gc: return constant floats for parts of complex constants

Fixes #1369.

R=rsc
CC=golang-dev
https://golang.org/cl/3731046

src/cmd/gc/go.h

@@ -1106,6 +1106,7 @@ Node*	nod(int op, Node *nleft, Node *nright);
 Node*	nodbool(int b);
 void	nodconst(Node *n, Type *t, int64 v);
 Node*	nodintconst(int64 v);
+Node*	nodfltconst(Mpflt *v);
 Node*	nodnil(void);
 int	parserline(void);
 Sym*	pkglookup(char *name, Pkg *pkg);

src/cmd/gc/subr.c

@@ -592,6 +592,21 @@ nodintconst(int64 v)
 	return c;
 }
 
+Node*
+nodfltconst(Mpflt* v)
+{
+	Node *c;
+
+	c = nod(OLITERAL, N, N);
+	c->addable = 1;
+	c->val.u.fval = mal(sizeof(*c->val.u.fval));
+	mpmovefltflt(c->val.u.fval, v);
+	c->val.ctype = CTFLT;
+	c->type = types[TIDEAL];
+	ullmancalc(c);
+	return c;
+}
+
 void
 nodconst(Node *n, Type *t, int64 v)
 {

src/cmd/gc/typecheck.c

@@ -829,6 +829,12 @@ reswitch:
 		case OIMAG:
 			if(!iscomplex[t->etype])
 				goto badcall1;
+			if(isconst(l, CTCPLX)){
+				if(n->op == OREAL)
+					n = nodfltconst(&l->val.u.cval->real);
+				else
+					n = nodfltconst(&l->val.u.cval->imag);
+			}
 			n->type = types[cplxsubtype(t->etype)];
 			goto ret;
 		}

test/fixedbugs/bug316.go

@@ -0,0 +1,17 @@
+// $G $D/$F.go || echo BUG: bug316
+
+// Copyright 2010 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 1369.
+
+package main
+
+const (
+	c = cmplx(1, 2)
+	r = real(c) // was: const initializer must be constant
+	i = imag(c) // was: const initializer must be constant
+)
+
+func main() {}