");
     CHECK_EQUAL(tfm::format("%c", 65), "A");
@@ -165,9 +170,17 @@ int unitTests()
     CHECK_EQUAL(tfm::format("%.4d", 10), "0010");
     CHECK_EQUAL(tfm::format("%10.4f", 1234.1234567890), " 1234.1235");
     CHECK_EQUAL(tfm::format("%.f", 10.1), "10");
-	CHECK_EQUAL(tfm::format("%.13a", 0.1), "0x1.999999999999ap-4");
-	CHECK_EQUAL(tfm::format("%14a", 1.671111047267913818359375), " 0x1.abcdefp+0");
-	CHECK_EQUAL(tfm::format("%.2s", "asdf"), "as"); // strings truncate to precision
+    // Per C++ spec, iostreams ignore the precision for "%a" to avoid precision
+    // loss. This is a printf incompatibility.
+#   ifndef _MSC_VER
+    CHECK_EQUAL(tfm::format("%.1a", 1.13671875), "0x1.23p+0");
+    CHECK_EQUAL(tfm::format("%14a", 1.671111047267913818359375), " 0x1.abcdefp+0");
+#   else
+    // MSVC workaround
+    CHECK_EQUAL(tfm::format("%.1a", 1.13671875), "0x1.2300000000000p+0");
+    CHECK_EQUAL(tfm::format("%21a", 1.671111047267913818359375), " 0x1.abcdef0000000p+0");
+#   endif
+    CHECK_EQUAL(tfm::format("%.2s", "asdf"), "as"); // strings truncate to precision
     CHECK_EQUAL(tfm::format("%.2s", std::string("asdf")), "as");
     // Test variable precision & width
     CHECK_EQUAL(tfm::format("%*.4f", 10, 1234.1234567890), " 1234.1235");