diff --git a/src/cmd/compile/internal/gc/const.go b/src/cmd/compile/internal/gc/const.go
index 3a9080e67d..f2035bf9a8 100644
--- a/src/cmd/compile/internal/gc/const.go
+++ b/src/cmd/compile/internal/gc/const.go
@@ -427,13 +427,13 @@ bad:
 func tocplx(v Val) Val {
 	switch u := v.U.(type) {
 	case *Mpint:
-		c := new(Mpcplx)
+		c := newMpcmplx()
 		c.Real.SetInt(u)
 		c.Imag.SetFloat64(0.0)
 		v.U = c
 
 	case *Mpflt:
-		c := new(Mpcplx)
+		c := newMpcmplx()
 		c.Real.Set(u)
 		c.Imag.SetFloat64(0.0)
 		v.U = c
@@ -845,7 +845,7 @@ Outer:
 	case CTCPLX:
 		x, y := x.U.(*Mpcplx), y.U.(*Mpcplx)
 
-		u := new(Mpcplx)
+		u := newMpcmplx()
 		u.Real.Set(&x.Real)
 		u.Imag.Set(&x.Imag)
 		switch op {
@@ -900,7 +900,7 @@ func unaryOp(op Op, x Val, t *types.Type) Val {
 
 		case CTCPLX:
 			x := x.U.(*Mpcplx)
-			u := new(Mpcplx)
+			u := newMpcmplx()
 			u.Real.Set(&x.Real)
 			u.Imag.Set(&x.Imag)
 			u.Real.Neg()
diff --git a/src/cmd/compile/internal/gc/mpfloat.go b/src/cmd/compile/internal/gc/mpfloat.go
index 846ce4cca7..b3a9af452a 100644
--- a/src/cmd/compile/internal/gc/mpfloat.go
+++ b/src/cmd/compile/internal/gc/mpfloat.go
@@ -32,12 +32,14 @@ type Mpcplx struct {
 	Imag Mpflt
 }
 
+// Use newMpflt (not new(Mpflt)!) to get the correct default precision.
 func newMpflt() *Mpflt {
 	var a Mpflt
 	a.Val.SetPrec(Mpprec)
 	return &a
 }
 
+// Use newMpcmplx (not new(Mpcplx)!) to get the correct default precision.
 func newMpcmplx() *Mpcplx {
 	var a Mpcplx
 	a.Real = *newMpflt()
diff --git a/src/cmd/compile/internal/gc/noder.go b/src/cmd/compile/internal/gc/noder.go
index 3aa303c0c1..3fab95b917 100644
--- a/src/cmd/compile/internal/gc/noder.go
+++ b/src/cmd/compile/internal/gc/noder.go
@@ -1327,7 +1327,7 @@ func (p *noder) basicLit(lit *syntax.BasicLit) Val {
 		return Val{U: x}
 
 	case syntax.ImagLit:
-		x := new(Mpcplx)
+		x := newMpcmplx()
 		x.Imag.SetString(strings.TrimSuffix(s, "i"))
 		return Val{U: x}
 
diff --git a/src/cmd/compile/internal/gc/swt_test.go b/src/cmd/compile/internal/gc/swt_test.go
index 74419596d2..2f73ef7b99 100644
--- a/src/cmd/compile/internal/gc/swt_test.go
+++ b/src/cmd/compile/internal/gc/swt_test.go
@@ -16,7 +16,7 @@ func nodrune(r rune) *Node {
 }
 
 func nodflt(f float64) *Node {
-	v := new(Mpflt)
+	v := newMpflt()
 	v.SetFloat64(f)
 	return nodlit(Val{v})
 }
diff --git a/src/cmd/compile/internal/gc/typecheck.go b/src/cmd/compile/internal/gc/typecheck.go
index 63e0d78273..e22fd6445a 100644
--- a/src/cmd/compile/internal/gc/typecheck.go
+++ b/src/cmd/compile/internal/gc/typecheck.go
@@ -1589,7 +1589,7 @@ func typecheck1(n *Node, top int) (res *Node) {
 
 		if l.Op == OLITERAL && r.Op == OLITERAL {
 			// make it a complex literal
-			c := new(Mpcplx)
+			c := newMpcmplx()
 			c.Real.Set(toflt(l.Val()).U.(*Mpflt))
 			c.Imag.Set(toflt(r.Val()).U.(*Mpflt))
 			setconst(n, Val{c})
diff --git a/test/fixedbugs/issue30243.go b/test/fixedbugs/issue30243.go
new file mode 100644
index 0000000000..51fd204cbc
--- /dev/null
+++ b/test/fixedbugs/issue30243.go
@@ -0,0 +1,27 @@
+// run
+
+// Copyright 2019 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.
+
+// Compile-time constants, even if they cannot be represented
+// accurately, should remain the same in operations that don't
+// affect their values.
+
+package main
+
+import "fmt"
+
+func main() {
+	const x = 0.01
+	const xi = 0.01i
+	const xc = complex(0, x)
+
+	if imag(xi) != x {
+		fmt.Printf("FAILED: %g != %g\n", imag(xi), x)
+	}
+
+	if xi != complex(0, x) {
+		fmt.Printf("FAILED: %g != %g\n", xi, complex(0, x))
+	}
+}