[dev.unified] cmd/compile: special case f(g()) calls in Unified IR

For f(g()) calls where g() is multi-valued, we may need to insert
implicit conversions to convert g()'s result values to f()'s parameter
types. This CL refactors code slightly so this will be easier to
handle.

Change-Id: I3a432220dcb62daecf9a66030e8fa1f097e95f95
Reviewed-on: https://go-review.googlesource.com/c/go/+/413362
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>

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

@@ -1735,8 +1735,15 @@ func (r *reader) expr() (res ir.Node) {
 			fun = typecheck.Callee(ir.NewSelectorExpr(pos, ir.OXDOT, fun, sym))
 		}
 		pos := r.pos()
-		args := r.exprs()
-		dots := r.Bool()
+		var args ir.Nodes
+		var dots bool
+		if r.Bool() { // f(g())
+			call := r.expr()
+			args = []ir.Node{call}
+		} else {
+			args = r.exprs()
+			dots = r.Bool()
+		}
 		n := typecheck.Call(pos, fun, args, dots)
 		switch n.Op() {
 		case ir.OAPPEND:

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

@@ -1505,8 +1505,14 @@ func (w *writer) expr(expr syntax.Expr) {
 		w.Code(exprCall)
 		writeFunExpr()
 		w.pos(expr)
-		w.exprs(expr.ArgList)
-		w.Bool(expr.HasDots)
+		if w.Bool(len(expr.ArgList) == 1 && isMultiValueExpr(w.p.info, expr.ArgList[0])) {
+			// f(g()) call
+			assert(!expr.HasDots)
+			w.expr(expr.ArgList[0])
+		} else {
+			w.exprs(expr.ArgList)
+			w.Bool(expr.HasDots)
+		}
 	}
 }
 
@@ -1999,6 +2005,19 @@ func isPkgQual(info *types2.Info, sel *syntax.SelectorExpr) bool {
 	return false
 }
 
+// isMultiValueExpr reports whether expr is a function call expression
+// that yields multiple values.
+func isMultiValueExpr(info *types2.Info, expr syntax.Expr) bool {
+	tv, ok := info.Types[expr]
+	assert(ok)
+	assert(tv.IsValue())
+	if tuple, ok := tv.Type.(*types2.Tuple); ok {
+		assert(tuple.Len() > 1)
+		return true
+	}
+	return false
+}
+
 // recvBase returns the base type for the given receiver parameter.
 func recvBase(recv *types2.Var) *types2.Named {
 	typ := recv.Type()