diff --git a/src/cmd/gofmt/testdata/go2numbers.golden b/src/cmd/gofmt/testdata/go2numbers.golden
new file mode 100644
index 0000000000..a227a58362
--- /dev/null
+++ b/src/cmd/gofmt/testdata/go2numbers.golden
@@ -0,0 +1,156 @@
+package p
+
+const (
+	_ = 0
+	_ = 0123
+	_ = 0123456
+
+	_ = 0_123
+	_ = 0123_456
+
+	// decimals
+	_ = 1
+	_ = 1234
+	_ = 1234567
+
+	_ = 1_234
+	_ = 1_234_567
+
+	// hexadecimals
+	_ = 0x0
+	_ = 0x1234
+	_ = 0xcafef00d
+
+	_ = 0X0
+	_ = 0X1234
+	_ = 0XCAFEf00d
+
+	_ = 0X_0
+	_ = 0X_1234
+	_ = 0X_CAFE_f00d
+
+	// octals
+	_ = 0o0
+	_ = 0o1234
+	_ = 0o01234567
+
+	_ = 0O0
+	_ = 0O1234
+	_ = 0O01234567
+
+	_ = 0o_0
+	_ = 0o_1234
+	_ = 0o0123_4567
+
+	_ = 0O_0
+	_ = 0O_1234
+	_ = 0O0123_4567
+
+	// binaries
+	_ = 0b0
+	_ = 0b1011
+	_ = 0b00101101
+
+	_ = 0B0
+	_ = 0B1011
+	_ = 0B00101101
+
+	_ = 0b_0
+	_ = 0b10_11
+	_ = 0b_0010_1101
+
+	// decimal floats
+	_ = 0.
+	_ = 123.
+	_ = 0123.
+
+	_ = .0
+	_ = .123
+	_ = .0123
+
+	_ = 0e0
+	_ = 123e+0
+	_ = 0123E-1
+
+	_ = 0e-0
+	_ = 123E+0
+	_ = 0123E123
+
+	_ = 0.e+1
+	_ = 123.E-10
+	_ = 0123.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 0.0
+	_ = 123.123
+	_ = 0123.0123
+
+	_ = 0.0e1
+	_ = 123.123E-10
+	_ = 0123.0123e+456
+
+	_ = 1_2_3.
+	_ = 0_123.
+
+	_ = 0_0e0
+	_ = 1_2_3e0
+	_ = 0_123e0
+
+	_ = 0e-0_0
+	_ = 1_2_3E+0
+	_ = 0123E1_2_3
+
+	_ = 0.e+1
+	_ = 123.E-1_0
+	_ = 01_23.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 1_2_3.123
+	_ = 0123.01_23
+
+	// hexadecimal floats
+	_ = 0x0.p+0
+	_ = 0Xdeadcafe.p-10
+	_ = 0x1234.P123
+
+	_ = 0x.1p-0
+	_ = 0X.deadcafep2
+	_ = 0x.1234P+10
+
+	_ = 0x0p0
+	_ = 0Xdeadcafep+1
+	_ = 0x1234P-10
+
+	_ = 0x0.0p0
+	_ = 0Xdead.cafep+1
+	_ = 0x12.34P-10
+
+	_ = 0Xdead_cafep+1
+	_ = 0x_1234P-10
+
+	_ = 0X_dead_cafe.p-10
+	_ = 0x12_34.P1_2_3
+
+	// imaginaries
+	_ = 0i
+	_ = 00i
+	_ = 1234i
+	_ = 1234567i
+
+	_ = 1_234i
+	_ = 1_234_567i
+
+	_ = 0.i
+	_ = 123.i
+	_ = 0123.i
+
+	_ = 0.e+1i
+	_ = 123.E-1_0i
+	_ = 01_23.e123i
+)
diff --git a/src/cmd/gofmt/testdata/go2numbers.input b/src/cmd/gofmt/testdata/go2numbers.input
new file mode 100644
index 0000000000..a227a58362
--- /dev/null
+++ b/src/cmd/gofmt/testdata/go2numbers.input
@@ -0,0 +1,156 @@
+package p
+
+const (
+	_ = 0
+	_ = 0123
+	_ = 0123456
+
+	_ = 0_123
+	_ = 0123_456
+
+	// decimals
+	_ = 1
+	_ = 1234
+	_ = 1234567
+
+	_ = 1_234
+	_ = 1_234_567
+
+	// hexadecimals
+	_ = 0x0
+	_ = 0x1234
+	_ = 0xcafef00d
+
+	_ = 0X0
+	_ = 0X1234
+	_ = 0XCAFEf00d
+
+	_ = 0X_0
+	_ = 0X_1234
+	_ = 0X_CAFE_f00d
+
+	// octals
+	_ = 0o0
+	_ = 0o1234
+	_ = 0o01234567
+
+	_ = 0O0
+	_ = 0O1234
+	_ = 0O01234567
+
+	_ = 0o_0
+	_ = 0o_1234
+	_ = 0o0123_4567
+
+	_ = 0O_0
+	_ = 0O_1234
+	_ = 0O0123_4567
+
+	// binaries
+	_ = 0b0
+	_ = 0b1011
+	_ = 0b00101101
+
+	_ = 0B0
+	_ = 0B1011
+	_ = 0B00101101
+
+	_ = 0b_0
+	_ = 0b10_11
+	_ = 0b_0010_1101
+
+	// decimal floats
+	_ = 0.
+	_ = 123.
+	_ = 0123.
+
+	_ = .0
+	_ = .123
+	_ = .0123
+
+	_ = 0e0
+	_ = 123e+0
+	_ = 0123E-1
+
+	_ = 0e-0
+	_ = 123E+0
+	_ = 0123E123
+
+	_ = 0.e+1
+	_ = 123.E-10
+	_ = 0123.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 0.0
+	_ = 123.123
+	_ = 0123.0123
+
+	_ = 0.0e1
+	_ = 123.123E-10
+	_ = 0123.0123e+456
+
+	_ = 1_2_3.
+	_ = 0_123.
+
+	_ = 0_0e0
+	_ = 1_2_3e0
+	_ = 0_123e0
+
+	_ = 0e-0_0
+	_ = 1_2_3E+0
+	_ = 0123E1_2_3
+
+	_ = 0.e+1
+	_ = 123.E-1_0
+	_ = 01_23.e123
+
+	_ = .0e-1
+	_ = .123E+10
+	_ = .0123E123
+
+	_ = 1_2_3.123
+	_ = 0123.01_23
+
+	// hexadecimal floats
+	_ = 0x0.p+0
+	_ = 0Xdeadcafe.p-10
+	_ = 0x1234.P123
+
+	_ = 0x.1p-0
+	_ = 0X.deadcafep2
+	_ = 0x.1234P+10
+
+	_ = 0x0p0
+	_ = 0Xdeadcafep+1
+	_ = 0x1234P-10
+
+	_ = 0x0.0p0
+	_ = 0Xdead.cafep+1
+	_ = 0x12.34P-10
+
+	_ = 0Xdead_cafep+1
+	_ = 0x_1234P-10
+
+	_ = 0X_dead_cafe.p-10
+	_ = 0x12_34.P1_2_3
+
+	// imaginaries
+	_ = 0i
+	_ = 00i
+	_ = 1234i
+	_ = 1234567i
+
+	_ = 1_234i
+	_ = 1_234_567i
+
+	_ = 0.i
+	_ = 123.i
+	_ = 0123.i
+
+	_ = 0.e+1i
+	_ = 123.E-1_0i
+	_ = 01_23.e123i
+)