[dev.regabi] cmd/compile: refactor type/value assertions

Small refactoring to make subsequent CLs clearer.

Passes toolstash-check.

Change-Id: I1a6ae599f491220d44aaabae0b7bed4aff46ee92
Reviewed-on: https://go-review.googlesource.com/c/go/+/272651
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/const.go

@@ -275,8 +275,8 @@ func convlit1(n *Node, t *types.Type, explicit bool, context func() string) *Nod
 		if v.U == nil {
 			break
 		}
-		n.SetVal(v)
 		n.Type = t
+		n.SetVal(v)
 		return n
 
 	case OPLUS, ONEG, OBITNOT, ONOT, OREAL, OIMAG:
@@ -979,9 +979,6 @@ func setconst(n *Node, v Val) {
 		Xoffset: BADWIDTH,
 	}
 	n.SetVal(v)
-	if vt := idealType(v.Ctype()); n.Type.IsUntyped() && n.Type != vt {
-		Fatalf("untyped type mismatch, have: %v, want: %v", n.Type, vt)
-	}
 
 	// Check range.
 	lno := setlineno(n)
@@ -1000,6 +997,22 @@ func setconst(n *Node, v Val) {
 	}
 }
 
+func assertRepresents(t *types.Type, v Val) {
+	if !represents(t, v) {
+		Fatalf("%v does not represent %v", t, v)
+	}
+}
+
+func represents(t *types.Type, v Val) bool {
+	if !t.IsUntyped() {
+		// TODO(mdempsky): Stricter handling of typed types.
+		return true
+	}
+
+	vt := idealType(v.Ctype())
+	return t == vt
+}
+
 func setboolconst(n *Node, v bool) {
 	setconst(n, Val{U: v})
 }
@@ -1013,8 +1026,8 @@ func setintconst(n *Node, v int64) {
 // nodlit returns a new untyped constant with value v.
 func nodlit(v Val) *Node {
 	n := nod(OLITERAL, nil, nil)
-	n.SetVal(v)
 	n.Type = idealType(v.Ctype())
+	n.SetVal(v)
 	return n
 }
 

src/cmd/compile/internal/gc/iexport.go

@@ -777,9 +777,7 @@ func constTypeOf(typ *types.Type) Ctype {
 }
 
 func (w *exportWriter) value(typ *types.Type, v Val) {
-	if vt := idealType(v.Ctype()); typ.IsUntyped() && typ != vt {
+	assertRepresents(typ, v)
-		Fatalf("exporter: untyped type mismatch, have: %v, want: %v", typ, vt)
-	}
 	w.typ(typ)
 
 	// Each type has only one admissible constant representation,

src/cmd/compile/internal/gc/syntax.go

@@ -251,6 +251,9 @@ func (n *Node) SetVal(v Val) {
 		Dump("have Opt", n)
 		Fatalf("have Opt")
 	}
+	if n.Op == OLITERAL {
+		assertRepresents(n.Type, v)
+	}
 	n.SetHasVal(true)
 	n.E = v.U
 }

src/cmd/compile/internal/gc/typecheck.go

@@ -3624,8 +3624,8 @@ func typecheckdef(n *Node) {
 			e = convlit(e, t)
 		}
 
-		n.SetVal(e.Val())
 		n.Type = e.Type
+		n.SetVal(e.Val())
 
 	case ONAME:
 		if n.Name.Param.Ntype != nil {