go/types: implement constant string(x) conversions

Fixes #4982.

R=adonovan, r
CC=golang-dev
https://golang.org/cl/7537043

src/pkg/go/types/conversions.go

@@ -30,12 +30,32 @@ func (check *checker) conversion(x *operand, conv *ast.CallExpr, typ Type, iota
 
 	if x.mode == constant && isConstType(typ) {
 		// constant conversion
-		// TODO(gri) implement this
+		typ := underlying(typ).(*Basic)
+		// For now just implement string(x) where x is an integer,
+		// as a temporary work-around for issue 4982, which is a
+		// common issue.
+		if typ.Kind == String {
+			switch {
+			case x.isInteger(check.ctxt):
+				codepoint, ok := x.val.(int64)
+				if !ok {
+					// absolute value too large (or unknown) for conversion;
+					// same as converting any other out-of-range value - let
+					// string(codepoint) do the work
+					codepoint = -1
+				}
+				x.val = string(codepoint)
+			case isString(x.typ):
+				// nothing to do
+			default:
+				goto ErrorMsg
+			}
+		}
+		// TODO(gri) verify the remaining conversions.
 	} else {
 		// non-constant conversion
 		if !x.isConvertible(check.ctxt, typ) {
-			check.invalidOp(conv.Pos(), "cannot convert %s to %s", x, typ)
-			goto Error
+			goto ErrorMsg
 		}
 		x.mode = value
 	}
@@ -45,8 +65,11 @@ func (check *checker) conversion(x *operand, conv *ast.CallExpr, typ Type, iota
 	x.typ = typ
 	return
 
+ErrorMsg:
+	check.invalidOp(conv.Pos(), "cannot convert %s to %s", x, typ)
 Error:
 	x.mode = invalid
+	x.expr = conv
 }
 
 func (x *operand) isConvertible(ctxt *Context, T Type) bool {

src/pkg/go/types/operand.go

@@ -205,6 +205,7 @@ func (x *operand) isAssignable(ctxt *Context, T Type) bool {
 }
 
 // isInteger reports whether x is a (typed or untyped) integer value.
+// TODO(gri) remove ctxt argument - it is not required for UntypedInt.
 func (x *operand) isInteger(ctxt *Context) bool {
 	return x.mode == invalid ||
 		isInteger(x.typ) ||

src/pkg/go/types/testdata/conversions.src

@@ -8,11 +8,29 @@ package conversions
 
 // argument count
 var (
-	_v0 = int /* ERROR "one argument" */ ()
-	_v1 = int /* ERROR "one argument" */ (1, 2)
+	_ = int /* ERROR "one argument" */ ()
+	_ = int /* ERROR "one argument" */ (1, 2)
 )
 
+func string_conversions() {
+	const A = string(65)
+	assert(A == "A")
+	const E = string(-1)
+	assert(E == "\uFFFD")
+	assert(E == string(1234567890))
+
+	type myint int
+	assert(A == string(myint(65)))
+
+	type mystring string
+	const _ mystring = mystring("foo")
+
+	const _ = string  /* ERROR "cannot convert" */ (true)
+	const _ = string  /* ERROR "cannot convert" */ (1.2)
+	const _ = string  /* ERROR "cannot convert" */ (nil)
+}
+
 // 
 var (
-	_v2 = int8(0)
+	_ = int8(0)
 )