gc: fix error for floating-point constant %

R=ken2
CC=golang-dev
https://golang.org/cl/5674108

src/cmd/gc/const.c

@@ -660,6 +660,14 @@ evconst(Node *n)
 		}
 		mpdivfltflt(v.u.fval, rv.u.fval);
 		break;
+	case TUP(OMOD, CTFLT):
+		// The default case above would print 'ideal % ideal',
+		// which is not quite an ideal error.
+		if(!n->diag) {
+			yyerror("illegal constant expression: floating-point %% operation");
+			n->diag = 1;
+		}
+		return;
 	case TUP(OADD, CTCPLX):
 		mpaddfltflt(&v.u.cval->real, &rv.u.cval->real);
 		mpaddfltflt(&v.u.cval->imag, &rv.u.cval->imag);

test/const1.go

@@ -16,11 +16,11 @@ const (
 	Int8   int8  = 101
 	Minus1 int8  = -1
 	Uint8  uint8 = 102
-	Const  = 103
+	Const        = 103
 
 	Float32    float32 = 104.5
 	Float64    float64 = 105.5
-	ConstFloat = 106.5
+	ConstFloat         = 106.5
 	Big        float64 = 1e300
 
 	String = "abc"
@@ -38,32 +38,33 @@ var (
 	a8 = Int8 * Const / 100      // ERROR "overflow"
 	a9 = Int8 * (Const / 100)    // OK
 
-	b1   = Uint8 * Uint8                                 // ERROR "overflow"
-	b2   = Uint8 * -1                                    // ERROR "overflow"
-	b3   = Uint8 - Uint8                                 // OK
-	b4   = Uint8 - Uint8 - Uint8                         // ERROR "overflow"
-	b5   = uint8(^0)                                     // ERROR "overflow"
-	b6   = ^uint8(0)                                     // OK
-	b7   = uint8(Minus1)                                 // ERROR "overflow"
-	b8   = uint8(int8(-1))                               // ERROR "overflow"
-	b8a  = uint8(-1)                                     // ERROR "overflow"
-	b9   byte                    = (1 << 10) >> 8        // OK
-	b10  byte                    = (1 << 10)             // ERROR "overflow"
-	b11  byte                    = (byte(1) << 10) >> 8  // ERROR "overflow"
-	b12  byte                    = 1000                  // ERROR "overflow"
-	b13  byte                    = byte(1000)            // ERROR "overflow"
-	b14  byte                    = byte(100) * byte(100) // ERROR "overflow"
-	b15  byte                    = byte(100) * 100       // ERROR "overflow"
-	b16  byte                    = byte(0) * 1000        // ERROR "overflow"
-	b16a byte                    = 0 * 1000              // OK
-	b17  byte                    = byte(0) * byte(1000)  // ERROR "overflow"
-	b18  byte                    = Uint8 / 0             // ERROR "division by zero"
+	b1        = Uint8 * Uint8         // ERROR "overflow"
+	b2        = Uint8 * -1            // ERROR "overflow"
+	b3        = Uint8 - Uint8         // OK
+	b4        = Uint8 - Uint8 - Uint8 // ERROR "overflow"
+	b5        = uint8(^0)             // ERROR "overflow"
+	b6        = ^uint8(0)             // OK
+	b7        = uint8(Minus1)         // ERROR "overflow"
+	b8        = uint8(int8(-1))       // ERROR "overflow"
+	b8a       = uint8(-1)             // ERROR "overflow"
+	b9   byte = (1 << 10) >> 8        // OK
+	b10  byte = (1 << 10)             // ERROR "overflow"
+	b11  byte = (byte(1) << 10) >> 8  // ERROR "overflow"
+	b12  byte = 1000                  // ERROR "overflow"
+	b13  byte = byte(1000)            // ERROR "overflow"
+	b14  byte = byte(100) * byte(100) // ERROR "overflow"
+	b15  byte = byte(100) * 100       // ERROR "overflow"
+	b16  byte = byte(0) * 1000        // ERROR "overflow"
+	b16a byte = 0 * 1000              // OK
+	b17  byte = byte(0) * byte(1000)  // ERROR "overflow"
+	b18  byte = Uint8 / 0             // ERROR "division by zero"
 
-	c1 float64     = Big
-	c2 float64     = Big * Big          // ERROR "overflow"
-	c3 float64     = float64(Big) * Big // ERROR "overflow"
-	c4 = Big * Big                      // ERROR "overflow"
-	c5 = Big / 0                        // ERROR "division by zero"
+	c1 float64 = Big
+	c2 float64 = Big * Big          // ERROR "overflow"
+	c3 float64 = float64(Big) * Big // ERROR "overflow"
+	c4         = Big * Big          // ERROR "overflow"
+	c5         = Big / 0            // ERROR "division by zero"
+	c6         = 1000 % 1e3         // ERROR "floating-point % operation"
 )
 
 func f(int)