cmd/compile/internal/noder: fix implicit conversion position

In go.dev/cl/413396, I implemented implicit conversions by setting the
conversion's position to the enclosing statement that necessitated the
conversion. However, users actually want the position information to
be at the expression itself, and this seems sensible anyway.

This was noticed because x/tools had a test for:

	fmt.Println(42)

and it was checking where the escape analysis diagnostic for
`42` (really `any(42)`) was reported.

Historically, we reported the column of the `4`; but CL 413396 caused
unified IR to instead report the column of the `(` instead (the
position associated with the call expression, which forced `42` to be
implicitly converted from `int` to `any`).

I chalk this mistake up to being accustomed to working with ir, where
we can't reliably use n.Pos() because of how ONAME positions work, so
I was trying to avoid relying on the implicitly converted expression's
own position.

Change-Id: I762076af6f65ebe6d444d64630722a5016dc2698
Reviewed-on: https://go-review.googlesource.com/c/go/+/422976
Reviewed-by: David Chase <[email protected]>
Run-TryBot: Matthew Dempsky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/cmd/compile/internal/noder/writer.go

@@ -1060,7 +1060,7 @@ func (w *writer) stmt1(stmt syntax.Stmt) {
 			if stmt.Op != syntax.Shl && stmt.Op != syntax.Shr {
 				typ = w.p.typeOf(stmt.Lhs)
 			}
-			w.implicitConvExpr(stmt, typ, stmt.Rhs)
+			w.implicitConvExpr(typ, stmt.Rhs)
 
 		default:
 			w.assignStmt(stmt, stmt.Lhs, stmt.Rhs)
@@ -1125,7 +1125,7 @@ func (w *writer) stmt1(stmt syntax.Stmt) {
 		w.Code(stmtSend)
 		w.pos(stmt)
 		w.expr(stmt.Chan)
-		w.implicitConvExpr(stmt, chanType.Elem(), stmt.Value)
+		w.implicitConvExpr(chanType.Elem(), stmt.Value)
 
 	case *syntax.SwitchStmt:
 		w.Code(stmtSwitch)
@@ -1376,7 +1376,7 @@ func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
 		}
 
 		if w.Bool(tag != nil) {
-			w.implicitConvExpr(tag, tagType, tag)
+			w.implicitConvExpr(tagType, tag)
 		}
 	}
 
@@ -1406,7 +1406,7 @@ func (w *writer) switchStmt(stmt *syntax.SwitchStmt) {
 			w.Sync(pkgbits.SyncExprs)
 			w.Len(len(cases))
 			for _, cas := range cases {
-				w.implicitConvExpr(cas, tagType, cas)
+				w.implicitConvExpr(tagType, cas)
 			}
 		}
 
@@ -1569,7 +1569,7 @@ func (w *writer) expr(expr syntax.Expr) {
 		w.Code(exprIndex)
 		w.expr(expr.X)
 		w.pos(expr)
-		w.implicitConvExpr(expr, keyType, expr.Index)
+		w.implicitConvExpr(keyType, expr.Index)
 		if keyType != nil {
 			w.rtype(xtyp)
 		}
@@ -1619,9 +1619,9 @@ func (w *writer) expr(expr syntax.Expr) {
 
 		w.Code(exprBinaryOp)
 		w.op(binOps[expr.Op])
-		w.implicitConvExpr(expr, commonType, expr.X)
+		w.implicitConvExpr(commonType, expr.X)
 		w.pos(expr)
-		w.implicitConvExpr(expr, commonType, expr.Y)
+		w.implicitConvExpr(commonType, expr.Y)
 
 	case *syntax.CallExpr:
 		tv, ok := w.p.info.Types[expr.Fun]
@@ -1814,23 +1814,23 @@ func (w *writer) multiExpr(pos poser, dstType func(int) types2.Type, exprs []syn
 	w.Bool(false) // N:N assignment
 	w.Len(len(exprs))
 	for i, expr := range exprs {
-		w.implicitConvExpr(pos, dstType(i), expr)
+		w.implicitConvExpr(dstType(i), expr)
 	}
 }
 
-// implicitConvExpr is like expr, but if dst is non-nil and different from
-// expr's type, then an implicit conversion operation is inserted at
-// pos.
-func (w *writer) implicitConvExpr(pos poser, dst types2.Type, expr syntax.Expr) {
+// implicitConvExpr is like expr, but if dst is non-nil and different
+// from expr's type, then an implicit conversion operation is inserted
+// at expr's position.
+func (w *writer) implicitConvExpr(dst types2.Type, expr syntax.Expr) {
 	src := w.p.typeOf(expr)
 	if dst != nil && !types2.Identical(src, dst) {
 		if !types2.AssignableTo(src, dst) {
-			w.p.fatalf(pos, "%v is not assignable to %v", src, dst)
+			w.p.fatalf(expr.Pos(), "%v is not assignable to %v", src, dst)
 		}
 		w.Code(exprConvert)
 		w.Bool(true) // implicit
 		w.typ(dst)
-		w.pos(pos)
+		w.pos(expr)
 		w.convRTTI(src, dst)
 		w.Bool(isTypeParam(dst))
 		// fallthrough
@@ -1882,12 +1882,12 @@ func (w *writer) compLit(lit *syntax.CompositeLit) {
 			if kv, ok := elem.(*syntax.KeyValueExpr); w.Bool(ok) {
 				// use position of expr.Key rather than of elem (which has position of ':')
 				w.pos(kv.Key)
-				w.implicitConvExpr(kv.Key, keyType, kv.Key)
+				w.implicitConvExpr(keyType, kv.Key)
 				elem = kv.Value
 			}
 		}
 		w.pos(elem)
-		w.implicitConvExpr(elem, elemType, elem)
+		w.implicitConvExpr(elemType, elem)
 	}
 }