[dev.typeparams] Fix the types of the OFUNCINST nodes in noder2

types2 doesn't actually give us the type of an instantiated
function/method after the type args have been applied. So, do a
substitution at the point that we create the OFUNCINST nodes.

We also needed to add in translation of the typeparams of a function
signature in the type substituter. If the type params of the function
become all concrete after the substitution, then we just drop them,
since the whole signature must now be concrete.

Change-Id: I6116d2aa248be6924ec9e6d8516678db45aa65c4
Reviewed-on: https://go-review.googlesource.com/c/go/+/336370
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Dan Scales <danscales@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

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

@@ -80,7 +80,7 @@ func (g *irgen) expr(expr syntax.Expr) ir.Node {
 	if n.Typecheck() != 1 && n.Typecheck() != 3 {
 		base.FatalfAt(g.pos(expr), "missed typecheck: %+v", n)
 	}
-	if !g.match(n.Type(), typ, tv.HasOk()) {
+	if n.Op() != ir.OFUNCINST && !g.match(n.Type(), typ, tv.HasOk()) {
 		base.FatalfAt(g.pos(expr), "expected %L to have type %v", n, typ)
 	}
 	return n
@@ -128,11 +128,14 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 				// includes the additional inferred type args
 				fun.(*ir.InstExpr).Targs = targs
 			} else {
-				// Create a function instantiation here, given
+				// Create a function instantiation here, given there
-				// there are only inferred type args (e.g.
+				// are only inferred type args (e.g. min(5,6), where
-				// min(5,6), where min is a generic function)
+				// min is a generic function). Substitute the type
+				// args for the type params in the uninstantiated function's
+				// type.
 				inst := ir.NewInstExpr(pos, ir.OFUNCINST, fun, targs)
-				typed(fun.Type(), inst)
+				newt := g.substType(fun.Type(), fun.Type().TParams(), targs)
+				typed(newt, inst)
 				fun = inst
 			}
 
@@ -169,7 +172,14 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 			panic("Incorrect argument for generic func instantiation")
 		}
 		n := ir.NewInstExpr(pos, ir.OFUNCINST, x, targs)
-		typed(g.typ(typ), n)
+		newt := g.typ(typ)
+		// Substitute the type args for the type params in the uninstantiated
+		// function's type. If there aren't enough type args, then the rest
+		// will be inferred at the call node, so don't try the substitution yet.
+		if x.Type().TParams().NumFields() == len(targs) {
+			newt = g.substType(g.typ(typ), x.Type().TParams(), targs)
+		}
+		typed(newt, n)
 		return n
 
 	case *syntax.SelectorExpr:
@@ -201,6 +211,28 @@ func (g *irgen) expr0(typ types2.Type, expr syntax.Expr) ir.Node {
 	}
 }
 
+// substType does a normal type substition, but tparams is in the form of a field
+// list, and targs is in terms of a slice of type nodes. substType records any newly
+// instantiated types into g.instTypeList.
+func (g *irgen) substType(typ *types.Type, tparams *types.Type, targs []ir.Node) *types.Type {
+	fields := tparams.FieldSlice()
+	tparams1 := make([]*types.Type, len(fields))
+	for i, f := range fields {
+		tparams1[i] = f.Type
+	}
+	targs1 := make([]*types.Type, len(targs))
+	for i, n := range targs {
+		targs1[i] = n.Type()
+	}
+	ts := typecheck.Tsubster{
+		Tparams: tparams1,
+		Targs:   targs1,
+	}
+	newt := ts.Typ(typ)
+	g.instTypeList = append(g.instTypeList, ts.InstTypeList...)
+	return newt
+}
+
 // selectorExpr resolves the choice of ODOT, ODOTPTR, OMETHVALUE (eventually
 // ODOTMETH & ODOTINTER), and OMETHEXPR and deals with embedded fields here rather
 // than in typecheck.go.

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

@@ -1130,7 +1130,10 @@ func (ts *Tsubster) Typ(t *types.Type) *types.Type {
 		newrecvs := ts.tstruct(t.Recvs(), false)
 		newparams := ts.tstruct(t.Params(), false)
 		newresults := ts.tstruct(t.Results(), false)
-		if newrecvs != t.Recvs() || newparams != t.Params() || newresults != t.Results() || targsChanged {
+		// Translate the tparams of a signature.
+		newtparams := ts.tstruct(t.TParams(), false)
+		if newrecvs != t.Recvs() || newparams != t.Params() ||
+			newresults != t.Results() || newtparams != t.TParams() || targsChanged {
 			// If any types have changed, then the all the fields of
 			// of recv, params, and results must be copied, because they have
 			// offset fields that are dependent, and so must have an
@@ -1148,7 +1151,16 @@ func (ts *Tsubster) Typ(t *types.Type) *types.Type {
 			if newresults == t.Results() {
 				newresults = ts.tstruct(t.Results(), true)
 			}
-			newt = types.NewSignature(t.Pkg(), newrecv, t.TParams().FieldSlice(), newparams.FieldSlice(), newresults.FieldSlice())
+			var tparamfields []*types.Field
+			if newtparams.HasTParam() {
+				tparamfields = newtparams.FieldSlice()
+			} else {
+				// Completely remove the tparams from the resulting
+				// signature, if the tparams are now concrete types.
+				tparamfields = nil
+			}
+			newt = types.NewSignature(t.Pkg(), newrecv, tparamfields,
+				newparams.FieldSlice(), newresults.FieldSlice())
 		}
 
 	case types.TINTER: