fix: instantiate metavariables when collecting a goal's constants (#13748)

This PR fixes premise selection silently dropping relevant premises when
the goal was reached via `induction`.

`MVarId.getRelevantConstants` and `MVarId.getConstants` walked the raw
goal type returned by `getType`, which after `induction` is `?motive
arg` with `?motive` an assigned-but-unsubstituted metavariable. The
constant fold treats the metavariable as an opaque leaf, so every
constant in the goal predicate is lost. Instantiating metavariables
first recovers them.

Thanks to Xavier Généreux for the report.

🤖 Prepared with Claude Code

---------

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/Lean/LibrarySuggestions/Basic.lean

@@ -101,16 +101,26 @@ end Lean.Expr
 
 open Lean Meta MVarId in
 public def Lean.MVarId.getConstants (g : MVarId) : MetaM NameSet := withContext g do
-  let mut c := (← g.getType).getUsedConstantsAsSet
+  -- `instantiateMVars` is needed so that constants are not lost behind assigned
+  -- metavariables, e.g. the `?motive` left in a goal reached via `induction`.
+  -- Note: this does not recover constants behind non-ground delayed-assigned
+  -- metavariables. Without evidence this matters for premise selection,
+  -- for now we avoid the extra complexity of walking the metavariable graph.
+  let mut c := (← instantiateMVars (← g.getType)).getUsedConstantsAsSet
   for t in (← getLocalHyps) do
-    c := c ∪ (← inferType t).getUsedConstantsAsSet
+    c := c ∪ (← instantiateMVars (← inferType t)).getUsedConstantsAsSet
   return c
 
 open Lean Meta MVarId in
 public def Lean.MVarId.getRelevantConstants (g : MVarId) : MetaM NameSet := withContext g do
-  let mut c ← (← g.getType).relevantConstantsAsSet
+  -- `instantiateMVars` is needed so that constants are not lost behind assigned
+  -- metavariables, e.g. the `?motive` left in a goal reached via `induction`.
+  -- Note: this does not recover constants behind non-ground delayed-assigned
+  -- metavariables. Without evidence this matters for premise selection,
+  -- for now we avoid the extra complexity of walking the metavariable graph.
+  let mut c ← (← instantiateMVars (← g.getType)).relevantConstantsAsSet
   for t in (← getLocalHyps) do
-    c := c ∪ (← (← inferType t).relevantConstantsAsSet)
+    c := c ∪ (← (← instantiateMVars (← inferType t)).relevantConstantsAsSet)
   return c
 
 @[expose] public section

tests/elab/library_suggestions_relevant_constants.lean

@@ -0,0 +1,27 @@
+import Lean.LibrarySuggestions.Basic
+
+/-!
+Regression test: `MVarId.getRelevantConstants` must instantiate metavariables
+in the goal type before collecting constants.
+
+A goal reached via `induction` has type `?motive arg`, where `?motive` is an
+assigned-but-unsubstituted metavariable. Without `instantiateMVars`, the
+constant fold treats `?motive` as an opaque leaf, so every constant occurring
+in the goal predicate is lost.
+-/
+
+open Lean Lean.Elab.Tactic
+
+axiom MyP : Nat → Prop
+
+example : ∀ n : Nat, MyP n := by
+  intro n
+  induction n with
+  | zero =>
+    run_tac do
+      let c ← (← getMainGoal).getRelevantConstants
+      unless c.contains ``MyP do
+        throwError "getRelevantConstants missed `MyP` hidden behind the \
+          induction motive metavariable"
+    sorry
+  | succ k ih => sorry

tests/elab/library_suggestions_relevant_constants.lean.out.expected

@@ -0,0 +1 @@
+library_suggestions_relevant_constants.lean:17:0-17:7: warning: declaration uses `sorry`