cmd/internal/gc: accept map literals with omitted key type

Fixes #10209.

Change-Id: I248434f9195c868befd1ed8a6000a9cac72d1df8
Reviewed-on: https://go-review.googlesource.com/10263Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -1080,7 +1080,7 @@ start_complit:
 	}
 
 keyval:
-	expr ':' complitexpr
+	complitexpr ':' complitexpr
 	{
 		$$ = Nod(OKEY, $1, $3);
 	}

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

@@ -3056,9 +3056,11 @@ func typecheckcomplit(np **Node) {
 				continue
 			}
 
-			typecheck(&l.Left, Erv)
-			defaultlit(&l.Left, t.Down)
-			l.Left = assignconv(l.Left, t.Down, "map key")
+			r = l.Left
+			pushtype(r, t.Down)
+			typecheck(&r, Erv)
+			defaultlit(&r, t.Down)
+			l.Left = assignconv(r, t.Down, "map key")
 			if l.Left.Op != OCONV {
 				keydup(l.Left, hash)
 			}

test/complit1.go

@@ -40,3 +40,17 @@ var (
 	_ = &T{i: 0, f: 0, s: "", next: {}} // ERROR "missing type in composite literal|omit types within composite literal"
 	_ = &T{0, 0, "", {}}                // ERROR "missing type in composite literal|omit types within composite literal"
 )
+
+type M map[T]T
+
+var (
+	_ = M{{i:1}: {i:2}}
+	_ = M{T{i:1}: {i:2}}
+	_ = M{{i:1}: T{i:2}}
+	_ = M{T{i:1}: T{i:2}}
+)
+
+type S struct { s [1]*M1 }
+type M1 map[S]int
+var _ = M1{{s:[1]*M1{&M1{{}:1}}}:2}
+