[dev.typeparams] cmd/compile: call transformArgs before early typecheckaste in noder

In the cases where we do an early call to typecheckaste() in noder to
expose CONVIFACE nodes, we need a preceding call to transformArgs().
This is needed to allow typecheckaste() to run correctly, in the case of
f(g()), where g has multiple return values.

I also cleaned up the code a bit and commented the code in Call(), and
we do the call to typecheckaste() in several more cases.

In stencil.go:stencil(), I moved the transformCall earlier for the
OCALLMETH/ODOTMETH case, just as I did in my previous CL for
OCALL/OFUNCINST. By doing this, transformArgs no longer needs to deal
with the extra dictionary args. Therefore, I was able to simply
transformArgs() to look like typecheckargs() again, and make use of
RewriteMultiValue directly.

Updates #47514

Change-Id: I49eb82ac05707e50c2e2fb03e39458a70491d406
Reviewed-on: https://go-review.googlesource.com/c/go/+/340531
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

5 files changed, 55 insertions, 91 deletions

diff --git a/src/cmd/compile/internal/noder/helpers.go b/src/cmd/compile/internal/noder/helpers.go
index 2b00a9d7a6..b9dbd030af 100644
--- a/src/cmd/compile/internal/noder/helpers.go
+++ b/src/cmd/compile/internal/noder/helpers.go
@@ -171,39 +171,34 @@ func Call(pos src.XPos, typ *types.Type, fun ir.Node, args []ir.Node, dots bool)
 		}
 	}
 
-	if fun.Type().HasTParam() {
+	if fun.Type().HasTParam() || fun.Op() == ir.OXDOT || fun.Op() == ir.OFUNCINST {
 		// If the fun arg is or has a type param, we can't do all the
-		// transformations, since we may not have needed properties yet.
-		// (e.g. number of return values, etc). However, if we do have the
-		// function type (even though it is parameterized), then can add in
-		// any needed CONVIFACE nodes. We can't do anything if fun is a type
-		// param (which is probably described by a structural constraint)
+		// transformations, since we may not have needed properties yet
+		// (e.g. number of return values, etc). The same applies if a fun
+		// which is an XDOT could not be transformed yet because of a generic
+		// type in the X of the selector expression.
+		//
+		// A function instantiation (even if fully concrete) shouldn't be
+		// transformed yet, because we need to add the dictionary during the
+		// transformation.
+		//
+		// However, if we have a function type (even though it is
+		// parameterized), then we can add in any needed CONVIFACE nodes via
+		// typecheckaste(). We need to call transformArgs() to deal first
+		// with the f(g(()) case where g returns multiple return values. We
+		// can't do anything if fun is a type param (which is probably
+		// described by a structural constraint)
 		if fun.Type().Kind() == types.TFUNC {
+			transformArgs(n)
 			typecheckaste(ir.OCALL, fun, n.IsDDD, fun.Type().Params(), n.Args, true)
 		}
 		return typed(typ, n)
 	}
 
-	if fun.Op() == ir.OXDOT {
-		if !fun.(*ir.SelectorExpr).X.Type().HasTParam() {
-			base.FatalfAt(pos, "Expecting type param receiver in %v", fun)
-		}
-		// For methods called in a generic function, don't do any extra
-		// transformations. We will do those later when we create the
-		// instantiated function and have the correct receiver type.
-		typed(typ, n)
-		return n
-	}
-	if fun.Op() != ir.OFUNCINST {
-		// If no type params, do the normal call transformations. This
-		// will convert OCALL to OCALLFUNC.
-		typed(typ, n)
-		transformCall(n)
-		return n
-	}
-
-	// Leave the op as OCALL, which indicates the call still needs typechecking.
+	// If no type params, do the normal call transformations. This
+	// will convert OCALL to OCALLFUNC.
 	typed(typ, n)
+	transformCall(n)
 	return n
 }
 
diff --git a/src/cmd/compile/internal/noder/stencil.go b/src/cmd/compile/internal/noder/stencil.go
index 5f2250d2f4..23e8090136 100644
--- a/src/cmd/compile/internal/noder/stencil.go
+++ b/src/cmd/compile/internal/noder/stencil.go
@@ -161,18 +161,21 @@ func (g *irgen) stencil() {
 					}
 				}
 
+				// Transform the Call now, which changes OCALL
+				// to OCALLFUNC and does typecheckaste/assignconvfn.
+				transformCall(call)
+
 				st := g.getInstantiation(gf, targs, true)
 				dictValue, usingSubdict := g.getDictOrSubdict(declInfo, n, gf, targs, true)
 				// We have to be using a subdictionary, since this is
 				// a generic method call.
 				assert(usingSubdict)
 
