From 6de7d8b254a1b422f7da27451b1efa07ff4bec4a Mon Sep 17 00:00:00 2001
From: James Shubin <james@shubin.ca>
Date: Wed, 31 Jul 2024 16:01:27 -0400
Subject: [PATCH] lang: funcs: Catch non-specific type build error

If you had ambiguous code, and specified an invalid type, this could
sneak through and become a runtime error, instead of a compile-time
error. We fix this and add a test.
---
 examples/lang/datetime2.mcl                         |  2 +-
 examples/lang/datetime3.mcl                         |  2 +-
 lang/funcs/struct_lookup_func.go                    |  8 ++++++++
 .../TestAstFunc2/unify-template.txtar               | 13 +++++++++++++
 4 files changed, 23 insertions(+), 2 deletions(-)
 create mode 100644 lang/interpret_test/TestAstFunc2/unify-template.txtar

diff --git a/examples/lang/datetime2.mcl b/examples/lang/datetime2.mcl
index 97ba4859..bdcaa280 100644
--- a/examples/lang/datetime2.mcl
+++ b/examples/lang/datetime2.mcl
@@ -8,7 +8,7 @@ $ayear = 60 * 60 * 24 * 365	# is a year in seconds (31536000)
 
 $tmplvalues = struct{year => $secplusone, load => $theload,}
 
-$theload = sys.load()->x1
+$theload float = sys.load()->x1	# ambiguous so we specify the type!
 
 if 5 > 3 {
 	file "/tmp/mgmt/datetime" {
diff --git a/examples/lang/datetime3.mcl b/examples/lang/datetime3.mcl
index 53de40fa..ccfbbf1f 100644
--- a/examples/lang/datetime3.mcl
+++ b/examples/lang/datetime3.mcl
@@ -9,7 +9,7 @@ $ayear = 60 * 60 * 24 * 365	# is a year in seconds (31536000)
 
 $tmplvalues = struct{year => $secplusone, load => $theload, vumeter => $vumeter,}
 
-$theload = sys.load()->x1
+$theload float = sys.load()->x1	# ambiguous so we specify the type!
 
 $vumeter = example.vumeter("====", 10, 0.9)
 
diff --git a/lang/funcs/struct_lookup_func.go b/lang/funcs/struct_lookup_func.go
index a9c8290e..d63bac75 100644
--- a/lang/funcs/struct_lookup_func.go
+++ b/lang/funcs/struct_lookup_func.go
@@ -193,6 +193,14 @@ func (obj *StructLookupFunc) Build(typ *types.Type) (*types.Type, error) {
 	if ix == -1 {
 		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
 	}
+	tF, exists := tStruct.Map[tStruct.Ord[ix]]
+	if !exists {
+		return nil, fmt.Errorf("field %s was not found in struct", obj.field)
+	}
+	// The return value must match the type of the field we're pulling out!
+	if err := typ.Out.Cmp(tF); err != nil {
+		return nil, fmt.Errorf("field %s type error: %+v", obj.field, err)
+	}
 
 	obj.Type = tStruct // struct type
 	obj.Out = typ.Out  // type of return value
diff --git a/lang/interpret_test/TestAstFunc2/unify-template.txtar b/lang/interpret_test/TestAstFunc2/unify-template.txtar
new file mode 100644
index 00000000..83343b14
--- /dev/null
+++ b/lang/interpret_test/TestAstFunc2/unify-template.txtar
@@ -0,0 +1,13 @@
+-- main.mcl --
+import "sys"
+
+$tmplvalues = struct{num => 42, load => $theload,}
+
+$theload bool = sys.load()->x1 # wrong type, make sure the compiler catches it!
+
+file "/tmp/datetime" {
+	state => $const.res.file.state.exists,
+	content => template("num: {{ .num }} seconds\nload average: {{ .load }}\n", $tmplvalues),
+}
+-- OUTPUT --
+# err: errUnify: error setting type: func() { <built-in:_struct_lookup> }, error: field x1 type error: base kind does not match (bool != float)