Diagnostics on 'infix' (declaration&use site)

compiler/testData/diagnostics/tests/Infix.kt

@@ -0,0 +1,26 @@
+class Pair<out A, out B>(val first: A, val second: B)
+
+class Example {
+    infix fun to(other: Example) = Pair(this, other)
+    fun toNonInfix(other: Example) = Pair(this, other)
+}
+
+infix fun Example.toExt(other: Example) = Pair(this, other)
+fun Example.toExtNonInfix(other: Example) = Pair(this, other)
+
+fun Example.withLambda(f: () -> Unit) = Pair(this, f)
+
+fun test() {
+    Example() to Example()
+    Example() <!INFIX_MODIFIER_REQUIRED!>toNonInfix<!> Example()
+    Example().toNonInfix(Example())
+
+    val a = Example()
+    val b = Example()
+
+    a toExt b
+    a <!INFIX_MODIFIER_REQUIRED!>toExtNonInfix<!> b
+    a.toExtNonInfix(b)
+
+    a <!INFIX_MODIFIER_REQUIRED!>withLambda<!> { }
+}