-				call.SetOp(ir.OCALL)
+				// Transform to a function call, by appending the
+				// dictionary and the receiver to the args.
+				call.SetOp(ir.OCALLFUNC)
 				call.X = st.Nname
 				call.Args.Prepend(dictValue, meth.X)
-				// Transform the Call now, which changes OCALL
-				// to OCALLFUNC and does typecheckaste/assignconvfn.
-				transformCall(call)
 				modified = true
 			}
 		})
diff --git a/src/cmd/compile/internal/noder/stmt.go b/src/cmd/compile/internal/noder/stmt.go
index 5af4a2da9c..1949f56095 100644
--- a/src/cmd/compile/internal/noder/stmt.go
+++ b/src/cmd/compile/internal/noder/stmt.go
@@ -129,6 +129,7 @@ func (g *irgen) stmt(stmt syntax.Stmt) ir.Node {
 				// Delay transforming the return statement if any of the
 				// return values have a type param.
 				if !ir.HasNamedResults(ir.CurFunc) {
+					transformArgs(n)
 					// But add CONVIFACE nodes where needed if
 					// any of the return values have interface type.
 					typecheckaste(ir.ORETURN, nil, false, ir.CurFunc.Type().Results(), n.Results, true)
diff --git a/src/cmd/compile/internal/noder/transform.go b/src/cmd/compile/internal/noder/transform.go
index ff113877df..e1eeb8e739 100644
--- a/src/cmd/compile/internal/noder/transform.go
+++ b/src/cmd/compile/internal/noder/transform.go
@@ -365,7 +365,7 @@ assignOK:
 	}
 }
 
-// Corresponds to, but slightly more general than, typecheck.typecheckargs.
+// Corresponds to typecheck.typecheckargs.  Really just deals with multi-value calls.
 func transformArgs(n ir.InitNode) {
 	var list []ir.Node
 	switch n := n.(type) {
@@ -379,76 +379,22 @@ func transformArgs(n ir.InitNode) {
 	case *ir.ReturnStmt:
 		list = n.Results
 	}
-
-	// Look to see if we have any multi-return functions as arguments.
-	extra := 0
-	for _, arg := range list {
-		t := arg.Type()
-		if t.IsFuncArgStruct() {
-			num := t.Fields().Len()
-			if num <= 1 {
-				base.Fatalf("multi-return type with only %d parts", num)
-			}
-			extra += num - 1
-		}
-	}
-	// If not, nothing to do.
-	if extra == 0 {
+	if len(list) != 1 {
 		return
 	}
 
-	// Rewrite f(..., g(), ...) into t1, ..., tN = g(); f(..., t1, ..., tN, ...).
+	t := list[0].Type()
+	if t == nil || !t.IsFuncArgStruct() {
+		return
+	}
 
 	// Save n as n.Orig for fmt.go.
 	if ir.Orig(n) == n {
 		n.(ir.OrigNode).SetOrig(ir.SepCopy(n))
 	}
 
-	// If we're outside of function context, then this call will
-	// be executed during the generated init function. However,
-	// init.go hasn't yet created it. Instead, associate the
-	// temporary variables with  InitTodoFunc for now, and init.go
-	// will reassociate them later when it's appropriate.
-	static := ir.CurFunc == nil
-	if static {
-		ir.CurFunc = typecheck.InitTodoFunc
-	}
-
-	// Expand multi-return function calls.
-	// The spec only allows a multi-return function as an argument
-	// if it is the only argument. This code must handle calls to
-	// stenciled generic functions which have extra arguments
-	// (like the dictionary) so it must handle a slightly more general
-	// cases, like f(n, g()) where g is multi-return.
-	newList := make([]ir.Node, 0, len(list)+extra)
-	for _, arg := range list {
-		t := arg.Type()
-		if t.IsFuncArgStruct() {
-			as := ir.NewAssignListStmt(base.Pos, ir.OAS2, nil, []ir.Node{arg})
-			for _, f := range t.FieldSlice() {
-				t := typecheck.Temp(f.Type)
-				as.PtrInit().Append(ir.NewDecl(base.Pos, ir.ODCL, t))
-				as.Lhs.Append(t)
-				newList = append(newList, t)
-			}
-			transformAssign(as, as.Lhs, as.Rhs)
-			as.SetTypecheck(1)
-			n.PtrInit().Append(as)
-		} else {
-			newList = append(newList, arg)
-		}
-	}
-
-	if static {
-		ir.CurFunc = nil
-	}
-
-	switch n := n.(type) {
-	case *ir.CallExpr:
-		n.Args = newList
-	case *ir.ReturnStmt:
-		n.Results = newList
-	}
+	// Rewrite f(g()) into t1, t2, ... = g(); f(t1, t2, ...).
+	typecheck.RewriteMultiValueCall(n, list[0])
 }
 
 // assignconvfn converts node n for assignment to type t. Corresponds to
diff --git a/test/typeparam/issue47514b.go b/test/typeparam/issue47514b.go
new file mode 100644
index 0000000000..5428a0edc5
--- /dev/null
+++ b/test/typeparam/issue47514b.go
@@ -0,0 +1,19 @@
+// run -gcflags=-G=3
+
+// Copyright 2021 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.
+
+package main
+
+func Do[T any](do func() (T, string)) {
+	_ = func() (T, string) {
+		return do()
+	}
+}
+
+func main() {
+	Do[int](func() (int, string) {
+		return 3, "3"
+	})
+}